nix-prefetch-git: fetch the fetching tag for NIX_PREFETCH_GIT_CHECKOUT_HOOK

Co-authored-by: Adam Dinwoodie <adam@dinwoodie.org>

pkgs/build-support/fetchgit/nix-prefetch-git

@@ -258,6 +258,13 @@ clone(){
         clean_git fetch origin 'refs/tags/*:refs/tags/*' || echo "warning: failed to fetch some tags" >&2
     fi
 
+    # Name "$ref" to make `git describe` work reproducibly in `NIX_PREFETCH_GIT_CHECKOUT_HOOK`.
+    # Name only when not leaving `.git` for compatibility purposes.
+    if [[ -n "$ref" ]] && [[ -z "$leaveDotGit" ]]; then
+        echo "refer to FETCH_HEAD as its original name $ref"
+        clean_git update-ref "$ref" FETCH_HEAD
+    fi
+
     # Checkout linked sources.
     if test -n "$fetchSubmodules"; then
         init_submodules

pkgs/build-support/fetchgit/tests.nix

@@ -34,6 +34,16 @@
     hash = "sha256-7DszvbCNTjpzGRmpIVAWXk20P0/XTrWZ79KSOGLrUWY=";
   };
 
+  describe-tag = testers.invalidateFetcherByDrvHash fetchgit {
+    name = "describe-tag-nix-source";
+    url = "https://github.com/NixOS/nix";
+    tag = "2.3.15";
+    hash = "sha256-y7l+46lVP2pzJwGON5qEV0EoxWofRoWAym5q9VXvpc8=";
+    postCheckout = ''
+      { git -C "$out" describe || echo "git describe failed"; } | tee "$out"/describe-output.txt
+    '';
+  };
+
   sparseCheckout = testers.invalidateFetcherByDrvHash fetchgit {
     name = "sparse-checkout-nix-source";
     url = "https://github.com/NixOS/nix";