From edd606ae62e213c2a30ff76b8eea4f75ea703d41 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B8rn=20Forsman?= <bjorn.forsman@gmail.com>
Date: Sat, 27 Mar 2021 14:15:28 +0100
Subject: [PATCH] fetchGit: don't prefix "refs/heads/" on ref = "HEAD"

This fixes builtins.fetchGit { url = ...; ref = "HEAD"; }, that works in
stable nix (v2.3.10), but is broken in nix master:

  $ ./result/bin/nix repl
  Welcome to Nix version 2.4pre19700101_dd77f71. Type :? for help.

  nix-repl> builtins.fetchGit { url = "https://github.com/NixOS/nix"; ref = "HEAD"; }
  fetching Git repository 'https://github.com/NixOS/nix'fatal: couldn't find remote ref refs/heads/HEAD
  error: program 'git' failed with exit code 128

The documentation for builtins.fetchGit says ref = "HEAD" is the
default, so it should also be supported to explicitly pass it.

I came across this issue because poetry2nix can use ref = "HEAD" in some
situations.

Fixes #4674.
---
 src/libfetchers/git.cc | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/libfetchers/git.cc b/src/libfetchers/git.cc
index 4f9db1bcd..b9a240b13 100644
--- a/src/libfetchers/git.cc
+++ b/src/libfetchers/git.cc
@@ -365,7 +365,9 @@ struct GitInputScheme : InputScheme
                         ? "refs/*"
                         : ref->compare(0, 5, "refs/") == 0
                             ? *ref
-                            : "refs/heads/" + *ref;
+                            : ref == "HEAD"
+                                ? *ref
+                                : "refs/heads/" + *ref;
                     runProgram("git", true, { "-C", repoDir, "fetch", "--quiet", "--force", "--", actualUrl, fmt("%s:%s", fetchRef, fetchRef) });
                 } catch (Error & e) {
                     if (!pathExists(localRefFile)) throw;