From c169ea59049f861aaba429f48b828d0820b74d1d Mon Sep 17 00:00:00 2001
From: Maximilian Bosch <maximilian@mbosch.me>
Date: Fri, 15 Nov 2019 13:04:42 +0100
Subject: [PATCH] builtins.fetchGit: Fix build when fetching a git worktree

Worktrees[1] are a feature of git which allow you to check out a ref in
a different directory.

While playing around with flakes I realized that git repositories in a
worktree checkout break when trying to build a flake:

```
$ git worktree add ../nixpkgs-flakes nixpkgs-flakes
$ cd ../nixpkgs-flakes
$ nix build .#hello
error: opening directory '/home/ma27/Projects/nixpkgs-flakes/.git/refs/heads': Not a directory
```

This issue has been fixed by determining with `git rev-parse --git-common-dir`
where the actual `.git` directory is.

Please note that this issue only exists on the `flakes` branch, fetching
worktree checkouts with Nix master seems to work fine.

[1] https://git-scm.com/docs/git-worktree
---
 src/libstore/fetchers/git.cc | 11 ++++++++++-
 tests/fetchGit.sh            |  8 ++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/src/libstore/fetchers/git.cc b/src/libstore/fetchers/git.cc
index 134d44ecd..9276b0993 100644
--- a/src/libstore/fetchers/git.cc
+++ b/src/libstore/fetchers/git.cc
@@ -217,7 +217,16 @@ struct GitInput : Input
 
             /* Check whether this repo has any commits. There are
                probably better ways to do this. */
-            bool haveCommits = !readDirectory(actualUrl + "/.git/refs/heads").empty();
+            auto gitDir = actualUrl + "/.git";
+            auto commonGitDir = chomp(runProgram(
+                "git",
+                true,
+                { "-C", actualUrl, "rev-parse", "--git-common-dir" }
+            ));
+            if (commonGitDir != ".git")
+                    gitDir = commonGitDir;
+
+            bool haveCommits = !readDirectory(gitDir + "/refs/heads").empty();
 
             try {
                 if (haveCommits) {
diff --git a/tests/fetchGit.sh b/tests/fetchGit.sh
index ac0d78f12..d44586977 100644
--- a/tests/fetchGit.sh
+++ b/tests/fetchGit.sh
@@ -25,8 +25,16 @@ rev1=$(git -C $repo rev-parse HEAD)
 
 echo world > $repo/hello
 git -C $repo commit -m 'Bla2' -a
+git -C $repo worktree add $TEST_ROOT/worktree
+echo hello >> $TEST_ROOT/worktree/hello
 rev2=$(git -C $repo rev-parse HEAD)
 
+# Fetch a worktree
+unset _NIX_FORCE_HTTP
+path0=$(nix eval --impure --raw --expr "(builtins.fetchGit file://$TEST_ROOT/worktree).outPath")
+export _NIX_FORCE_HTTP=1
+[[ $(tail -n 1 $path0/hello) = "hello" ]]
+
 # Fetch the default branch.
 path=$(nix eval --impure --raw --expr "(builtins.fetchGit file://$repo).outPath")
 [[ $(cat $path/hello) = world ]]