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-rw-r--r--nix/bufCheck/default.nix31
-rw-r--r--nix/buildGo/README.md27
-rw-r--r--nix/buildGo/default.nix80
-rw-r--r--nix/buildGo/example/default.nix8
-rw-r--r--nix/buildGo/example/thing.proto10
-rw-r--r--nix/buildGo/external/main.go7
-rw-r--r--nix/buildGo/proto.nix87
-rw-r--r--nix/buildLisp/default.nix1
-rw-r--r--nix/buildkite/default.nix169
-rwxr-xr-xnix/buildkite/fetch-parent-targets.sh61
-rw-r--r--nix/dependency-analyzer/default.nix53
-rw-r--r--nix/lazy-deps/default.nix48
-rw-r--r--nix/nix-1p/README.md105
-rw-r--r--nix/readTree/README.md13
-rw-r--r--nix/readTree/default.nix70
-rw-r--r--nix/readTree/tests/default.nix10
-rw-r--r--nix/readTree/tests/test-tree-traversal/skip-tree/a/default.nix1
-rw-r--r--nix/readTree/tests/test-tree-traversal/skip-tree/b/.skip-tree1
-rw-r--r--nix/readTree/tests/test-tree-traversal/skip-tree/b/default.nix1
-rw-r--r--nix/renderMarkdown/default.nix19
-rw-r--r--nix/sparseTree/default.nix42
-rw-r--r--nix/writeTree/OWNERS1
-rw-r--r--nix/writeTree/default.nix43
-rw-r--r--nix/writeTree/tests/default.nix93
24 files changed, 610 insertions, 371 deletions
diff --git a/nix/bufCheck/default.nix b/nix/bufCheck/default.nix
index 25a8865d8d..ec98cfc376 100644
--- a/nix/bufCheck/default.nix
+++ b/nix/bufCheck/default.nix
@@ -1,9 +1,26 @@
-# Check protobuf syntax and breaking.
+# Check protobuf breaking. Lints already happen in individual targets.
 #
-{ depot, pkgs, ... }:
+{ depot, pkgs, lib, ... }:
 
-pkgs.writeShellScriptBin "ci-buf-check" ''
-  ${depot.third_party.nixpkgs.buf}/bin/buf lint .
-  # Report-only
-  ${depot.third_party.nixpkgs.buf}/bin/buf breaking . --against "./.git#ref=HEAD~1" || true
-''
+let
+  inherit (depot.nix) bufCheck;# self reference
+
+  script = pkgs.writeShellScriptBin "ci-buf-check" ''
+    export PATH="$PATH:${pkgs.lib.makeBinPath [ pkgs.buf ]}"
+    # Report-only
+    (cd $(git rev-parse --show-toplevel) && (buf breaking . --against "./.git#ref=HEAD~1" || true))
+  '';
+in
+
+script.overrideAttrs (old: {
+  meta = lib.recursiveUpdate old.meta {
+    # Protobuf check step executed in the buildkite pipeline which
+    # validates that changes to .proto files between revisions
+    # don't cause backwards-incompatible or otherwise flawed changes.
+    ci.extraSteps.protoCheck = {
+      alwaysRun = true;
+      label = ":water_buffalo: protoCheck";
+      command = pkgs.writeShellScript "ci-buf-check-step" "exec ${depot.nix.bufCheck}/bin/ci-buf-check";
+    };
+  };
+})
diff --git a/nix/buildGo/README.md b/nix/buildGo/README.md
index 37e0c06933..e9667c039a 100644
--- a/nix/buildGo/README.md
+++ b/nix/buildGo/README.md
@@ -2,8 +2,7 @@ buildGo.nix
 ===========
 
 This is an alternative [Nix][] build system for [Go][]. It supports building Go
-libraries and programs, and even automatically generating Protobuf & gRPC
-libraries.
+libraries and programs.
 
 *Note:* This will probably end up being folded into [Nixery][].
 
