From 60dd23ab8f5d772bee38200395e54d640499d443 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?P=C5=99emysl=20Janouch?= <p.janouch@gmail.com>
Date: Wed, 13 Jan 2016 00:19:20 +0100
Subject: [PATCH] Make writing files a bit safer

Especially configuration files.
---
 liberty.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/liberty.c b/liberty.c
index 9d5efd3..62eab4e 100644
--- a/liberty.c
+++ b/liberty.c
@@ -3379,11 +3379,11 @@ write_file (const char *filename, const struct str *data, struct error **e)
 		return false;
 	}
 
-	errno = 0;
 	fwrite (data->str, data->len, 1, fp);
+	bool success = !ferror (fp) && !fflush (fp) && !fsync (fileno (fp));
 	fclose (fp);
 
-	if (errno)
+	if (!success)
 	{
 		error_set (e, "writing to `%s' failed: %s", filename, strerror (errno));
 		return false;
@@ -3391,6 +3391,22 @@ write_file (const char *filename, const struct str *data, struct error **e)
 	return true;
 }
 
+/// Wrapper for write_file() that makes sure that the new data has been written
+/// to disk in its entirety before overriding the old file
+static bool
+write_file_safe (const char *filename, const struct str *data, struct error **e)
+{
+	// XXX: ideally we would also open the directory, use *at() versions
+	//   of functions and call fsync() on the directory as appropriate
+	char *temp = xstrdup_printf ("%s.new", filename);
+	bool success = write_file (temp, data, e);
+	if (success && !(success = !rename (temp, filename)))
+		error_set (e, "could not rename `%s' to `%s': %s",
+			temp, filename, strerror (errno));
+	free (temp);
+	return success;
+}
+
 // --- Simple configuration ----------------------------------------------------
 
 // The keys are stripped of surrounding whitespace, the values are not.
@@ -3482,7 +3498,7 @@ write_configuration_file (const char *path_hint, const struct str *data,
 		str_append (&path, "/" PROGRAM_NAME "/" PROGRAM_NAME ".conf");
 	}
 
-	if (!write_file (path.str, data, e))
+	if (!write_file_safe (path.str, data, e))
 	{
 		str_free (&path);
 		return NULL;