From bc73faf88f3629eb05ea111bfe303f4cae678797 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=B6rg=20Thalheim?= <joerg@thalheim.io>
Date: Thu, 12 Jun 2025 17:40:54 +0200
Subject: [PATCH] local-store: Fix potential race in build log creation

Add atomic counter to temporary log file names and retry loop to
handle EEXIST errors. If another process creates the log file first,
we gracefully exit since the log already exists.
---
 src/libstore/local-store.cc | 31 ++++++++++++++++++++++++++++---
 1 file changed, 28 insertions(+), 3 deletions(-)

diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 76fadba86..fc7798644 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -24,6 +24,7 @@
 #include <algorithm>
 #include <cstring>
 
+#include <atomic>
 #include <memory>
 #include <new>
 #include <sys/types.h>
@@ -1666,11 +1667,35 @@ void LocalStore::addBuildLog(const StorePath & drvPath, std::string_view log)
 
     createDirs(dirOf(logPath));
 
-    auto tmpFile = fmt("%s.tmp.%d", logPath, getpid());
+    /* Retry loop for temporary log file creation to handle race conditions */
+    while (true) {
+        auto tmpFile = makeTempPath(dirOf(logPath), baseNameOf(logPath) + ".tmp");
 
-    writeFile(tmpFile, compress("bzip2", log));
+        try {
+            writeFile(tmpFile, compress("bzip2", log));
+        } catch (Error & e) {
+            e.addTrace({}, "writing build log to '%s'", tmpFile);
+            throw;
+        }
 
-    std::filesystem::rename(tmpFile, logPath);
+        try {
+            std::filesystem::rename(tmpFile, logPath);
+            break; /* Success! */
+        } catch (std::filesystem::filesystem_error & e) {
+            try {
+                std::filesystem::remove(tmpFile);
+            } catch (...) {
+                /* Ignore errors removing the temp file */
+            }
+
+            if (e.code() == std::errc::file_exists) {
+                /* Another process created the log file. That's fine, we're done. */
+                break;
+            }
+
+            throw SysError("renaming temporary file '%1%' to '%2%'", tmpFile, logPath);
+        }
+    }
 }
 
 std::optional<std::string> LocalStore::getVersion()