optimise-store: Fix race condition in temporary link creation

Replace rand() with atomic counter for unique temp link names and add
retry loop to handle EEXIST errors when multiple processes optimize
the store simultaneously.

src/libstore/optimise-store.cc

@@ -223,7 +223,9 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
        its timestamp back to 0. */
     MakeReadOnly makeReadOnly(mustToggle ? dirOfPath : "");
 
-    std::filesystem::path tempLink = fmt("%1%/.tmp-link-%2%-%3%", config->realStoreDir, getpid(), rand());
+    /* Retry loop for temporary link creation to handle race conditions */
+    while (true) {
+        Path tempLink = makeTempPath(config->realStoreDir.get(), ".tmp-link");
 
         try {
             std::filesystem::create_hard_link(linkPath, tempLink);
@@ -237,24 +239,37 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
                     printInfo("'%1%' has maximum number of links", linkPath);
                 return;
             }
-        throw;
+            throw SysError("creating temporary link '%1%'", tempLink);
         }
 
         /* Atomically replace the old file with the new hard link. */
         try {
             std::filesystem::rename(tempLink, path);
+            break; /* Success! */
         } catch (std::filesystem::filesystem_error & e) {
+            try {
                 std::filesystem::remove(tempLink);
-            printError("unable to unlink '%1%'", tempLink);
+            } catch (...) {
+                /* Ignore errors removing the temp link */
+            }
+
             if (e.code() == std::errc::too_many_links) {
                 /* Some filesystems generate too many links on the rename,
                    rather than on the original link.  (Probably it
                    temporarily increases the st_nlink field before
                    decreasing it again.) */
-            debug("'%s' has reached maximum number of links", linkPath);
+                debug("%s has reached maximum number of links", linkPath);
                 return;
             }
-        throw;
+
+            if (e.code() == std::errc::file_exists) {
+                /* Race condition: another process created the same temp link.
+                   Try again with a different name. */
+                continue;
+            }
+
+            throw SysError("renaming temporary link '%1%' to '%2%'", tempLink, path);
+        }
     }
 
     stats.filesLinked++;