Add library for atomically writing files

... by using the write-rename dance. This allows processes
waiting for pid or similar files to rely on the information
being available as soon as the file is available.

1 files changed, 66 insertions, 0 deletions

diff --git a/src/buildtool/file_system/atomic.cpp b/src/buildtool/file_system/atomic.cpp
new file mode 100644
index 00000000..4d7ddc25
--- /dev/null
+++ b/src/buildtool/file_system/atomic.cpp
@@ -0,0 +1,66 @@
+// Copyright 2025 Huawei Cloud Computing Technology Co., Ltd.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "src/buildtool/file_system/atomic.hpp"
+
+#ifdef __unix__
+#include <unistd.h>
+#else
+#error "Non-unix is not supported yet"
+#endif
+
+#include <cstddef>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+
+#include "fmt/core.h"
+
+auto FileSystemAtomic::WriteFile(std::string const& filename,
+                                 std::string const& content) noexcept -> bool {
+    // As there is no high-level replacement of mkstemp(3), we fall back to
+    // using the libc functions.
+    auto filename_tmp_template = fmt::format("{}.XXXXXX", filename);
+    char* filename_tmp = strdup(filename_tmp_template.c_str());
+    if (filename_tmp == nullptr) {
+        return false;
+    }
+    auto fd = mkstemp(filename_tmp);
+    if (fd == -1) {
+        free(filename_tmp);  // NOLINT
+        return false;
+    }
+    size_t to_write = content.length();
+    const char* buf = content.c_str();
+    while (to_write > 0) {
+        auto written = write(fd, buf, to_write);
+        if (written < 0) {
+            (void)close(fd);
+            free(filename_tmp);  // NOLINT
+            return false;
+        }
+        to_write -= static_cast<size_t>(written);
+        buf = &buf[written];  // NOLINT
+    }
+    if (close(fd) != 0) {
+        free(filename_tmp);  // NOLINT
+        return false;
+    }
+    if (rename(filename_tmp, filename.c_str()) != 0) {
+        free(filename_tmp);  // NOLINT
+        return false;
+    }
+    free(filename_tmp);  // NOLINT
+    return true;
+}