From 02098d2073e2f7c06b6d05c6749ae2b76b7f57d5 Mon Sep 17 00:00:00 2001
From: Graham Christensen <graham.christensen@target.com>
Date: Fri, 17 Aug 2018 10:29:32 -0400
Subject: [PATCH] fetchGit: use a better caching scheme

The current usage technically works by putting multiple different
repos in to the same git directory. However, it is very slow as
Git tries very hard to find common commits between the two
repositories. If the two repositories are large (like Nixpkgs and
another long-running project,) it is maddeningly slow.

This change busts the cache for existing deployments, but users
will be promptly repaid in per-repository performance.
---
 src/libexpr/primops/fetchGit.cc | 4 +++-
 tests/fetchGit.sh               | 4 ++--
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/src/libexpr/primops/fetchGit.cc b/src/libexpr/primops/fetchGit.cc
index 7aa98e0bf..aeb2df5f8 100644
--- a/src/libexpr/primops/fetchGit.cc
+++ b/src/libexpr/primops/fetchGit.cc
@@ -3,6 +3,7 @@
 #include "download.hh"
 #include "store-api.hh"
 #include "pathlocks.hh"
+#include "hash.hh"
 
 #include <sys/time.h>
 
@@ -84,9 +85,10 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
     if (rev != "" && !std::regex_match(rev, revRegex))
         throw Error("invalid Git revision '%s'", rev);
 
-    Path cacheDir = getCacheDir() + "/nix/git";
+    Path cacheDir = getCacheDir() + "/nix/gitv2/" + hashString(htSHA256, uri).to_string(Base32, false);
 
     if (!pathExists(cacheDir)) {
+        createDirs(dirOf(cacheDir));
         runProgram("git", true, { "init", "--bare", cacheDir });
     }
 
diff --git a/tests/fetchGit.sh b/tests/fetchGit.sh
index 530ac7bb8..4c46bdf04 100644
--- a/tests/fetchGit.sh
+++ b/tests/fetchGit.sh
@@ -9,7 +9,7 @@ clearStore
 
 repo=$TEST_ROOT/git
 
-rm -rf $repo ${repo}-tmp $TEST_HOME/.cache/nix/git
+rm -rf $repo ${repo}-tmp $TEST_HOME/.cache/nix/gitv2
 
 git init $repo
 git -C $repo config user.email "foobar@example.com"
@@ -129,7 +129,7 @@ path5=$(nix eval --raw "(builtins.fetchGit { url = $repo; ref = \"dev\"; }).outP
 
 
 # Nuke the cache
-rm -rf $TEST_HOME/.cache/nix/git
+rm -rf $TEST_HOME/.cache/nix/gitv2
 
 # Try again, but without 'git' on PATH
 NIX=$(command -v nix)