fetchgitlocal: use output of `git show` and `git diff` to tell nix whether the source has changed.

this is way more hacky than i was hoping...

1 files changed, 15 insertions, 2 deletions

diff --git a/pkgs/build-support/fetchgitlocal/default.nix b/pkgs/build-support/fetchgitlocal/default.nix
index 04e6aafc8a1..43fc4b1179d 100644
--- a/pkgs/build-support/fetchgitlocal/default.nix
+++ b/pkgs/build-support/fetchgitlocal/default.nix
@@ -1,6 +1,19 @@
-{ runCommand, git }: src:
+{ runCommand, git, nix }: src:
 
-runCommand "local-git-export" {} ''
+let hash = import (runCommand "head-hash.nix"
+  { dummy = builtins.currentTime;
+    preferLocalBuild = true; }
+''
+  cd ${toString src}
+  (${git}/bin/git show && ${git}/bin/git diff) > $out
+  hash=$(${nix}/bin/nix-hash $out)
+  echo "\"$hash\"" > $out
+''); in
+
+runCommand "local-git-export"
+  { dummy = hash;
+    preferLocalBuild = true; }
+''
   cd ${toString src}
   mkdir -p "$out"
   for file in $(${git}/bin/git ls-files); do