From 201cf5faf647fb12e1b14c41119ea8cb163502bc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Isma=C3=ABl=20Bouya?= <ismael.bouya@fretlink.com>
Date: Mon, 25 Jan 2021 16:46:44 +0100
Subject: [PATCH] Add "branch" information for builtins.fetchGit

This is the counterpart of nix-tools proposed change:
https://github.com/input-output-hk/nix-tools/pull/99
---
 lib/stack-cache-generator.nix | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/lib/stack-cache-generator.nix b/lib/stack-cache-generator.nix
index 6c6f3e86fd..8756913ab5 100644
--- a/lib/stack-cache-generator.nix
+++ b/lib/stack-cache-generator.nix
@@ -13,6 +13,13 @@
   if sha256map != null
     then { location, tag, ...}: sha256map."${location}"."${tag}"
     else _: null
+, branchMap    ? null
+                     # A way to specify in which branch a git commit can
+                     # be found
+, lookupBranch ?
+  if branchMap != null
+    then { location, tag, ...}: branchMap."${location}"."${tag}" or null
+    else _: null
 , resolverSha256 ? null
 , nix-tools ? pkgs.haskell-nix.internal-nix-tools # When building stack projects we use the internal nix-tools (compiled with a fixed GHC version)
 , ...
@@ -84,15 +91,19 @@ concatMap (dep:
                   location = dep.url;
                   tag = dep.rev;
                 };
+            branch = lookupBranch {
+              location = dep.url;
+              tag = dep.rev;
+            };
             pkgsrc =
               if !is-private && sha256 != null
                 then pkgs.fetchgit {
                   inherit (dep) url rev;
                   inherit sha256;
                 }
-                else builtins.fetchGit {
+                else builtins.fetchGit ({
                   inherit (dep) url rev;
-                };
+                } // pkgs.lib.optionalAttrs (branch != null) { ref = branch; });
         in map (subdir: {
                 name = cabalName "${pkgsrc}/${subdir}";
                 inherit (dep) url rev;