Store cached head in cached git repo

The previous head caching implementation stored two paths in the local
cache; one for the cached git repo and another textfile containing the
resolved HEAD ref. This commit instead stores the resolved HEAD by
setting the HEAD ref in the local cache appropriately.

src/libfetchers/git.cc

@@ -10,6 +10,7 @@
 #include "fetch-settings.hh"
 
 #include <regex>
+#include <string.h>
 #include <sys/time.h>
 #include <sys/wait.h>
 
@@ -37,7 +38,19 @@ bool isCacheFileWithinTtl(const time_t now, const struct stat& st) {
     return st.st_mtime + settings.tarballTtl > now;
 }
 
-Path getCachePath(std::string key) {
+bool touchCacheFile(const Path& path, const time_t& touch_time)
+{
+  struct timeval times[2];
+  times[0].tv_sec = touch_time;
+  times[0].tv_usec = 0;
+  times[1].tv_sec = touch_time;
+  times[1].tv_usec = 0;
+
+  return lutimes(path.c_str(), times) == 0;
+}
+
+Path getCachePath(std::string key)
+{
     return getCacheDir() + "/nix/gitv3/" +
         hashString(htSHA256, key).to_string(Base32, false);
 }
@@ -80,32 +93,42 @@ std::optional<std::string> readHead(const Path & path)
     return std::nullopt;
 }
 
-std::optional<std::string> readHeadCached(std::string actualUrl)
+// Persist the HEAD ref from the remote repo in the local cached repo.
+bool storeCachedHead(const std::string& actualUrl, const std::string& headRef)
+{
+    Path cacheDir = getCachePath(actualUrl);
+    try {
+        runProgram("git", true, { "-C", cacheDir, "symbolic-ref", "--", "HEAD", headRef });
+    } catch (ExecError &e) {
+        if (!WIFEXITED(e.status)) throw;
+        return false;
+    }
+    /* No need to touch refs/HEAD, because `git symbolic-ref` updates the mtime. */
+    return true;
+}
+
+std::optional<std::string> readHeadCached(const std::string& actualUrl)
 {
     // Create a cache path to store the branch of the HEAD ref. Append something
     // in front of the URL to prevent collision with the repository itself.
-    Path cachePath = getCachePath("<ref>|" + actualUrl);
+    Path cacheDir = getCachePath(actualUrl);
+    Path headRefFile = cacheDir + "/HEAD";
+
     time_t now = time(0);
     struct stat st;
     std::optional<std::string> cachedRef;
-    if (stat(cachePath.c_str(), &st) == 0 &&
-        // The file may be empty, because writeFile() is not atomic.
-        st.st_size > 0) {
-
-        // The cached ref is persisted unconditionally (see below).
-        cachedRef = readFile(cachePath);
-        if (isCacheFileWithinTtl(now, st)) {
+    if (stat(headRefFile.c_str(), &st) == 0) {
+        cachedRef = readHead(cacheDir);
+        if (cachedRef != std::nullopt &&
+            *cachedRef != gitInitialBranch &&
+            isCacheFileWithinTtl(now, st)) {
             debug("using cached HEAD ref '%s' for repo '%s'", *cachedRef, actualUrl);
             return cachedRef;
         }
     }
 
     auto ref = readHead(actualUrl);
-
     if (ref) {
-        debug("storing cached HEAD ref '%s' for repo '%s' at '%s'", *ref, actualUrl, cachePath);
-        createDirs(dirOf(cachePath));
-        writeFile(cachePath, *ref);
         return ref;
     }
 
@@ -438,15 +461,6 @@ struct GitInputScheme : InputScheme
             }
         }
 
-        if (!input.getRef()) {
-            auto head = isLocal ? readHead(actualUrl) : readHeadCached(actualUrl);
-            if (!head) {
-                warn("could not read HEAD ref from repo at '%s', using 'master'", actualUrl);
-                head = "master";
-            }
-            input.attrs.insert_or_assign("ref", *head);
-        }
-
         const Attrs unlockedAttrs({
             {"type", cacheType},
             {"name", name},
@@ -457,14 +471,30 @@ struct GitInputScheme : InputScheme
         Path repoDir;
 
         if (isLocal) {
+            if (!input.getRef()) {
+                auto head = readHead(actualUrl);
+                if (!head) {
+                    warn("could not read HEAD ref from repo at '%s', using 'master'", actualUrl);
+                    head = "master";
+                }
+                input.attrs.insert_or_assign("ref", *head);
+            }
 
             if (!input.getRev())
                 input.attrs.insert_or_assign("rev",
                     Hash::parseAny(chomp(runProgram("git", true, { "-C", actualUrl, "--git-dir", gitDir, "rev-parse", *input.getRef() })), htSHA1).gitRev());
 
             repoDir = actualUrl;
-
         } else {
+            const bool useHeadRef = !input.getRef();
+            if (useHeadRef) {
+                auto head = readHeadCached(actualUrl);
+                if (!head) {
+                    warn("could not read HEAD ref from repo at '%s', using 'master'", actualUrl);
+                    head = "master";
+                }
+                input.attrs.insert_or_assign("ref", *head);
+            }
 
             if (auto res = getCache()->lookup(store, unlockedAttrs)) {
                 auto rev2 = Hash::parseAny(getStrAttr(res->first, "rev"), htSHA1);
@@ -538,13 +568,10 @@ struct GitInputScheme : InputScheme
                     warn("could not update local clone of Git repository '%s'; continuing with the most recent version", actualUrl);
                 }
 
-                struct timeval times[2];
-                times[0].tv_sec = now;
-                times[0].tv_usec = 0;
-                times[1].tv_sec = now;
-                times[1].tv_usec = 0;
-
-                utimes(localRefFile.c_str(), times);
+                if (!touchCacheFile(localRefFile, now))
+                    warn("could not update mtime for file '%s': %s", localRefFile, strerror(errno));
+                if (useHeadRef && !storeCachedHead(actualUrl, *input.getRef()))
+                    warn("could not update cached head '%s' for '%s'", *input.getRef(), actualUrl);
             }
 
             if (!input.getRev())