Fix: materialize propagatedNativeBuildInputs file for runtimeInputs propagation (#231)

* Fix: write nix-support/propagated-native-build-inputs file

writeShellApplication uses writeTextFile internally, which does NOT
run stdenv.mkDerivation's fixupPhase. Setting propagatedNativeBuildInputs
via overrideAttrs only adds the attribute to the derivation but never
materializes the $out/nix-support/propagated-native-build-inputs file
that setup.sh's findInputs actually reads at runtime.

Without this file, curl was invisible to the shell environment despite
being set as a propagated dep — the wrapper script itself had curl on
its internal PATH, but other programs (like GHC's stage0 cabal) could
not find it.

Fix: explicitly create the nix-support file in postInstall.

* Fix: use buildCommand instead of postInstall for nix-support file

writeTextFile sets `buildCommand` in the derivation, which causes
stdenv's genericBuild to skip the entire phase system — installPhase,
postInstall, fixupPhase — none of them execute. The previous commit
used postInstall which was silently ignored, leaving the wrapper
output without nix-support/propagated-native-build-inputs.

Append the file creation directly to buildCommand, which is the only
code path the builder actually runs.

Author: angerman

writers.nix

@@ -32,9 +32,25 @@
 #
 # ─── The fix ───────────────────────────────────────────────────────────────────
 #
-# We use overrideAttrs to add propagatedNativeBuildInputs to the wrapper
-# derivation. When stdenv's setup.sh processes the wrapper from
-# nativeBuildInputs, it:
+# We use overrideAttrs to:
+#
+#   1. Set propagatedNativeBuildInputs on the derivation
+#   2. Write the $out/nix-support/propagated-native-build-inputs file
+#
+# Step 2 is critical: writeShellApplication uses writeTextFile internally,
+# which sets `buildCommand` in the derivation. When buildCommand is set,
+# stdenv's genericBuild skips the entire phase system — installPhase,
+# postInstall, fixupPhase — none of them run. The fixupPhase is what
+# normally materializes the propagatedNativeBuildInputs derivation
+# attribute into the $out/nix-support/propagated-native-build-inputs file
+# that setup.sh's findInputs reads at runtime. Without the file on disk,
+# setting the attribute alone has no effect — setup.sh never sees it.
+#
+# We therefore append the file creation directly to buildCommand, since
+# that is the only code path the builder actually executes.
+#
+# With the file in place, when stdenv's setup.sh processes the wrapper
+# from buildInputs or nativeBuildInputs, it:
 #
 #   1. Adds $wrapper/bin to PATH  (the wrapper itself)
 #   2. Reads $wrapper/nix-support/propagated-native-build-inputs
@@ -64,8 +80,23 @@
 { pkgs }:
 {
   writeShellApplicationWithRuntime = args@{ runtimeInputs ? [], ... }:
+    let
+      allPropagated = runtimeInputs;
+      # Space-separated store paths for the propagation file, matching the
+      # format that stdenv's setup.sh findInputs expects.
+      propagatedPathsStr = builtins.concatStringsSep " "
+        (map (dep: "${dep}") allPropagated);
+    in
     (pkgs.writeShellApplication args).overrideAttrs (old: {
       propagatedNativeBuildInputs =
-        (old.propagatedNativeBuildInputs or []) ++ runtimeInputs;
+        (old.propagatedNativeBuildInputs or []) ++ allPropagated;
+
+      # writeTextFile uses `buildCommand` which bypasses stdenv's entire
+      # phase system — neither postInstall nor fixupPhase ever runs.
+      # We must append our file creation directly to buildCommand.
+      buildCommand = (old.buildCommand or "") + ''
+        mkdir -p $out/nix-support
+        echo "${propagatedPathsStr}" > $out/nix-support/propagated-native-build-inputs
+      '';
     });
 }