Merge pull request 'Race condition: concurrent builds clobber each other's external dep clones' (#3) from claude/issue-2 into master

Reviewed-on: #3

implementations/Crafter.Build-External.cpp

@@ -103,9 +103,13 @@ std::string FetchGit(const GitSource& source, const fs::path& cloneDir) {
         }
     }
 
+    // Random per-invocation tmp suffix so concurrent crafter-build processes
+    // resolving to the same cache key don't share an in-flight `.tmp`
+    // directory — git's pack tempfiles would get yanked when one process
+    // removes or renames the dir out from under the other.
+    std::random_device rd;
     fs::path tmpDir = cloneDir;
-    tmpDir += ".tmp";
+    tmpDir += std::format(".tmp.{:08x}{:08x}", rd(), rd());
-    if (fs::exists(tmpDir)) fs::remove_all(tmpDir);
 
     if (auto err = runGit(std::format("git clone --recursive {} {}", ShellQuote(source.url), ShellQuote(tmpDir.string())))) {
         if (fs::exists(tmpDir)) fs::remove_all(tmpDir);
@@ -128,6 +132,10 @@ std::string FetchGit(const GitSource& source, const fs::path& cloneDir) {
     fs::rename(tmpDir, cloneDir, ec);
     if (ec) {
         fs::remove_all(tmpDir);
+        // If a concurrent build won the race, cloneDir now exists with the
+        // same source revision (cache key includes url+branch+commit), so
+        // discard our clone and reuse theirs.
+        if (fs::exists(cloneDir)) return "";
         return std::format("rename {} -> {} failed: {}", tmpDir.string(), cloneDir.string(), ec.message());
     }
     return "";