From 5ac7077dd031108a9a133bba8ff44be7cef9140c Mon Sep 17 00:00:00 2001
From: catbot <catbot@bot.local>
Date: Wed, 27 May 2026 01:55:29 +0000
Subject: [PATCH] fix external cache race on concurrent builds
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Two crafter-build invocations resolving to the same external dep both
cloned into <cache>/<name>-<hash>.tmp, corrupting each other's pack
tempfiles. Use a random per-invocation tmp suffix and treat a failed
rename whose destination now exists as the loser-of-the-race case —
discard the local clone and reuse the winner's.
---
 implementations/Crafter.Build-External.cpp | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/implementations/Crafter.Build-External.cpp b/implementations/Crafter.Build-External.cpp
index e9d1689..fc6c1ec 100644
--- a/implementations/Crafter.Build-External.cpp
+++ b/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";
-    if (fs::exists(tmpDir)) fs::remove_all(tmpDir);
+    tmpDir += std::format(".tmp.{:08x}{:08x}", rd(), rd());
 
     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 "";