From 8b217d531305433df01c789dab4dc81d58f05eba Mon Sep 17 00:00:00 2001
From: Max Kellermann <max@duempel.org>
Date: Mon, 5 Jan 2015 20:24:59 +0100
Subject: fs/io/FileOutputStream: use O_TMPFILE if available

The Linux feature allows writing new files to an invisible file, and
then replace the old file.  This preserves the old file if we get
interrupted by some event.
---
 src/fs/io/FileOutputStream.cxx | 66 +++++++++++++++++++++++++++++++++++++-----
 1 file changed, 59 insertions(+), 7 deletions(-)

(limited to 'src/fs/io/FileOutputStream.cxx')

diff --git a/src/fs/io/FileOutputStream.cxx b/src/fs/io/FileOutputStream.cxx
index 2951149b9..7a6416557 100644
--- a/src/fs/io/FileOutputStream.cxx
+++ b/src/fs/io/FileOutputStream.cxx
@@ -79,14 +79,47 @@ FileOutputStream::Cancel()
 #include <unistd.h>
 #include <errno.h>
 
+#ifdef HAVE_LINKAT
+#ifndef O_TMPFILE
+/* supported since Linux 3.11 */
+#define __O_TMPFILE 020000000
+#define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
+#include <stdio.h>
+#endif
+
+/**
+ * Open a file using Linux's O_TMPFILE for writing the given file.
+ */
+static int
+OpenTempFile(Path path)
+{
+	const auto directory = path.GetDirectoryName();
+	if (directory.IsNull())
+		return -1;
+
+	return OpenFile(directory, O_TMPFILE|O_WRONLY, 0666);
+}
+
+#endif /* HAVE_LINKAT */
+
 FileOutputStream::FileOutputStream(Path _path, Error &error)
-	:path(_path),
-	 fd(OpenFile(path,
-		     O_WRONLY|O_CREAT|O_TRUNC,
-		     0666))
+	:path(_path)
 {
-	if (fd < 0)
-		error.FormatErrno("Failed to create %s", path.c_str());
+#ifdef HAVE_LINKAT
+	/* try Linux's O_TMPFILE first */
+	fd = OpenTempFile(path);
+	is_tmpfile = fd >= 0;
+	if (!is_tmpfile) {
+#endif
+		/* fall back to plain POSIX */
+		fd = OpenFile(path,
+			      O_WRONLY|O_CREAT|O_TRUNC,
+			      0666);
+		if (fd < 0)
+			error.FormatErrno("Failed to create %s", path.c_str());
+#ifdef HAVE_LINKAT
+	}
+#endif
 }
 
 bool
@@ -112,6 +145,22 @@ FileOutputStream::Commit(Error &error)
 {
 	assert(IsDefined());
 
+#if HAVE_LINKAT
+	if (is_tmpfile) {
+		RemoveFile(path);
+
+		/* hard-link the temporary file to the final path */
+		char fd_path[64];
+		snprintf(fd_path, sizeof(fd_path), "/proc/self/fd/%d", fd);
+		if (linkat(AT_FDCWD, fd_path, AT_FDCWD, path.c_str(),
+			   AT_SYMLINK_FOLLOW) < 0) {
+			error.FormatErrno("Failed to commit %s", path.c_str());
+			close(fd);
+			return false;
+		}
+	}
+#endif
+
 	bool success = close(fd) == 0;
 	fd = -1;
 	if (!success)
@@ -128,7 +177,10 @@ FileOutputStream::Cancel()
 	close(fd);
 	fd = -1;
 
-	RemoveFile(path);
+#ifdef HAVE_LINKAT
+	if (!is_tmpfile)
+#endif
+		RemoveFile(path);
 }
 
 #endif
-- 
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