@@ -33,7 +32,6 @@ Given a program layout like this:
 ├── lib          <-- some library component
 │   ├── bar.go
 │   └── foo.go
-├── api.proto    <-- gRPC API definition
 ├── main.go      <-- program implementation
 └── default.nix  <-- build instructions
 ```
@@ -44,11 +42,6 @@ The contents of `default.nix` could look like this:
 { buildGo }:
 
 let
-  api = buildGo.grpc {
-    name  = "someapi";
-    proto = ./api.proto;
-  };
-
   lib = buildGo.package {
     name = "somelib";
     srcs = [
@@ -58,7 +51,7 @@ let
   };
 in buildGo.program {
   name = "my-program";
-  deps = [ api lib ];
+  deps = [ lib ];
 
   srcs = [
     ./main.go
@@ -105,22 +98,6 @@ in buildGo.program {
   | `src`     | `path`         | Path to the source **directory**              | yes       |
   | `deps`    | `list<drv>`    | List of dependencies (i.e. other Go packages) | no        |
 
-  For some examples of how `buildGo.external` is used, check out
-  [`proto.nix`](./proto.nix).
-
-* `buildGo.proto`: Build a Go library out of the specified Protobuf definition.
-
-  | parameter   | type        | use                                              | required? |
-  |-------------|-------------|--------------------------------------------------|-----------|
-  | `name`      | `string`    | Name for the resulting library                   | yes       |
-  | `proto`     | `path`      | Path to the Protobuf definition file             | yes       |
-  | `path`      | `string`    | Import path for the resulting Go library         | no        |
-  | `extraDeps` | `list<drv>` | Additional Go dependencies to add to the library | no        |
-
-* `buildGo.grpc`: Build a Go library out of the specified gRPC definition.
-
-  The parameters are identical to `buildGo.proto`.
-
 ## Current status
 
 This project is work-in-progress. Crucially it is lacking the following features:
diff --git a/nix/buildGo/default.nix b/nix/buildGo/default.nix
index 97b8bd2264..c93642a127 100644
--- a/nix/buildGo/default.nix
+++ b/nix/buildGo/default.nix
@@ -22,10 +22,8 @@ let
     replaceStrings
     toString;
 
-  inherit (pkgs) lib runCommand fetchFromGitHub protobuf symlinkJoin;
-
-  # TODO: Adapt to Go 1.19 changes
-  go = pkgs.go_1_18;
+  inherit (pkgs) lib runCommand fetchFromGitHub protobuf symlinkJoin go;
+  goStdlib = buildStdlib go;
 
   # Helpers for low-level Go compiler invocations
   spaceOut = lib.concatStringsSep " ";
@@ -44,8 +42,6 @@ let
 
   xFlags = x_defs: spaceOut (map (k: "-X ${k}=${x_defs."${k}"}") (attrNames x_defs));
 
-  pathToName = p: replaceStrings [ "/" ] [ "_" ] (toString p);
-
   # Add an `overrideGo` attribute to a function result that works
   # similar to `overrideAttrs`, but is used specifically for the
   # arguments passed to Go builders.
@@ -53,16 +49,52 @@ let
     overrideGo = new: makeOverridable f (orig // (new orig));
   };
 
+  buildStdlib = go: runCommand "go-stdlib-${go.version}"
+    {
+      nativeBuildInputs = [ go ];
+    } ''
+    HOME=$NIX_BUILD_TOP/home
+    mkdir $HOME
+
+    goroot="$(go env GOROOT)"
+    cp -R "$goroot/src" "$goroot/pkg" .
+
+    chmod -R +w .
+    GODEBUG=installgoroot=all GOROOT=$NIX_BUILD_TOP go install -v --trimpath std
+
+    mkdir $out
+    cp -r pkg/*_*/* $out
+
+    find $out -name '*.a' | while read -r ARCHIVE_FULL; do
+      ARCHIVE="''${ARCHIVE_FULL#"$out/"}"
+      PACKAGE="''${ARCHIVE%.a}"
+      echo "packagefile $PACKAGE=$ARCHIVE_FULL"
+    done > $out/importcfg
+  '';
+
+  importcfgCmd = { name, deps, out ? "importcfg" }: ''
+    echo "# nix buildGo ${name}" > "${out}"
+    cat "${goStdlib}/importcfg" >> "${out}"
+    ${lib.concatStringsSep "\n" (map (dep: ''
+      find "${dep}" -name '*.a' | while read -r pkgp; do
+        relpath="''${pkgp#"${dep}/"}"
+        pkgname="''${relpath%.a}"
+        echo "packagefile $pkgname=$pkgp"
+      done >> "${out}"
+    '') deps)}
+  '';
+
   # High-level build functions
 
   # Build a Go program out of the specified files and dependencies.
   program = { name, srcs, deps ? [ ], x_defs ? { } }:
     let uniqueDeps = allDeps (map (d: d.gopkg) deps);
     in runCommand name { } ''
-      ${go}/bin/go tool compile -o ${name}.a -trimpath=$PWD -trimpath=${go} ${includeSources uniqueDeps} ${spaceOut srcs}
+      ${importcfgCmd { inherit name; deps = uniqueDeps; }}
+      ${go}/bin/go tool compile -o ${name}.a -importcfg=importcfg -trimpath=$PWD -trimpath=${go} -p main ${includeSources uniqueDeps} ${spaceOut srcs}
       mkdir -p $out/bin
       export GOROOT_FINAL=go
-      ${go}/bin/go tool link -o $out/bin/${name} -buildid nix ${xFlags x_defs} ${includeLibs uniqueDeps} ${name}.a
+      ${go}/bin/go tool link -o $out/bin/${name} -importcfg=importcfg -buildid nix ${xFlags x_defs} ${includeLibs uniqueDeps} ${name}.a
     '';
 
   # Build a Go library assembled out of the specified files.
@@ -79,8 +111,8 @@ let
       # This is required for several popular packages (e.g. x/sys).
       ifAsm = do: lib.optionalString (sfiles != [ ]) do;
       asmBuild = ifAsm ''
-        ${go}/bin/go tool asm -trimpath $PWD -I $PWD -I ${go}/share/go/pkg/include -D GOOS_linux -D GOARCH_amd64 -gensymabis -o ./symabis ${spaceOut sfiles}
-        ${go}/bin/go tool asm -trimpath $PWD -I $PWD -I ${go}/share/go/pkg/include -D GOOS_linux -D GOARCH_amd64 -o ./asm.o ${spaceOut sfiles}
+        ${go}/bin/go tool asm -p ${path} -trimpath $PWD -I $PWD -I ${go}/share/go/pkg/include -D GOOS_linux -D GOARCH_amd64 -gensymabis -o ./symabis ${spaceOut sfiles}
+        ${go}/bin/go tool asm -p ${path} -trimpath $PWD -I $PWD -I ${go}/share/go/pkg/include -D GOOS_linux -D GOARCH_amd64 -o ./asm.o ${spaceOut sfiles}
       '';
       asmLink = ifAsm "-symabis ./symabis -asmhdr $out/go_asm.h";
       asmPack = ifAsm ''
@@ -91,7 +123,8 @@ let
         mkdir -p $out/${path}
         ${srcList path (map (s: "${s}") srcs)}
         ${asmBuild}
-        ${go}/bin/go tool compile -pack ${asmLink} -o $out/${path}.a -trimpath=$PWD -trimpath=${go} -p ${path} ${includeSources uniqueDeps} ${spaceOut srcs}
+        ${importcfgCmd { inherit name; deps = uniqueDeps; }}
+        ${go}/bin/go tool compile -pack ${asmLink} -o $out/${path}.a -importcfg=importcfg -trimpath=$PWD -trimpath=${go} -p ${path} ${includeSources uniqueDeps} ${spaceOut srcs}
         ${asmPack}
       '').overrideAttrs (_: {
         passthru = {
@@ -111,33 +144,14 @@ let
   # named "gopkg", and an attribute named "gobin" for binaries.
   external = import ./external { inherit pkgs program package; };
 
-  # Import support libraries needed for protobuf & gRPC support
-  protoLibs = import ./proto.nix {
-    inherit external;
-  };
-
-  # Build a Go library out of the specified protobuf definition.
-  proto = { name, proto, path ? name, goPackage ? name, extraDeps ? [ ] }: (makeOverridable package) {
-    inherit name path;
-    deps = [ protoLibs.goProto.proto.gopkg ] ++ extraDeps;
-    srcs = lib.singleton (runCommand "goproto-${name}.pb.go" { } ''
-      cp ${proto} ${baseNameOf proto}
-      ${protobuf}/bin/protoc --plugin=${protoLibs.goProto.protoc-gen-go.gopkg}/bin/protoc-gen-go \
-        --go_out=plugins=grpc,import_path=${baseNameOf path}:. ${baseNameOf proto}
-      mv ./${goPackage}/*.pb.go $out
-    '');
-  };
-
-  # Build a Go library out of the specified gRPC definition.
-  grpc = args: proto (args // { extraDeps = [ protoLibs.goGrpc.gopkg ]; });
-
 in
 {
   # Only the high-level builder functions are exposed, but made
   # overrideable.
   program = makeOverridable program;
   package = makeOverridable package;
-  proto = makeOverridable proto;
-  grpc = makeOverridable grpc;
   external = makeOverridable external;
+
+  # re-expose the Go version used
+  inherit go;
 }
diff --git a/nix/buildGo/example/default.nix b/nix/buildGo/example/default.nix
index 08da075e18..6756bf39e2 100644
--- a/nix/buildGo/example/default.nix
+++ b/nix/buildGo/example/default.nix
@@ -19,13 +19,6 @@ let
     ];
   };
 
-  # Example use of buildGo.proto, which generates a Go library from a
-  # Protobuf definition file.
-  exampleProto = buildGo.proto {
-    name = "exampleproto";
-    proto = ./thing.proto;
-  };
-
   # Example use of buildGo.program, which builds an executable using
   # the specified name and dependencies (which in turn must have been
   # created via buildGo.package etc.)
@@ -39,7 +32,6 @@ buildGo.program {
 
   deps = [
     examplePackage
-    exampleProto
   ];
 
   x_defs = {
diff --git a/nix/buildGo/example/thing.proto b/nix/buildGo/example/thing.proto
deleted file mode 100644
index 0f6d6575e0..0000000000
--- a/nix/buildGo/example/thing.proto
+++ /dev/null
@@ -1,10 +0,0 @@
-// Copyright 2019 Google LLC.
-// SPDX-License-Identifier: Apache-2.0
-
-syntax = "proto3";
-package example;
-
-message Thing {
-  string id = 1;
-  string kind_of_thing = 2;
-}
diff --git a/nix/buildGo/external/main.go b/nix/buildGo/external/main.go
index a77c43b371..4402a8eb86 100644
--- a/nix/buildGo/external/main.go
+++ b/nix/buildGo/external/main.go
@@ -10,7 +10,6 @@ import (
 	"flag"
 	"fmt"
 	"go/build"
-	"io/ioutil"
 	"log"
 	"os"
 	"path"
@@ -74,8 +73,8 @@ func findGoDirs(at string) ([]string, error) {
 	}
 
 	goDirs := []string{}
-	for k, _ := range dirSet {
-		goDirs = append(goDirs, k)
+	for goDir := range dirSet {
+		goDirs = append(goDirs, goDir)
 	}
 
 	return goDirs, nil
@@ -148,7 +147,7 @@ func analysePackage(root, source, importpath string, stdlib map[string]bool) (pk
 }
 
 func loadStdlibPkgs(from string) (pkgs map[string]bool, err error) {
-	f, err := ioutil.ReadFile(from)
+	f, err := os.ReadFile(from)
 	if err != nil {
 		return
 	}
diff --git a/nix/buildGo/proto.nix b/nix/buildGo/proto.nix
deleted file mode 100644
index 6c37f758ce..0000000000
--- a/nix/buildGo/proto.nix
+++ /dev/null
@@ -1,87 +0,0 @@
-# Copyright 2019 Google LLC.
-# SPDX-License-Identifier: Apache-2.0
-#
-# This file provides derivations for the dependencies of a gRPC
-# service in Go.
-
-{ external }:
-
-let
-  inherit (builtins) fetchGit map;
-in
-rec {
-  goProto = external {
