Cache calls to GitRepo::getWorkdirInfo()

A command like `nix flake metadata` was causing about 4 calls to
getWorkdirInfo(), which is slow for large repos (even when they're not
dirty).

src/libfetchers/git-utils.cc

@@ -5,6 +5,7 @@
 #include "signals.hh"
 #include "users.hh"
 #include "fs-sink.hh"
+#include "sync.hh"
 
 #include <git2/attr.h>
 #include <git2/blob.h>
@@ -1276,4 +1277,17 @@ ref<GitRepo> getTarballCache()
     return GitRepo::openRepo(repoDir, true, true);
 }
 
+GitRepo::WorkdirInfo GitRepo::getCachedWorkdirInfo(const std::filesystem::path & path)
+{
+    static Sync<std::unordered_map<std::filesystem::path, WorkdirInfo>> _cache;
+    {
+        auto cache(_cache.lock());
+        auto i = cache->find(path);
+        if (i != cache->end()) return i->second;
+    }
+    auto workdirInfo = GitRepo::openRepo(path)->getWorkdirInfo();
+    _cache.lock()->emplace(path, workdirInfo);
+    return workdirInfo;
+}
+
 }

src/libfetchers/git-utils.hh

@@ -70,6 +70,8 @@ struct GitRepo
 
     virtual WorkdirInfo getWorkdirInfo() = 0;
 
+    static WorkdirInfo getCachedWorkdirInfo(const std::filesystem::path & path);
+
     /* Get the ref that HEAD points to. */
     virtual std::optional<std::string> getWorkdirRef() = 0;
 

src/libfetchers/git.cc

@@ -431,7 +431,7 @@ struct GitInputScheme : InputScheme
         // If this is a local directory and no ref or revision is
         // given, then allow the use of an unclean working tree.
         if (!input.getRef() && !input.getRev() && repoInfo.isLocal)
-            repoInfo.workdirInfo = GitRepo::openRepo(repoInfo.url)->getWorkdirInfo();
+            repoInfo.workdirInfo = GitRepo::getCachedWorkdirInfo(repoInfo.url);
 
         return repoInfo;
     }