buildEnv: improve file check to avoid false-positives

The original change in #55372 was supposed to fix the case where a store
path which is a file should be placed into `buildEnv` which broke with a
fairly misleading Perl error.

Unfortunately this introduced a regression, `findFiles` can have targets
that are files if the file isn't a store path. Rather than adding more
obscure checks with probably further regressions, I figured that it's
better to replicate the behavior of `lib.isStorePath` and explicitly
check if the store path is a file and break in this case only.

This should also fix recent staging issues.

2 files changed, 19 insertions, 4 deletions

diff --git a/pkgs/build-support/buildenv/builder.pl b/pkgs/build-support/buildenv/builder.pl
index 1f77edf86cb..b699d762d29 100755
--- a/pkgs/build-support/buildenv/builder.pl
+++ b/pkgs/build-support/buildenv/builder.pl
@@ -26,6 +26,13 @@ sub isInPathsToLink {
     return 0;
 }
 
+# Similar to `lib.isStorePath`
+sub isStorePath {
+    my $path = shift;
+    my $storePath = "@storeDir@";
+
+    return substr($path, 0, 1) eq "/" && dirname($path) eq $storePath;
+}
 
 # For each activated package, determine what symlinks to create.
 
@@ -84,8 +91,9 @@ sub checkCollision {
 sub findFiles {
     my ($relName, $target, $baseName, $ignoreCollisions, $checkCollisionContents, $priority) = @_;
 
-    if (-f $target) {
-        die "Path $target is a file and can't be merged into an environment using pkgs.buildEnv!";
+    # The store path must not be a file
+    if (-f $target && isStorePath $target) {
+        die "The store path $target is a file and can't be merged into an environment using pkgs.buildEnv!";
     }
 
     # Urgh, hacky...
diff --git a/pkgs/build-support/buildenv/default.nix b/pkgs/build-support/buildenv/default.nix
index 41a1e67ef42..f858048d694 100644
--- a/pkgs/build-support/buildenv/default.nix
+++ b/pkgs/build-support/buildenv/default.nix
@@ -2,7 +2,7 @@
 # a fork of the buildEnv in the Nix distribution.  Most changes should
 # eventually be merged back into the Nix distribution.
 
-{ buildPackages, runCommand, lib }:
+{ buildPackages, runCommand, lib, substituteAll }:
 
 lib.makeOverridable
 ({ name
@@ -43,6 +43,13 @@ lib.makeOverridable
 , meta ? {}
 }:
 
+let
+  builder = substituteAll {
+    src = ./builder.pl;
+    inherit (builtins) storeDir;
+  };
+in
+
 runCommand name
   rec {
     inherit manifest ignoreCollisions checkCollisionContents passthru
@@ -67,6 +74,6 @@ runCommand name
     passAsFile = if builtins.stringLength pkgs >= 128*1024 then [ "pkgs" ] else null;
   }
   ''
-    ${buildPackages.perl}/bin/perl -w ${./builder.pl}
+    ${buildPackages.perl}/bin/perl -w ${builder}
     eval "$postBuild"
   '')