-    path = "github.com/golang/protobuf";
-    src = fetchGit {
-      url = "https://github.com/golang/protobuf";
-      rev = "ed6926b37a637426117ccab59282c3839528a700";
-    };
-  };
-
-  xnet = external {
-    path = "golang.org/x/net";
-
-    src = fetchGit {
-      url = "https://go.googlesource.com/net";
-      rev = "ffdde105785063a81acd95bdf89ea53f6e0aac2d";
-    };
-
-    deps = [
-      xtext.secure.bidirule
-      xtext.unicode.bidi
-      xtext.unicode.norm
-    ];
-  };
-
-  xsys = external {
-    path = "golang.org/x/sys";
-    src = fetchGit {
-      url = "https://go.googlesource.com/sys";
-      rev = "bd437916bb0eb726b873ee8e9b2dcf212d32e2fd";
-    };
-  };
-
-  xtext = external {
-    path = "golang.org/x/text";
-    src = fetchGit {
-      url = "https://go.googlesource.com/text";
-      rev = "cbf43d21aaebfdfeb81d91a5f444d13a3046e686";
-    };
-  };
-
-  genproto = external {
-    path = "google.golang.org/genproto";
-    src = fetchGit {
-      url = "https://github.com/google/go-genproto";
-      # necessary because https://github.com/NixOS/nix/issues/1923
-      ref = "main";
-      rev = "83cc0476cb11ea0da33dacd4c6354ab192de6fe6";
-    };
-
-    deps = with goProto; [
-      proto
-      ptypes.any
-    ];
-  };
-
-  goGrpc = external {
-    path = "google.golang.org/grpc";
-    deps = ([
-      xnet.trace
-      xnet.http2
-      xsys.unix
-      xnet.http2.hpack
-      genproto.googleapis.rpc.status
-    ] ++ (with goProto; [
-      proto
-      ptypes
-      ptypes.duration
-      ptypes.timestamp
-    ]));
-
-    src = fetchGit {
-      url = "https://github.com/grpc/grpc-go";
-      rev = "d8e3da36ac481ef00e510ca119f6b68177713689";
-    };
-  };
-}
diff --git a/nix/buildLisp/default.nix b/nix/buildLisp/default.nix
index eb85f0ff51..0d68a2818b 100644
--- a/nix/buildLisp/default.nix
+++ b/nix/buildLisp/default.nix
@@ -513,7 +513,6 @@ let
 
       # See https://ccl.clozure.com/docs/ccl.html#building-definitions
       faslExt =
-        /**/
         if targetPlatform.isPower && targetPlatform.is32bit then "pfsl"
         else if targetPlatform.isPower && targetPlatform.is64bit then "p64fsl"
         else if targetPlatform.isx86_64 && targetPlatform.isLinux then "lx64fsl"
diff --git a/nix/buildkite/default.nix b/nix/buildkite/default.nix
index b04dc55a0b..9abba9408a 100644
--- a/nix/buildkite/default.nix
+++ b/nix/buildkite/default.nix
@@ -27,64 +27,112 @@ let
 
   inherit (pkgs) lib runCommand writeText;
   inherit (depot.nix.readTree) mkLabel;
+
+  inherit (depot.nix) dependency-analyzer;
 in
 rec {
-  # Creates a Nix expression that yields the target at the specified
-  # location in the repository.
-  #
-  # This makes a distinction between normal targets (which physically
-  # exist in the repository) and subtargets (which are "virtual"
-  # targets exposed by a physical one) to make it clear in the build
-  # output which is which.
-  mkBuildExpr = target:
+  # Create a unique key for the buildkite pipeline based on the given derivation
+  # or drvPath. A consequence of using such keys is that every derivation may
+  # only be exposed as a single, unique step in the pipeline.
+  keyForDrv = drvOrPath:
+    let
+      drvPath =
+        if lib.isDerivation drvOrPath then drvOrPath.drvPath
+        else if lib.isString drvOrPath then drvOrPath
+        else builtins.throw "keyForDrv: expected string or derivation";
+
+      # Only use the drv hash to prevent escaping problems. Buildkite also has a
+      # limit of 100 characters on keys.
+    in
+    "drv-" + (builtins.substring 0 32
+      (builtins.baseNameOf (unsafeDiscardStringContext drvPath))
+    );
+
+  # Given an arbitrary attribute path generate a Nix expression which obtains
+  # this from the root of depot (assumed to be ./.). Attributes may be any
+  # Nix strings suitable as attribute names, not just Nix literal-safe strings.
+  mkBuildExpr = attrPath:
     let
       descend = expr: attr: "builtins.getAttr \"${attr}\" (${expr})";
-      targetExpr = foldl' descend "import ./. {}" target.__readTree;
-      subtargetExpr = descend targetExpr target.__subtarget;
     in
-    if target ? __subtarget then subtargetExpr else targetExpr;
+    foldl' descend "import ./. {}" attrPath;
 
   # Determine whether to skip a target if it has not diverged from the
   # HEAD branch.
-  shouldSkip = parentTargetMap: label: drvPath:
+  shouldSkip = { parentTargetMap ? { }, label, drvPath }:
     if (hasAttr label parentTargetMap) && parentTargetMap."${label}".drvPath == drvPath
     then "Target has not changed."
     else false;
 
-  # Create build command for a derivation target.
-  mkBuildCommand = target: drvPath: concatStringsSep " " [
+  # Create build command for an attribute path pointing to a derivation.
+  mkBuildCommand = { attrPath, drvPath, outLink ? "result" }: concatStringsSep " " [
+    # If the nix build fails, the Nix command's exit status should be used.
+    "set -o pipefail;"
+
     # First try to realise the drvPath of the target so we don't evaluate twice.
     # Nix has no concept of depending on a derivation file without depending on
     # at least one of its `outPath`s, so we need to discard the string context
     # if we don't want to build everything during pipeline construction.
-    "(nix-store --realise '${drvPath}' --add-root result --indirect && readlink result)"
+    #
+    # To make this more uniform with how nix-build(1) works, we call realpath(1)
+    # on nix-store(1)'s output since it has the habit of printing the path of the
+    # out link, not the store path.
+    "(nix-store --realise '${drvPath}' --add-root '${outLink}' --indirect | xargs -r realpath)"
 
     # Since we don't gcroot the derivation files, they may be deleted by the
     # garbage collector. In that case we can reevaluate and build the attribute
     # using nix-build.
-    "|| (test ! -f '${drvPath}' && nix-build -E '${mkBuildExpr target}' --show-trace)"
+    "|| (test ! -f '${drvPath}' && nix-build -E '${mkBuildExpr attrPath}' --show-trace --out-link '${outLink}')"
   ];
 
+  # Attribute path of a target relative to the depot root. Needs to take into
+  # account whether the target is a physical target (which corresponds to a path
+  # in the filesystem) or the subtarget of a physical target.
+  targetAttrPath = target:
+    target.__readTree
+    ++ lib.optionals (target ? __subtarget) [ target.__subtarget ];
+
+  # Given a derivation (identified by drvPath) that is part of the list of
+  # targets passed to mkPipeline, determine all derivations that it depends on
+  # and are also part of the pipeline. Finally, return the keys of the steps
+  # that build them. This is used to populate `depends_on` in `mkStep`.
+  #
+  # See //nix/dependency-analyzer for documentation on the structure of `targetDepMap`.
+  getTargetPipelineDeps = targetDepMap: drvPath:
+    # Sanity check: We should only call this function on targets explicitly
+    # passed to mkPipeline. Thus it should have been passed as a “known” drv to
+    # dependency-analyzer.
+    assert targetDepMap.${drvPath}.known;
+    builtins.map keyForDrv targetDepMap.${drvPath}.knownDeps;
+
   # Create a pipeline step from a single target.
-  mkStep = headBranch: parentTargetMap: target: cancelOnBuildFailing:
+  mkStep = { headBranch, parentTargetMap, targetDepMap, target, cancelOnBuildFailing }:
     let
       label = mkLabel target;
       drvPath = unsafeDiscardStringContext target.drvPath;
-      shouldSkip' = shouldSkip parentTargetMap;
     in
     {
       label = ":nix: " + label;
-      key = hashString "sha1" label;
-      skip = shouldSkip' label drvPath;
-      command = mkBuildCommand target drvPath;
+      key = keyForDrv target;
+      skip = shouldSkip { inherit label drvPath parentTargetMap; };
+      command = mkBuildCommand {
+        attrPath = targetAttrPath target;
+        inherit drvPath;
+      };
       env.READTREE_TARGET = label;
       cancel_on_build_failing = cancelOnBuildFailing;
 
       # Add a dependency on the initial static pipeline step which
       # always runs. This allows build steps uploaded in batches to
       # start running before all batches have been uploaded.
-      depends_on = ":init:";
-    };
+      depends_on = [ ":init:" ]
+      ++ getTargetPipelineDeps targetDepMap drvPath
+      ++ lib.optionals (target ? meta.ci.buildkiteExtraDeps) target.meta.ci.buildkiteExtraDeps;
+    } // lib.optionalAttrs (target ? meta.timeout) {
+      timeout_in_minutes = target.meta.timeout / 60;
+      # Additional arguments to set on the step.
+      # Keep in mind these *overwrite* existing step args, not extend. Use with caution.
+    } // lib.optionalAttrs (target ? meta.ci.buildkiteExtraStepArgs) target.meta.ci.buildkiteExtraStepArgs;
 
   # Helper function to inelegantly divide a list into chunks of at
   # most n elements.
@@ -182,18 +230,24 @@ rec {
       # logic/optimisation depends on knowing whether is executing.
       buildEnabled = elem "build" enabledPhases;
 
+      # Dependency relations between the `drvTargets`. See also //nix/dependency-analyzer.
+      targetDepMap = dependency-analyzer (dependency-analyzer.drvsToPaths drvTargets);
+
       # Convert a target into all of its steps, separated by build
       # phase (as phases end up in different chunks).
       targetToSteps = target:
         let
-          step = mkStep headBranch parentTargetMap target cancelOnBuildFailing;
+          mkStepArgs = {
+            inherit headBranch parentTargetMap targetDepMap target cancelOnBuildFailing;
+          };
+          step = mkStep mkStepArgs;
 
           # Same step, but with an override function applied. This is
           # used in mkExtraStep if the extra step needs to modify the
           # parent derivation somehow.
           #
           # Note that this will never affect the label.
-          overridable = f: mkStep headBranch parentTargetMap (f target) cancelOnBuildFailing;
+          overridable = f: mkStep (mkStepArgs // { target = (f target); });
 
