buildEnv: support pulling in closures of paths

This will become important for Steam.

pkgs/build-support/buildenv/builder.pl

@@ -255,6 +255,21 @@ while (scalar(keys %postponed) > 0) {
     }
 }
 
+my $extraPathsFilePath = $ENV{"extraPathsFrom"};
+if ($extraPathsFilePath) {
+    open FILE, $extraPathsFilePath or die "cannot open extra paths file $extraPathsFilePath: $!";
+
+    while(my $line = <FILE>) {
+        chomp $line;
+        addPkg($line,
+               $ENV{"ignoreCollisions"} eq "1",
+               $ENV{"checkCollisionContents"} eq "1",
+               1000)
+            if -d $line;
+    }
+
+    close FILE;
+}
 
 # Create the symlinks.
 my $nrLinks = 0;

pkgs/build-support/buildenv/default.nix

@@ -1,7 +1,7 @@
 # buildEnv creates a tree of symlinks to the specified paths.  This is
 # a fork of the hardcoded buildEnv in the Nix distribution.
 
-{ buildPackages, runCommand, lib, substituteAll }:
+{ buildPackages, runCommand, lib, substituteAll, writeClosure }:
 
 let
   builder = substituteAll {
@@ -23,6 +23,9 @@ lib.makeOverridable
 , # Whether to ignore collisions or abort.
   ignoreCollisions ? false
 
+, # Whether to include closures of all input paths.
+  includeClosures ? false
+
 , # If there is a collision, check whether the contents and permissions match
   # and only if not, throw a collision error.
   checkCollisionContents ? true
@@ -49,13 +52,8 @@ lib.makeOverridable
 , passthru ? {}
 , meta ? {}
 }:
-
-runCommand name
-  rec {
-    inherit manifest ignoreCollisions checkCollisionContents passthru
-            meta pathsToLink extraPrefix postBuild
-            nativeBuildInputs buildInputs;
-    pkgs = builtins.toJSON (map (drv: {
+let
+  chosenOutputs = map (drv: {
     paths =
       # First add the usual output(s): respect if user has chosen explicitly,
       # and otherwise use `meta.outputsToInstall`. The attribute is guaranteed
@@ -69,7 +67,16 @@ runCommand name
       ++ lib.filter (p: p!=null)
         (builtins.map (outName: drv.${outName} or null) extraOutputsToInstall);
     priority = drv.meta.priority or lib.meta.defaultPriority;
-    }) paths);
+  }) paths;
+
+  pathsForClosure = lib.flatten (map (p: p.paths) chosenOutputs);
+in runCommand name
+  rec {
+    inherit manifest ignoreCollisions checkCollisionContents passthru
+            meta pathsToLink extraPrefix postBuild
+            nativeBuildInputs buildInputs;
+    pkgs = builtins.toJSON chosenOutputs;
+    extraPathsFrom = lib.optional includeClosures (writeClosure pathsForClosure);
     preferLocalBuild = true;
     allowSubstitutes = false;
     # XXX: The size is somewhat arbitrary