io/linux/ProcPath: new library

src/io/FileOutputStream.cxx

@@ -4,13 +4,13 @@
 #include "FileOutputStream.hxx"
 #include "lib/fmt/PathFormatter.hxx"
 #include "lib/fmt/SystemError.hxx"
-#include "lib/fmt/ToBuffer.hxx"
 
 #ifdef _WIN32
 #include <tchar.h>
 #endif
 
 #ifdef __linux__
+#include "io/linux/ProcPath.hxx"
 #include <fcntl.h>
 #endif
 
@@ -281,8 +281,7 @@ try {
 		unlinkat(directory_fd.Get(), GetPath().c_str(), 0);
 
 		/* hard-link the temporary file to the final path */
-		if (linkat(-1,
-			   FmtBuffer<64>("/proc/self/fd/{}", fd.Get()),
+		if (linkat(-1, ProcFdPath(fd),
 			   directory_fd.Get(), path.c_str(),
 			   AT_SYMLINK_FOLLOW) < 0)
 			throw FmtErrno("Failed to commit {}", path);

src/io/linux/ProcPath.hxx

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: BSD-2-Clause
+// Copyright CM4all GmbH
+// author: Max Kellermann <mk@cm4all.com>
+
+#pragma once
+
+#include "lib/fmt/ToBuffer.hxx"
+#include "io/FileDescriptor.hxx"
+#include "util/StringBuffer.hxx"
+
+/**
+ * Build the path to the "/proc/self/fd/" magic link of the given file
+ * descriptor.
+ */
+[[gnu::const]]
+inline StringBuffer<32>
+ProcFdPath(FileDescriptor fd) noexcept
+{
+	return FmtBuffer<32>("/proc/self/fd/{}", fd.Get());
+}
+
+/**
+ * Build the path to the "/proc/self/fdinfo/" file of the given file
+ * descriptor.
+ */
+[[gnu::const]]
+inline StringBuffer<32>
+ProcFdinfoPath(FileDescriptor fd) noexcept
+{
+	return FmtBuffer<32>("/proc/self/fdinfo/{}", fd.Get());
+}