           # Split extra steps by phase.
           splitExtraSteps = lib.groupBy ({ phase, ... }: phase)
@@ -202,7 +256,7 @@ rec {
 
           extraSteps = mapAttrs
             (_: steps:
-              map (mkExtraStep buildEnabled) steps)
+              map (mkExtraStep (targetAttrPath target) buildEnabled) steps)
             splitExtraSteps;
         in
         if !buildEnabled then extraSteps
@@ -252,9 +306,7 @@ rec {
 
         # Include the attrPath in the output to reconstruct the drv
         # without parsing the human-readable label.
-        attrPath = target.__readTree ++ lib.optionals (target ? __subtarget) [
-          target.__subtarget
-        ];
+        attrPath = targetAttrPath target;
       };
     })
     drvTargets)));
@@ -278,10 +330,6 @@ rec {
   #     confirmation. These steps always run after the main build is
   #     done and have no influence on CI status.
   #
-  #   postBuild (optional): If set to true, this step will run after
-  #     all primary build steps (that is, after status has been reported
-  #     back to CI).
-  #
   #   needsOutput (optional): If set to true, the parent derivation
   #     will be built in the working directory before running the
   #     command. Output will be available as 'result'.
@@ -332,12 +380,7 @@ rec {
     , alwaysRun ? false
     , prompt ? false
     , softFail ? false
-
-      # TODO(tazjin): Default to 'build' after 2022-10-01.
-    , phase ? if (isNull postBuild || !postBuild) then "build" else "release"
-
-      # TODO(tazjin): Turn into hard-failure after 2022-10-01.
-    , postBuild ? null
+    , phase ? "build"
     , skip ? false
     , agents ? null
     }:
@@ -368,32 +411,12 @@ rec {
         skip
         agents;
 
-      # //nix/buildkite is growing a new feature for adding different
-      # "build phases" which supersedes the previous `postBuild`
-      # boolean API.
-      #
-      # To help users transition, emit warnings if the old API is used.
-      phase = lib.warnIfNot (isNull postBuild) ''
-        In step '${label}' (from ${parentLabel}):
-
-        Please note: The CI system is introducing support for running
-        steps in different build phases.
-
-        The currently supported phases are 'build' (all Nix targets,
-        extra steps such as tests that feed into the build results,
-        etc.) and 'release' (steps that run after builds and tests
-        have already completed).
-
-        This replaces the previous boolean `postBuild` API in extra
-        step definitions. Please remove the `postBuild` parameter from
-        this step and instead set `phase = ${phase};`.
-      ''
-        validPhase;
+      phase = validPhase;
 
       prompt = lib.throwIf (prompt != false && phase == "build") ''
         In step '${label}' (from ${parentLabel}):
 
-        The 'prompt' feature can only be used by steps in the "release"
+        The 'prompt' feature can not be used by steps in the "build"
         phase, because CI builds should not be gated on manual human
         approvals.
       ''
@@ -402,9 +425,13 @@ rec {
 
   # Create the Buildkite configuration for an extra step, optionally
   # wrapping it in a gate group.
-  mkExtraStep = buildEnabled: cfg:
+  mkExtraStep = parentAttrPath: buildEnabled: cfg:
     let
+      # ATTN: needs to match an entry in .gitignore so that the tree won't get dirty
+      commandScriptLink = "nix-buildkite-extra-step-command-script";
+
       step = {
+        key = "extra-step-" + hashString "sha1" "${cfg.label}-${cfg.parentLabel}";
         label = ":gear: ${cfg.label} (from ${cfg.parentLabel})";
         skip =
           let
@@ -429,8 +456,22 @@ rec {
             "echo '~~~ Preparing build output of ${cfg.parentLabel}'"
           }
           ${lib.optionalString cfg.needsOutput cfg.parent.command}
-          echo '+++ Running extra step command'
-          exec ${cfg.command}
+          echo '--- Building extra step script'
+          command_script="$(${
+            # Using command substitution in this way assumes the script drv only has one output
+            assert builtins.length cfg.command.outputs == 1;
+            mkBuildCommand {
+              # script is exposed at <parent>.meta.ci.extraSteps.<key>.command
+              attrPath =
+                parentAttrPath
+                ++ [ "meta" "ci" "extraSteps" cfg.key "command" ];
+              drvPath = unsafeDiscardStringContext cfg.command.drvPath;
+              # make sure it doesn't conflict with result (from needsOutput)
+              outLink = commandScriptLink;
+            }
+          })"
+          echo '+++ Running extra step script'
+          exec "$command_script"
         '';
 
         soft_fail = cfg.softFail;
diff --git a/nix/buildkite/fetch-parent-targets.sh b/nix/buildkite/fetch-parent-targets.sh
index 8afac1e5ec..08c2d1f3ab 100755
--- a/nix/buildkite/fetch-parent-targets.sh
+++ b/nix/buildkite/fetch-parent-targets.sh
@@ -2,43 +2,54 @@
 set -ueo pipefail
 
 # Each Buildkite build stores the derivation target map as a pipeline
-# artifact. This script determines the most appropriate commit (the
-# fork point of the current chain from HEAD) and fetches the artifact.
+# artifact. To reduce the amount of work done by CI, each CI build is
+# diffed against the latest such derivation map found for the
+# repository.
 #
-# New builds can be based on HEAD before the pipeline for the last
-# commit has finished, in which case it is possible that the fork
-# point has no derivation map. To account for this, up to 3 commits
-# prior to HEAD are also queried to find a map.
+# Note that this does not take into account when the currently
+# processing CL was forked off from the canonical branch, meaning that
+# things like nixpkgs updates in between will cause mass rebuilds in
+# any case.
 #
 # If no map is found, the failure mode is not critical: We simply
 # build all targets.
 
+readonly REPO_ROOT=$(git rev-parse --show-toplevel)
+
 : ${DRVMAP_PATH:=pipeline/drvmap.json}
 : ${BUILDKITE_TOKEN_PATH:=~/buildkite-token}
 
-git fetch -v origin "${BUILDKITE_PIPELINE_DEFAULT_BRANCH}"
-
-FIRST=$(git merge-base FETCH_HEAD "${BUILDKITE_COMMIT}")
-SECOND=$(git rev-parse "$FIRST~1")
-THIRD=$(git rev-parse "$FIRST~2")
-
-function most_relevant_builds {
+# Runs a fairly complex Buildkite GraphQL query that attempts to fetch all
+# pipeline-gen steps from the default branch, as long as one appears within the
+# last 50 builds or so. The query restricts build states to running or passed
+# builds, which means that it *should* be unlikely that nothing is found.
+#
+# There is no way to filter this more loosely (e.g. by saying "any recent build
+# matching these conditions").
+#
+# The returned data structure is complex, and disassembled by a JQ script that
+# first filters out all builds with no matching jobs (e.g. builds that are still
+# in progress), and then filters those down to builds with artifacts, and then
+# to drvmap artifacts specifically.
+#
+# If a recent drvmap was found, this returns its download URL. Otherwise, it
+# returns the string "null".
+function latest_drvmap_url {
     set -u
     curl 'https://graphql.buildkite.com/v1' \
          --silent \
          -H "Authorization: Bearer $(cat ${BUILDKITE_TOKEN_PATH})" \
-         -d "{\"query\": \"query { pipeline(slug: \\\"$BUILDKITE_ORGANIZATION_SLUG/$BUILDKITE_PIPELINE_SLUG\\\") { builds(commit: [\\\"$FIRST\\\",\\\"$SECOND\\\",\\\"$THIRD\\\"]) { edges { node { uuid }}}}}\"}" | \
-         jq -r '.data.pipeline.builds.edges[] | .node.uuid'
+         -H "Content-Type: application/json" \
+         -d "{\"query\": \"{ pipeline(slug: \\\"$BUILDKITE_ORGANIZATION_SLUG/$BUILDKITE_PIPELINE_SLUG\\\") { builds(first: 50, branch: [\\\"%default\\\"], state: [RUNNING, PASSED]) { edges { node { jobs(passed: true, first: 1, type: [COMMAND], step: {key: [\\\"pipeline-gen\\\"]}) { edges { node { ... on JobTypeCommand { url artifacts { edges { node { downloadURL path }}}}}}}}}}}}\"}" | tee out.json | \
+        jq -r '[.data.pipeline.builds.edges[] | select((.node.jobs.edges | length) > 0) | .node.jobs.edges[] | .node.artifacts[][] | select(.node.path == "pipeline/drvmap.json")][0].node.downloadURL'
 }
 
-mkdir -p tmp
-for build in $(most_relevant_builds); do
-    echo "Checking artifacts for build $build"
-    buildkite-agent artifact download --build "${build}" "${DRVMAP_PATH}" 'tmp/' || true
+readonly DOWNLOAD_URL=$(latest_drvmap_url)
 
-    if [[ -f "tmp/${DRVMAP_PATH}" ]]; then
-        echo "Fetched target map from build ${build}"
-        mv "tmp/${DRVMAP_PATH}" tmp/parent-target-map.json
-        break
-    fi
-done
+if [[ ${DOWNLOAD_URL} != "null" ]]; then
+    mkdir -p tmp
+    curl -o tmp/parent-target-map.json ${DOWNLOAD_URL} && echo "downloaded parent derivation map" \
+            || echo "failed to download derivation map!"
+else
+    echo "no derivation map found!"
+fi
diff --git a/nix/dependency-analyzer/default.nix b/nix/dependency-analyzer/default.nix
index 54ff72912e..2ec8d7b5b9 100644
--- a/nix/dependency-analyzer/default.nix
+++ b/nix/dependency-analyzer/default.nix
@@ -16,30 +16,35 @@ let
   #
   # TODO(sterni): clean this up and expose it
   directDrvDeps =
-    if lib.versionAtLeast builtins.nixVersion "2.6"
-    then
-    # Since https://github.com/NixOS/nix/pull/1643, Nix apparently »preserves
-    # string context« through a readFile invocation. This has the side effect
-    # that it becomes possible to query the actual references a store path has.
-    # Not a 100% sure this is intended, but _very_ convenient for us here.
-      drvPath:
-      # if the passed path is not a derivation we can't necessarily get its
-      # dependencies, since it may not be representable as a Nix string due to
-      # NUL bytes, e.g. compressed patch files imported into the Nix store.
-      if builtins.match "^.+\\.drv$" drvPath == null
-      then [ ]
-      else builtins.attrNames (builtins.getContext (builtins.readFile drvPath))
-    else
