docs(users/Profpatsch/lib): move split-stdin to lib & document r/3045

Change-Id: I39e81ed766cb209ded5309ea962a59a6f1c811c9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3285
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

2 files changed, 24 insertions, 7 deletions

diff --git a/users/Profpatsch/blog/default.nix b/users/Profpatsch/blog/default.nix
index ca1fbfbfb4e4..1558344b1b7c 100644
--- a/users/Profpatsch/blog/default.nix
+++ b/users/Profpatsch/blog/default.nix
@@ -162,11 +162,6 @@ let
     ''
   ];
 
-  split-stdin = depot.nix.writeExecline "split-stdin" { argMode = "env"; } [
-    "pipeline" [ "runblock" "1" bins.bash "-c" ''${bins.tee} >("$@")'' "bash-split-stdin" ]
-    "runblock" "-r" "1"
-  ];
-
   capture-stdin = depot.nix.writers.rustSimple {
     name = "capture-stdin";
     dependencies = [ depot.users.Profpatsch.execline.exec-helpers ];
@@ -231,7 +226,6 @@ in depot.nix.utils.drvTargets {
    inherit
     router
     notes-server
-    split-stdin
     index
     router-lookup
     ;
diff --git a/users/Profpatsch/lib.nix b/users/Profpatsch/lib.nix
index 5d5fb01294cb..e3d59b7d8beb 100644
--- a/users/Profpatsch/lib.nix
+++ b/users/Profpatsch/lib.nix
@@ -1,9 +1,11 @@
 { depot, pkgs, ... }:
 let
-  bins = depot.nix.getBins pkgs.coreutils [ "printf" "echo" "cat" "printenv" ]
+  bins = depot.nix.getBins pkgs.coreutils [ "printf" "echo" "cat" "printenv" "tee" ]
+      // depot.nix.getBins pkgs.bash [ "bash" ]
       // depot.nix.getBins pkgs.fdtools [ "multitee" ]
       ;
 
+  # Print `msg` and and argv to stderr, then execute into argv
   debugExec = msg: depot.nix.writeExecline "debug-exec" {} [
     "if" [
       "fdmove" "-c" "1" "2"
@@ -13,10 +15,12 @@ let
     "$@"
   ];
 
+  # Print stdin to stderr and stdout
   eprint-stdin = depot.nix.writeExecline "eprint-stdin" {} [
     "pipeline" [ bins.multitee "0-1,2" ] "$@"
   ];
 
+  # Assume the input on stdin is netencode, pretty print it to stderr and forward it to stdout
   eprint-stdin-netencode = depot.nix.writeExecline "eprint-stdin-netencode" {} [
     "pipeline" [
       # move stdout to 3
@@ -30,6 +34,7 @@ let
     "$@"
   ];
 
+  # print the given environment variable in $1 to stderr, then execute into the rest of argv
   eprintenv = depot.nix.writeExecline "eprintenv" { readNArgs = 1; } [
     "ifelse" [ "fdmove" "-c" "1" "2" bins.printenv "$1" ]
     [ "$@" ]
@@ -37,6 +42,23 @@ let
     "$@"
   ];
 
+  # Split stdin into two commands, given by a block and the rest of argv
+  #
+  # Example (execline):
+  #
+  #   pipeline [ echo foo ]
+  #   split-stdin [ fdmove 1 2 foreground [ cat ] echo "bar" ] cat
+  #
+  #   stdout: foo\n
+  #   stderr: foo\nbar\n
+  split-stdin = depot.nix.writeExecline "split-stdin" { argMode = "env"; } [
+    "pipeline" [
+      # this is horrible yes but the quickest way I knew how to implement it
+      "runblock" "1" bins.bash "-c" ''${bins.tee} >("$@")'' "bash-split-stdin"
+    ]
+    "runblock" "-r" "1"
+  ];
+
   # remove everything but a few selected environment variables
   runInEmptyEnv = keepVars:
     let
@@ -53,6 +75,7 @@ in {
     eprint-stdin
     eprint-stdin-netencode
     eprintenv
+    split-stdin
     runInEmptyEnv
     ;
 }