feat(nix/buildkite): Check target map of parent to determine skips r/3602

This changes the logic for build pipeline generation to inspect
an (optional) parentTargetMap attribute which contains the derivation
map of a target commit.

Targets that existed in a parent commit with the same drv hash will be
skipped, as they are not considered to have changed.

This does not yet wire up any logic for retrieving the target map from
storage, meaning that at this commit all targets are always built.

The intention is that we will have logic to fetch the target
map (initially from Buildkite artefact storage), which we then pass to
the depot via externalArgs when actually generating the pipeline.

Change-Id: I3373c60aaf4b56b94c6ab64e2e5eef68dea9287c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4946
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

3 files changed, 31 insertions, 25 deletions

diff --git a/default.nix b/default.nix
index 1cf62e94afb0..db2c5035599c 100644
--- a/default.nix
+++ b/default.nix
@@ -2,7 +2,9 @@
 # (see //nix/readTree for details) and constructing a matching attribute set
 # tree.
 
-{ nixpkgsBisectPath ? null, nixpkgsConfig ? {}, ... }@args:
+{ nixpkgsBisectPath ? null
+, parentTargetMap ? null
+, nixpkgsConfig ? {}, ... }@args:
 
 let
   inherit (builtins)
diff --git a/nix/buildkite/default.nix b/nix/buildkite/default.nix
index 5a61aef3b779..c34861123c0d 100644
--- a/nix/buildkite/default.nix
+++ b/nix/buildkite/default.nix
@@ -16,6 +16,7 @@ let
     filter
     foldl'
     getEnv
+    hasAttr
     length
     listToAttrs
     mapAttrs
@@ -46,26 +47,24 @@ in rec {
       then "${label}:${target.__subtarget}"
       else label;
 
-  # Skip build steps if their out path has already been built.
-  skip = headBranch: target: let
-    shouldSkip =
-      # Only skip in real Buildkite builds
-      (getEnv "BUILDKITE_BUILD_ID" != "") &&
-      # Always build everything for the canon branch.
-      (getEnv "BUILDKITE_BRANCH" != headBranch) &&
-      # Discard string context to avoid realising the store path during
-      # pipeline construction.
-      (pathExists (unsafeDiscardStringContext target.outPath));
-    in if shouldSkip then "Target was already built." else false;
+  # Determine whether to skip a target if it has not diverged from the
+  # HEAD branch.
+  shouldSkip = parentTargetMap: label: drvPath:
+    if (hasAttr label parentTargetMap) && parentTargetMap."${label}".drvPath == drvPath
+    then "Target has not changed."
+    else false;
 
   # Create a pipeline step from a single target.
-  mkStep = headBranch: skipIfBuilt: target: {
-    label = ":nix: ${mkLabel target}";
-    skip = if skipIfBuilt then skip headBranch target else false;
-
-    command = let
-      drvPath = unsafeDiscardStringContext target.drvPath;
-    in concatStringsSep " " [
+  mkStep = headBranch: parentTargetMap: target:
+  let
+    label = mkLabel target;
+    drvPath = unsafeDiscardStringContext target.drvPath;
+    shouldSkip' = shouldSkip parentTargetMap;
+  in {
+    label = ":nix: " + label;
+    skip = shouldSkip' label drvPath;
+
+    command = concatStringsSep " " [
       # 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
@@ -124,9 +123,10 @@ in rec {
     # possible, in order, without any concurrency restrictions.
     drvTargets,
 
-    # Should build steps be skipped (on non-HEAD builds) if the output
-    # path has already been built?
-    skipIfBuilt ? false,
+    # Derivation map of a parent commit. Only targets which no longer
+    # correspond to the content of this map will be built. Passing an
+    # empty map will always build all targets.
+    parentTargetMap ? {},
 
     # A list of plain Buildkite step structures to run alongside the
     # build for all drvTargets, but before proceeding with any
@@ -141,7 +141,7 @@ in rec {
     # Can be used for status reporting steps and the like.
     postBuildSteps ? []
   }: let
-    mkStep' = mkStep headBranch skipIfBuilt;
+    mkStep' = mkStep headBranch parentTargetMap;
     steps =
       # Add build steps for each derivation target.
       (map mkStep' drvTargets)
diff --git a/ops/pipelines/depot.nix b/ops/pipelines/depot.nix
index 8cc4b5691f59..232d229b90d8 100644
--- a/ops/pipelines/depot.nix
+++ b/ops/pipelines/depot.nix
@@ -1,6 +1,6 @@
 # This file configures the primary build pipeline used for the
 # top-level list of depot targets.
-{ depot, pkgs, ... }:
+{ depot, pkgs, externalArgs, ... }:
 
 let
   # Protobuf check step which validates that changes to .proto files
@@ -17,11 +17,15 @@ let
     command = "${depot.tools.depotfmt.check}";
     label = ":evergreen_tree: (tools/depotfmt)";
   };
+
   pipeline = depot.nix.buildkite.mkPipeline {
     headBranch = "refs/heads/canon";
     drvTargets = depot.ci.targets;
-    skipIfBuilt = true;
     additionalSteps = [ depotfmtCheck protoCheck ];
+
+    parentTargetMap = if (externalArgs ? parentTargetMap)
+      then builtins.fromJSON (builtins.readFile externalArgs.parentTargetMap)
+      else {};
   };
 
   drvmap = depot.nix.buildkite.mkDrvmap depot.ci.targets;