-    # For Nix < 2.6 we have to rely on HACK, namely grepping for quoted store
-    # path references in the file. In the future this should be replaced by
-    # a proper derivation parser.
-      drvPath: builtins.concatLists (
-        builtins.filter builtins.isList (
-          builtins.split
-            "\"(${lib.escapeRegex builtins.storeDir}/[[:alnum:]+._?=-]+.drv)\""
-            (builtins.readFile drvPath)
-        )
-      );
+    let
+      getDeps =
+        if lib.versionAtLeast builtins.nixVersion "2.6"
+        then
+        # Since https://github.com/NixOS/nix/pull/1643, Nix apparently »preserves
+        # string context« through a readFile invocation. This has the side effect
+        # that it becomes possible to query the actual references a store path has.
+        # Not a 100% sure this is intended, but _very_ convenient for us here.
+          drvPath:
+          builtins.attrNames (builtins.getContext (builtins.readFile drvPath))
+        else
+        # For Nix < 2.6 we have to rely on HACK, namely grepping for quoted
+        # store path references in the file. In the future this should be
+        # replaced by a proper derivation parser.
+          drvPath: builtins.concatLists (
+            builtins.filter builtins.isList (
+              builtins.split
+                "\"(${lib.escapeRegex builtins.storeDir}/[[:alnum:]+._?=-]+.drv)\""
+                (builtins.readFile drvPath)
+            )
+          );
+    in
+    drvPath:
+    # if the passed path is not a derivation we can't necessarily get its
+    # dependencies, since it may not be representable as a Nix string due to
+    # NUL bytes, e.g. compressed patch files imported into the Nix store.
+    if builtins.match "^.+\\.drv$" drvPath == null
+    then [ ]
+    else getDeps drvPath;
 
   # Maps a list of derivation to the list of corresponding `drvPath`s.
   #
diff --git a/nix/lazy-deps/default.nix b/nix/lazy-deps/default.nix
index 3cce48d8a5..fbdb30b38e 100644
--- a/nix/lazy-deps/default.nix
+++ b/nix/lazy-deps/default.nix
@@ -9,11 +9,11 @@
 # evaluation, and expects both `git` and `nix-build` to exist in the
 # user's $PATH. If required, this can be done in the shell
 # configuration invoking this function.
-{ pkgs, ... }:
+{ pkgs, lib, ... }:
 
 let
   inherit (builtins) attrNames attrValues mapAttrs;
-  inherit (pkgs.lib) concatStringsSep;
+  inherit (lib) fix concatStringsSep;
 
   # Create the case statement for a command invocations, optionally
   # overriding the `TARGET_TOOL` variable.
@@ -28,20 +28,21 @@ let
 
   invocations = tools: concatStringsSep "\n" (attrValues (mapAttrs invoke tools));
 in
+fix (self:
 
 # Attribute set of tools that should be lazily-added to the $PATH.
-
-  # The name of each attribute is used as the command name (on $PATH).
-  # It must contain the keys 'attr' (containing the Nix attribute path
-  # to the tool's derivation from the top-level), and may optionally
-  # contain the key 'cmd' to override the name of the binary inside the
-  # derivation.
+#
+# The name of each attribute is used as the command name (on $PATH).
+# It must contain the keys 'attr' (containing the Nix attribute path
+# to the tool's derivation from the top-level), and may optionally
+# contain the key 'cmd' to override the name of the binary inside the
+# derivation.
 tools:
 
-pkgs.writeTextFile {
-  name = "lazy-dispatch";
-  executable = true;
-  destination = "/bin/__dispatch";
+pkgs.runCommandNoCC "lazy-dispatch"
+{
+  passthru.overrideDeps = newTools: self (tools // newTools);
+  passthru.tools = tools;
 
   text = ''
     #!${pkgs.runtimeShell}
@@ -68,8 +69,23 @@ pkgs.writeTextFile {
     exec "''${TARGET_TOOL}" "''${@}"
   '';
 
-  checkPhase = ''
-    ${pkgs.stdenv.shellDryRun} "$target"
-    ${concatStringsSep "\n" (map link (attrNames tools))}
-  '';
+  # Access this to get a compatible nix-shell
+  passthru.devShell = pkgs.mkShellNoCC {
+    name = "${self.name}-shell";
+    packages = [ self ];
+  };
 }
+  ''
+    # Write the dispatch code
+    target=$out/bin/__dispatch
+    mkdir -p "$(dirname "$target")"
+    echo "$text" > $target
+    chmod +x $target
+
+    # Add symlinks from all the tools to the dispatch
+    ${concatStringsSep "\n" (map link (attrNames tools))}
+
+    # Check that it's working-ish
+    ${pkgs.stdenv.shellDryRun} $target
+  ''
+)
diff --git a/nix/nix-1p/README.md b/nix/nix-1p/README.md
index e7cf1e2d90..309eddb51e 100644
--- a/nix/nix-1p/README.md
+++ b/nix/nix-1p/README.md
@@ -1,3 +1,8 @@
+> [!TIP]
+> Are you interested in hacking on Nix projects for a week, together
+> with other Nix users? Do you have time at the end of August? Great,
+> come join us at [Volga Sprint](https://volgasprint.org/)!
+
 Nix - A One Pager
 =================
 
@@ -9,6 +14,9 @@ Unless otherwise specified, the word "Nix" refers only to the language below.
 Please file an issue if something in here confuses you or you think something
 important is missing.
 
+If you have Nix installed, you can try the examples below by running `nix repl`
+and entering code snippets there.
+
 <!-- markdown-toc start - Don't edit this section. Run M-x markdown-toc-refresh-toc -->
 **Table of Contents**
 
@@ -16,6 +24,7 @@ important is missing.
 - [Language constructs](#language-constructs)
     - [Primitives / literals](#primitives--literals)
     - [Operators](#operators)
+        - [`//` (merge) operator](#-merge-operator)
     - [Variable bindings](#variable-bindings)
     - [Functions](#functions)
         - [Multiple arguments (currying)](#multiple-arguments-currying)
@@ -46,8 +55,7 @@ Nix is:
     any dependency between operations is established by depending on *data* from
     previous operations.
 
-    Everything in Nix is an expression, meaning that every directive returns
-    some kind of data.
+    Any valid piece of Nix code is an *expression* that returns a value.
 
     Evaluating a Nix expression *yields a single data structure*, it does not
     execute a sequence of operations.
@@ -110,23 +118,52 @@ rec { a = 15; b = a * 2; }
 
 Nix has several operators, most of which are unsurprising:
 
-| Syntax               | Description                                                                 |
-|----------------------|-----------------------------------------------------------------------------|
-| `+`, `-`, `*`, `/`   | Numerical operations                                                        |
-| `+`                  | String concatenation                                                        |
-| `++`                 | List concatenation                                                          |
-| `==`                 | Equality                                                                    |
-| `>`, `>=`, `<`, `<=` | Ordering comparators                                                        |
-| `&&`                 | Logical `AND`                                                               |
-| <code>&vert;&vert;</code> | Logical `OR`                                                           |
-| `e1 -> e2`           | Logical implication (i.e. <code>!e1 &vert;&vert; e2</code>)                 |
-| `!`                  | Boolean negation                                                            |
-| `set.attr`           | Access attribute `attr` in attribute set `set`                              |
-| `set ? attribute`    | Test whether attribute set contains an attribute                            |
-| `left // right`      | Merge `left` & `right` attribute sets, with the right set taking precedence |
-
-Make sure to understand the `//`-operator, as it is used quite a lot and is
-probably the least familiar one.
+| Syntax                    | Description                                                                 |
+|---------------------------|-----------------------------------------------------------------------------|
+| `+`, `-`, `*`, `/`        | Numerical operations                                                        |
+| `+`                       | String concatenation                                                        |
+| `++`                      | List concatenation                                                          |
+| `==`                      | Equality                                                                    |
+| `>`, `>=`, `<`, `<=`      | Ordering comparators                                                        |
+| `&&`                      | Logical `AND`                                                               |
+| <code>&vert;&vert;</code> | Logical `OR`                                                                |
+| `e1 -> e2`                | Logical implication (i.e. <code>!e1 &vert;&vert; e2</code>)                 |
+| `!`                       | Boolean negation                                                            |
+| `set.attr`                | Access attribute `attr` in attribute set `set`                              |
+| `set ? attribute`         | Test whether attribute set contains an attribute                            |
+| `left // right`           | Merge `left` & `right` attribute sets, with the right set taking precedence |
+
+
+### `//` (merge) operator
+
+The `//`-operator is used pervasively in Nix code. You should familiarise
+yourself with it, as it is likely also the least familiar one.
+
+It merges the left and right attribute sets given to it:
+
+```nix
+{ a = 1; } // { b = 2; }
+
+# yields { a = 1; b = 2; }
+```
+
+Values from the right side take precedence:
+
+```nix
+{ a = "left"; } // { a = "right"; }
+
+# yields { a = "right"; }
+```
+
+The merge operator does *not* recursively merge attribute sets;
+
+```nix
+{ a = { b = 1; }; } // { a = { c = 2; }; }
+
+# yields { a = { c = 2; }; }
+```
+
+Helper functions for recursive merging exist in the [`lib` library](#pkgslib).
 
 ## Variable bindings
 
@@ -323,10 +360,18 @@ let attrs = { a = 15; b = 2; };
 in with attrs; a + b # 'a' and 'b' become variables in the scope following 'with'
 ```
 
