io/FileOutputStream: write to temporary file if O_TMPFILE is not available

src/io/FileOutputStream.cxx

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2014-2018 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2014-2022 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -31,6 +31,10 @@
 #include "system/Error.hxx"
 #include "util/StringFormat.hxx"
 
+#ifdef _WIN32
+#include <tchar.h>
+#endif
+
 #ifdef __linux__
 #include <fcntl.h>
 #endif
@@ -81,8 +85,22 @@ FileOutputStream::Open()
 #ifdef _WIN32
 
 inline void
-FileOutputStream::OpenCreate([[maybe_unused]] bool visible)
+FileOutputStream::OpenCreate(bool visible)
 {
+	if (!visible) {
+		/* attempt to create a temporary file */
+		tmp_path = path.WithSuffix(_T(".tmp"));
+		Delete(tmp_path);
+
+		handle = CreateFile(tmp_path.c_str(), GENERIC_WRITE, 0, nullptr,
+				    CREATE_NEW,
+				    FILE_ATTRIBUTE_NORMAL|FILE_FLAG_WRITE_THROUGH,
+				    nullptr);
+		if (handle != INVALID_HANDLE_VALUE)
+			return;
+
+	}
+
 	handle = CreateFile(path.c_str(), GENERIC_WRITE, 0, nullptr,
 			    CREATE_ALWAYS,
 			    FILE_ATTRIBUTE_NORMAL|FILE_FLAG_WRITE_THROUGH,
@@ -149,10 +167,17 @@ FileOutputStream::Sync()
 
 void
 FileOutputStream::Commit()
-{
+try {
 	assert(IsDefined());
 
 	Close();
+
+	if (tmp_path != nullptr)
+		RenameOrThrow(tmp_path, path);
+} catch (...) {
+	if (tmp_path != nullptr)
+		Delete(tmp_path);
+	throw;
 }
 
 #else
@@ -200,6 +225,22 @@ FileOutputStream::OpenCreate([[maybe_unused]] bool visible)
 	}
 #endif
 
+	if (!visible) {
+		/* attempt to create a temporary file */
+		tmp_path = path + ".tmp";
+		Delete(tmp_path);
+
+		if (fd.Open(
+#ifdef __linux__
+			    directory_fd,
+#endif
+			    tmp_path.c_str(),
+			    O_WRONLY|O_CREAT|O_EXCL,
+			    0666))
+			return;
+
+	}
+
 	/* fall back to plain POSIX */
 	if (!fd.Open(
 #ifdef __linux__
@@ -263,7 +304,7 @@ FileOutputStream::Sync()
 
 void
 FileOutputStream::Commit()
-{
+try {
 	assert(IsDefined());
 
 #ifdef HAVE_O_TMPFILE
@@ -283,6 +324,13 @@ FileOutputStream::Commit()
 	if (!Close()) {
 		throw FormatErrno("Failed to commit %s", path.c_str());
 	}
+
+	if (tmp_path != nullptr)
+		RenameOrThrow(tmp_path, path);
+} catch (...) {
+	if (tmp_path != nullptr)
+		Delete(tmp_path);
+	throw;
 }
 
 #endif
@@ -294,6 +342,11 @@ FileOutputStream::Cancel() noexcept
 
 	Close();
 
+	if (tmp_path != nullptr) {
+		Delete(tmp_path);
+		return;
+	}
+
 	switch (mode) {
 	case Mode::CREATE:
 #ifdef HAVE_O_TMPFILE
@@ -310,6 +363,27 @@ FileOutputStream::Cancel() noexcept
 	}
 }
 
+inline void
+FileOutputStream::RenameOrThrow([[maybe_unused]] Path old_path,
+				[[maybe_unused]] Path new_path) const
+{
+	assert(old_path != nullptr);
+	assert(new_path != nullptr);
+
+#ifdef _WIN32
+	if (!MoveFileEx(old_path.c_str(), new_path.c_str(),
+			MOVEFILE_REPLACE_EXISTING))
+		throw MakeLastError("Failed to rename file");
+#elif defined(__linux__)
+	if (renameat(directory_fd.Get(), tmp_path.c_str(),
+		     directory_fd.Get(), path.c_str()) < 0)
+		throw MakeErrno("Failed to rename file");
+#else
+	if (rename(tmp_path.c_str(), path.c_str()))
+		throw MakeErrno("Failed to rename file");
+#endif
+}
+
 inline void
 FileOutputStream::Delete(Path delete_path) const noexcept
 {

src/io/FileOutputStream.hxx

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2014-2018 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2014-2022 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -68,6 +68,13 @@ class Path;
 class FileOutputStream final : public OutputStream {
 	const AllocatedPath path;
 
+	/**
+	 * If a temporary file is being written to, then this is its
+	 * path.  Commit() will rename it to the path specified in the
+	 * constructor.
+	 */
+	AllocatedPath tmp_path{nullptr};
+
 #ifdef __linux__
 	const FileDescriptor directory_fd;
 #endif
@@ -206,6 +213,7 @@ private:
 #endif
 	}
 
+	void RenameOrThrow(Path old_path, Path new_path) const;
 	void Delete(Path delete_path) const noexcept;
 };
 

test/net/meson.build

@@ -19,6 +19,7 @@ test(
     include_directories: inc,
     dependencies: [
       net_dep,
+      fs_dep,
       gtest_dep,
     ],
   ),