Fix cross-process race in CPU compile cache

mlx/backend/cpu/compiled.cpp

@@ -5,6 +5,7 @@
 #include <fstream>
 #include <list>
 #include <mutex>
+#include <random>
 #include <shared_mutex>
 
 #include <fmt/format.h>
@@ -116,9 +117,14 @@ void* compile(
   }
 
   if (!lib_exists) {
-    // Open source file and write source code to it
-    std::string source_file_name = kernel_file_name + ".cpp";
-    auto source_file_path = (output_dir / source_file_name).string();
+    // Build under a unique name, then move the files into place. Other
+    // processes then never load a partly written library.
+    auto tmp_name = fmt::format("tmp_{:08x}", std::random_device{}());
+    std::string source_file_name = tmp_name + ".cpp";
+    std::string tmp_lib_name =
+        tmp_name + std::filesystem::path(shared_lib_name).extension().string();
+    auto source_file_path = output_dir / source_file_name;
+    auto tmp_lib_path = output_dir / tmp_lib_name;
 
     std::ofstream source_file(source_file_path);
     source_file << source_code;
@@ -127,14 +133,27 @@ void* compile(
     try {
       JitCompiler::exec(
           JitCompiler::build_command(
-              output_dir, source_file_name, shared_lib_name));
+              output_dir, source_file_name, tmp_lib_name));
     } catch (const std::exception& error) {
       throw std::runtime_error(
           fmt::format(
               "[Compile::eval_cpu] Failed to compile function {0}: {1}",
               kernel_name,
               error.what()));
     }
+
+    std::error_code error;
+    std::filesystem::rename(
+        source_file_path, output_dir / (kernel_file_name + ".cpp"), error);
+    std::filesystem::rename(tmp_lib_path, shared_lib_path, error);
+    if (error) {
+      // On Windows the rename can fail while another process uses the file.
+      if (std::filesystem::exists(shared_lib_path, error)) {
+        std::filesystem::remove(tmp_lib_path, error);
+      } else {
+        shared_lib_path = tmp_lib_path.string();
+      }
+    }
   }
 
   // load library