+The scope of a `with`-"block" is the expression immediately following the
+semicolon, i.e.:
+
+```nix
+let attrs = { /* some attributes */ };
+in with attrs; (/* this is the scope of the `with` */)
+```
+
 ## `import` / `NIX_PATH` / `<entry>`
 
-Nix files can import each other by using the `import` keyword and a literal
-path:
+Nix files can import each other by using the builtin `import` function and a
+literal path:
 
 ```nix
 # assuming there is a file lib.nix with some useful functions
@@ -334,6 +379,8 @@ let myLib = import ./lib.nix;
 in myLib.usefulFunction 42
 ```
 
+The `import` function will read and evaluate the file, and return its Nix value.
+
 Nix files often begin with a function header to pass parameters into the rest of
 the file, so you will often see imports of the form `import ./some-file { ... }`.
 
@@ -587,15 +634,15 @@ details (note: the convention has moved away from using `self` in favor of
 updated to reflect this).
 
 [currying]: https://en.wikipedia.org/wiki/Currying
-[builtins]: https://nixos.org/nix/manual/#ssec-builtins
+[builtins]: https://nixos.org/manual/nix/stable/language/builtins
 [nixpkgs]: https://github.com/NixOS/nixpkgs
 [lib-src]: https://github.com/NixOS/nixpkgs/tree/master/lib
 [nixdoc]: https://github.com/tazjin/nixdoc
-[lib-manual]: https://nixos.org/nixpkgs/manual/#sec-functions-library
-[channels]: https://nixos.org/nix/manual/#sec-channels
-[trivial builders]: https://github.com/NixOS/nixpkgs/blob/master/pkgs/build-support/trivial-builders.nix
-[smkd]: https://nixos.org/nixpkgs/manual/#chap-stdenv
-[drv-manual]: https://nixos.org/nix/manual/#ssec-derivation
+[lib-manual]: https://nixos.org/manual/nixpkgs/stable/#sec-functions-library
+[channels]: https://nixos.org/manual/nix/stable/command-ref/files/channels
+[trivial builders]: https://github.com/NixOS/nixpkgs/blob/master/pkgs/build-support/trivial-builders/default.nix
+[smkd]: https://nixos.org/manual/nixpkgs/stable/#chap-stdenv
+[drv-manual]: https://nixos.org/manual/nix/stable/language/derivations
 [fp]: https://github.com/NixOS/nixpkgs/blob/master/lib/fixed-points.nix
-[on overrides]: https://nixos.org/nixpkgs/manual/#sec-overrides
-[on overlays]: https://nixos.org/nixpkgs/manual/#chap-overlays
+[on overrides]: https://nixos.org/manual/nixpkgs/stable/#chap-overrides
+[on overlays]: https://nixos.org/manual/nixpkgs/stable/#chap-overlays
diff --git a/nix/readTree/README.md b/nix/readTree/README.md
index f8bbe2255e..5d430d1cfc 100644
--- a/nix/readTree/README.md
+++ b/nix/readTree/README.md
@@ -52,14 +52,17 @@ true;` attribute merged into it.
 `readTree` will follow any subdirectories of a tree and import all Nix files,
 with some exceptions:
 
