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.

3 files changed, 104 insertions, 0 deletions

diff --git a/src/buildtool/file_system/TARGETS b/src/buildtool/file_system/TARGETS
index dc7774ac..a6ae625f 100644
--- a/src/buildtool/file_system/TARGETS
+++ b/src/buildtool/file_system/TARGETS
@@ -209,4 +209,12 @@
     ]
   , "stage": ["src", "buildtool", "file_system"]
   }
+, "atomic":
+  { "type": ["@", "rules", "CC", "library"]
+  , "name": ["atomic"]
+  , "hdrs": ["atomic.hpp"]
+  , "srcs": ["atomic.cpp"]
+  , "private-deps": [["@", "fmt", "", "fmt"]]
+  , "stage": ["src", "buildtool", "file_system"]
+  }
 }
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;
+}
diff --git a/src/buildtool/file_system/atomic.hpp b/src/buildtool/file_system/atomic.hpp
new file mode 100644
index 00000000..40c6e4ce
--- /dev/null
+++ b/src/buildtool/file_system/atomic.hpp
@@ -0,0 +1,30 @@
+// 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.
+
+#ifndef INCLUDED_SRC_BUILDTOOL_FILE_SYSTEM_ATOMIC_HPP
+#define INCLUDED_SRC_BUILDTOOL_FILE_SYSTEM_ATOMIC_HPP
+
+#include <string>
+
+class FileSystemAtomic {
+
+  public:
+    /// \brief Write file atomically
+    /// First write the contents to a temporary file in the same directory; only
+    /// once completed, rename the fie to the final destination.
+    static auto WriteFile(std::string const& filename,
+                          std::string const& content) noexcept -> bool;
+};
+
+#endif