+* If a folder contains a `default.nix` file, no *sibling* Nix files will be
+  imported - however children are traversed as normal.
+* If a folder contains a `default.nix` it is loaded and, if it
+  evaluates to a set, *merged* with the children. If it evaluates to
+  anything other than a set, else the children are *not traversed*.
+* A folder can opt out from readTree completely by containing a
+  `.skip-tree` file. The content of the file is not read. These
+  folders will be missing completely from the readTree structure.
 * A folder can declare that its children are off-limit by containing a
   `.skip-subtree` file. Since the content of the file is not checked, it can be
   useful to leave a note for a human in the file.
-* If a folder contains a `default.nix` file, no *sibling* Nix files will be
-  imported - however children are traversed as normal.
-* If a folder contains a `default.nix` it is loaded and, if it evaluates to a
-  set, *merged* with the children. If it evaluates to anything else the children
-  are *not traversed*.
 * The `default.nix` of the top-level folder on which readTree is
   called is **not** read to avoid infinite recursion (as, presumably,
   this file is where readTree itself is called).
diff --git a/nix/readTree/default.nix b/nix/readTree/default.nix
index e243e85517..4a745ce33c 100644
--- a/nix/readTree/default.nix
+++ b/nix/readTree/default.nix
@@ -2,7 +2,7 @@
 # Copyright (c) 2020-2021 The TVL Authors
 # SPDX-License-Identifier: MIT
 #
-# Provides a function to automatically read a a filesystem structure
+# Provides a function to automatically read a filesystem structure
 # into a Nix attribute set.
 #
 # Called with an attribute set taking the following arguments:
@@ -41,7 +41,8 @@ let
   readDirVisible = path:
     let
       children = readDir path;
-      isVisible = f: f == ".skip-subtree" || (substring 0 1 f) != ".";
+      # skip hidden files, except for those that contain special instructions to readTree
+      isVisible = f: f == ".skip-subtree" || f == ".skip-tree" || (substring 0 1 f) != ".";
       names = filter isVisible (attrNames children);
     in
     listToAttrs (map
@@ -86,16 +87,39 @@ let
       pathType = builtins.typeOf importedFile;
     in
     if pathType != "lambda"
-    then builtins.throw "readTree: trying to import ${toString path}, but it’s a ${pathType}, you need to make it a function like { depot, pkgs, ... }"
+    then throw "readTree: trying to import ${toString path}, but it’s a ${pathType}, you need to make it a function like { depot, pkgs, ... }"
     else importedFile (filter parts (argsWithPath args parts));
 
   nixFileName = file:
     let res = match "(.*)\\.nix" file;
     in if res == null then null else head res;
 
-  readTree = { args, initPath, rootDir, parts, argsFilter, scopedArgs }:
+  # Internal implementation of readTree, which handles things like the
+  # skipping of trees and subtrees.
+  #
+  # This method returns an attribute sets with either of two shapes:
+  #
+  # { ok = ...; }    # a tree was read successfully
+  # { skip = true; } # a tree was skipped
+  #
+  # The higher-level `readTree` method assembles the final attribute
+  # set out of these results at the top-level, and the internal
+  # `children` implementation unwraps and processes nested trees.
+  readTreeImpl = { args, initPath, rootDir, parts, argsFilter, scopedArgs }:
     let
       dir = readDirVisible initPath;
+
+      # Determine whether any part of this tree should be skipped.
+      #
+      # Adding a `.skip-subtree` file will still allow the import of
+      # the current node's "default.nix" file, but stop recursion
+      # there.
+      #
+      # Adding a `.skip-tree` file will completely ignore the folder
+      # in which this file is located.
+      skipTree = hasAttr ".skip-tree" dir;
+      skipSubtree = skipTree || hasAttr ".skip-subtree" dir;
+
       joinChild = c: initPath + ("/" + c);
 
       self =
@@ -103,19 +127,17 @@ let
         then { __readTree = [ ]; }
         else importFile args scopedArgs initPath parts argsFilter;
 
-      # Import subdirectories of the current one, unless the special
-      # `.skip-subtree` file exists which makes readTree ignore the
-      # children.
+      # Import subdirectories of the current one, unless any skip
+      # instructions exist.
       #
       # This file can optionally contain information on why the tree
       # should be ignored, but its content is not inspected by
       # readTree
       filterDir = f: dir."${f}" == "directory";
-      children = if hasAttr ".skip-subtree" dir then [ ] else
-      map
+      filteredChildren = map
         (c: {
           name = c;
-          value = readTree {
+          value = readTreeImpl {
             inherit argsFilter scopedArgs;
             args = args;
             initPath = (joinChild c);
@@ -125,9 +147,15 @@ let
         })
         (filter filterDir (attrNames dir));
 
+      # Remove skipped children from the final set, and unwrap the
+      # result set.
+      children =
+        if skipSubtree then [ ]
+        else map ({ name, value }: { inherit name; value = value.ok; }) (filter (child: child.value ? ok) filteredChildren);
+
       # Import Nix files
       nixFiles =
-        if hasAttr ".skip-subtree" dir then [ ]
+        if skipSubtree then [ ]
         else filter (f: f != null) (map nixFileName (attrNames dir));
       nixChildren = map
         (c:
@@ -154,9 +182,23 @@ let
       );
 
     in
-    if isAttrs nodeValue
-    then merge nodeValue (allChildren // (marker parts allChildren))
-    else nodeValue;
+    if skipTree
+    then { skip = true; }
+    else {
+      ok =
+        if isAttrs nodeValue
+        then merge nodeValue (allChildren // (marker parts allChildren))
+        else nodeValue;
+    };
+
+  # Top-level implementation of readTree itself.
+  readTree = args:
+    let
+      tree = readTreeImpl args;
+    in
+    if tree ? skip
+    then throw "Top-level folder has a .skip-tree marker and could not be read by readTree!"
+    else tree.ok;
 
   # Helper function to fetch subtargets from a target. This is a
   # temporary helper to warn on the use of the `meta.targets`
diff --git a/nix/readTree/tests/default.nix b/nix/readTree/tests/default.nix
index fcca141714..6f9eb02eff 100644
--- a/nix/readTree/tests/default.nix
+++ b/nix/readTree/tests/default.nix
@@ -41,6 +41,16 @@ let
   };
 
   traversal-logic = it "corresponds to the traversal logic in the README" [
+    (assertEq "skip-tree/a is read"
+      tree-tl.skip-tree.a
+      "a is read normally")
+    (assertEq "skip-tree does not contain b"
+      (builtins.attrNames tree-tl.skip-tree)
+      [ "__readTree" "__readTreeChildren" "a" ])
+    (assertEq "skip-tree children list does not contain b"
+      tree-tl.skip-tree.__readTreeChildren
+      [ "a" ])
+
     (assertEq "skip subtree default.nix is read"
       tree-tl.skip-subtree.but
       "the default.nix is still read")
diff --git a/nix/readTree/tests/test-tree-traversal/skip-tree/a/default.nix b/nix/readTree/tests/test-tree-traversal/skip-tree/a/default.nix
new file mode 100644
index 0000000000..186488be3c
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/skip-tree/a/default.nix
@@ -0,0 +1 @@
+_: "a is read normally"
diff --git a/nix/readTree/tests/test-tree-traversal/skip-tree/b/.skip-tree b/nix/readTree/tests/test-tree-traversal/skip-tree/b/.skip-tree
new file mode 100644
index 0000000000..34936b45d1
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/skip-tree/b/.skip-tree
@@ -0,0 +1 @@
+b subfolder should be skipped completely
diff --git a/nix/readTree/tests/test-tree-traversal/skip-tree/b/default.nix b/nix/readTree/tests/test-tree-traversal/skip-tree/b/default.nix
new file mode 100644
index 0000000000..7903f8e95a
--- /dev/null
+++ b/nix/readTree/tests/test-tree-traversal/skip-tree/b/default.nix
@@ -0,0 +1 @@
+throw "b is skipped completely"
diff --git a/nix/renderMarkdown/default.nix b/nix/renderMarkdown/default.nix
index c5e830c834..8759ada0fe 100644
--- a/nix/renderMarkdown/default.nix
+++ b/nix/renderMarkdown/default.nix
@@ -3,6 +3,19 @@
 
 with depot.nix.yants;
 
-defun [ path drv ] (file: pkgs.runCommand "${file}.rendered.html" { } ''
-  cat ${file} | ${depot.tools.cheddar}/bin/cheddar --about-filter ${file} > $out
-'')
+let
+  args = struct "args" {
+    path = path;
+    tagfilter = option bool;
+  };
+in
+defun [ (either path args) drv ]
+  (arg: pkgs.runCommand "${arg.path or arg}.rendered.html" { }
+    (
+      let
+        tagfilter = if (arg.tagfilter or true) then "" else "--no-tagfilter";
+      in
+      ''
+        cat ${arg.path or arg} | ${depot.tools.cheddar}/bin/cheddar --about-filter ${tagfilter} ${arg.path or arg} > $out
+      ''
+    ))
diff --git a/nix/sparseTree/default.nix b/nix/sparseTree/default.nix
index bf56a5348c..35fa459e1c 100644
--- a/nix/sparseTree/default.nix
+++ b/nix/sparseTree/default.nix
@@ -2,22 +2,33 @@
 # and directories if they are listed in a supplied list:
 #
 # # A very minimal depot
-# sparseTree ./depot [
-#   ./default.nix
-#   ./depot/nix/readTree/default.nix
-#   ./third_party/nixpkgs
-#   ./third_party/overlays
-# ]
+# sparseTree {
+#   root = ./depot;
+#   paths = [
+#     ./default.nix
+#     ./depot/nix/readTree/default.nix
+#     ./third_party/nixpkgs
+#     ./third_party/overlays
+#   ];
+# }
 { pkgs, lib, ... }:
 
-# root path to use as a reference point
-root:
-# list of paths below `root` that should be
-# included in the resulting directory
-#
-# If path, need to refer to the actual file / directory to be included.
-# If a string, it is treated as a string relative to the root.
-paths:
+{
+  # root path to use as a reference point
+  root
+, # list of paths below `root` that should be
+  # included in the resulting directory
+  #
+  # If path, need to refer to the actual file / directory to be included.
+  # If a string, it is treated as a string relative to the root.
+  paths
+, # (optional) name to use for the derivation
+  #
+  # This should always be set when using roots that do not have
+  # controlled names, such as when passing the top-level of a git
+  # repository (e.g. `depot.path.origSrc`).
+  name ? builtins.baseNameOf root
+}:
 
 let
   rootLength = builtins.stringLength (toString root);
@@ -45,7 +56,6 @@ let
     let
       withLeading = p: if builtins.substring 0 1 p == "/" then p else "/" + p;
       fullPath =
-        /**/
         if builtins.isPath path then path
         else if builtins.isString path then (root + withLeading path)
         else builtins.throw "Unsupported path type ${builtins.typeOf path}";
@@ -64,7 +74,7 @@ in
 
 # TODO(sterni): teach readTree to also read symlinked directories,
   # so we ln -sT instead of cp -aT.
-pkgs.runCommand "sparse-${builtins.baseNameOf root}" { } (
+pkgs.runCommand "sparse-${name}" { } (
   lib.concatMapStrings
     ({ src, dst }: ''
       mkdir -p "$(dirname "$out${dst}")"
diff --git a/nix/writeTree/OWNERS b/nix/writeTree/OWNERS
new file mode 100644
index 0000000000..b381c4e660
--- /dev/null
+++ b/nix/writeTree/OWNERS
@@ -0,0 +1 @@
+aspen
diff --git a/nix/writeTree/default.nix b/nix/writeTree/default.nix
new file mode 100644
index 0000000000..0c7c2a130f
--- /dev/null
+++ b/nix/writeTree/default.nix
@@ -0,0 +1,43 @@
+{ depot, lib, pkgs, ... }:
+let
+  inherit (lib) fix pipe mapAttrsToList isAttrs concatLines isString isDerivation isPath;
+
+  # TODO(sterni): move to //nix/utils with clearer naming and alternative similar to lib.types.path
+  isPathLike = value:
+    isPath value
+    || isDerivation value
+    || (isString value && builtins.hasContext value);
+
+  esc = s: lib.escapeShellArg /* ensure paths import into store */ "${s}";
+
+  writeTreeAtPath = path: tree:
+    ''
+      mkdir -p "$out/"${esc path}
+    ''
+    + pipe tree [
+      (mapAttrsToList (k: v:
+        if isPathLike v then
+          "cp -R --reflink=auto ${v} \"$out/\"${esc path}/${esc k}"
+        else if lib.isAttrs v then
+          writeTreeAtPath (path + "/" + k) v
+        else
+          throw "invalid type (expected path, derivation, string with context, or attrs)"))
+      concatLines
+    ];
+
+  /* Create a directory tree specified by a Nix attribute set structure.
+
+     Each value in `tree` should either be a file, a directory, or another tree
+     attribute set. Those paths will be written to a directory tree
+     corresponding to the structure of the attribute set.
+
+     Type: string -> attrSet -> derivation
+  */
+  writeTree = name: tree:
+    pkgs.runCommandLocal name { } (writeTreeAtPath "" tree);
+in
+
+# __functor trick so readTree can add the tests attribute
+{
+  __functor = _: writeTree;
+}
diff --git a/nix/writeTree/tests/default.nix b/nix/writeTree/tests/default.nix
new file mode 100644
index 0000000000..c5858ee96e
--- /dev/null
+++ b/nix/writeTree/tests/default.nix
@@ -0,0 +1,93 @@
+{ depot, pkgs, lib, ... }:
+
+let
+  inherit (pkgs) runCommand writeText writeTextFile;
+  inherit (depot.nix) writeTree;
+
+  checkTree = name: tree: expected:
+    runCommand "writeTree-test-${name}"
+      {
+        nativeBuildInputs = [ pkgs.buildPackages.lr ];
+        passAsFile = [ "expected" ];
+        inherit expected;
+      } ''
+      actualPath="$NIX_BUILD_TOP/actual"
+      cd ${lib.escapeShellArg (writeTree name tree)}
+      lr . > "$actualPath"
+      diff -u "$expectedPath" "$actualPath" | tee "$out"
+    '';
+in
+
+depot.nix.readTree.drvTargets {
+  empty = checkTree "empty" { }
+    ''
+      .
+    '';
+
+  simple-paths = checkTree "simple"
+    {
+      writeTree = {
+        meta = {
+          "owners.txt" = ../OWNERS;
+        };
+        "code.nix" = ../default.nix;
+        all-tests = ./.;
+        nested.dirs.eval-time = builtins.toFile "owothia" ''
+          hold me owo
+        '';
+      };
+    }
+    ''
+      .
+      ./writeTree
+      ./writeTree/all-tests
+      ./writeTree/all-tests/default.nix
+      ./writeTree/code.nix
+      ./writeTree/meta
+      ./writeTree/meta/owners.txt
+      ./writeTree/nested
+      ./writeTree/nested/dirs
+      ./writeTree/nested/dirs/eval-time
+    '';
+
+  empty-dirs = checkTree "empty-dirs"
+    {
+      this.dir.is.empty = { };
+      so.is.this.one = { };
+    }
+    ''
+      .
+      ./so
+      ./so/is
+      ./so/is/this
+      ./so/is/this/one
+      ./this
+      ./this/dir
+      ./this/dir/is
+      ./this/dir/is/empty
+    '';
+
+  drvs = checkTree "drvs"
+    {
+      file-drv = writeText "road.txt" ''
+        Any road followed precisely to its end leads precisely nowhere.
+      '';
+      dir-drv = writeTextFile {
+        name = "dir-of-text";
+        destination = "/text/in/more/dirs.txt";
+        text = ''
+          Climb the mountain just a little bit to test that it’s a mountain.
+          From the top of the mountain, you cannot see the mountain.
+        '';
+      };
+    }
+    ''
+      .
+      ./dir-drv
+      ./dir-drv/text
+      ./dir-drv/text/in
+      ./dir-drv/text/in/more
+      ./dir-drv/text/in/more/dirs.txt
+      ./file-drv
+    '';
+}