git-apply: be a lot more careful when writing files

We write them under another name and rename them to their destination,
so that if something bad happens in the middle, we won't have caused any
bigger harm.

Also, this makes the writing be NFS "intr" safe, and as a side effects
makes sure that if the target is hardlinked (or symlinked) we will have
broken the link.

apply.c

@@ -1245,31 +1245,65 @@ static void create_subdirectories(const char *path)
 	free(buf);
 }
 
+static int try_create_file(const char *path, unsigned int mode, const char *buf, unsigned long size)
+{
+	int fd;
+
+	if (S_ISLNK(mode))
+		return symlink(buf, path);
+	fd = open(path, O_CREAT | O_EXCL | O_WRONLY | O_TRUNC, (mode & 0100) ? 0777 : 0666);
+	if (fd < 0)
+		return -1;
+	while (size) {
+		int written = write(fd, buf, size);
+		if (written < 0) {
+			if (errno == EINTR || errno == EAGAIN)
+				continue;
+			die("writing file %s: %s", path, strerror(errno));
+		}
+		if (!written)
+			die("out of space writing file %s", path);
+		buf += written;
+		size -= written;
+	}
+	if (close(fd) < 0)
+		die("closing file %s: %s", path, strerror(errno));
+	return 0;
+}
+
 /*
  * We optimistically assume that the directories exist,
  * which is true 99% of the time anyway. If they don't,
  * we create them and try again.
  */
-static int create_regular_file(const char *path, unsigned int mode)
+static void create_one_file(const char *path, unsigned mode, const char *buf, unsigned long size)
 {
-	int ret = open(path, O_WRONLY | O_CREAT | O_TRUNC, mode);
+	if (!try_create_file(path, mode, buf, size))
+		return;
 
-	if (ret < 0 && errno == ENOENT) {
+	if (errno == ENOENT) {
 		create_subdirectories(path);
-		ret = open(path, O_WRONLY | O_CREAT | O_TRUNC, mode);
+		if (!try_create_file(path, mode, buf, size))
+			return;
 	}
-	return ret;
-}
 
-static int create_symlink(const char *buf, const char *path)
-{
-	int ret = symlink(buf, path);
+	if (errno == EEXIST) {
+		unsigned int nr = getpid();
 
-	if (ret < 0 && errno == ENOENT) {
-		create_subdirectories(path);
-		ret = symlink(buf, path);
+		for (;;) {
+			const char *newpath;
+			newpath = mkpath("%s~%u", path, nr);
+			if (!try_create_file(newpath, mode, buf, size)) {
+				if (!rename(newpath, path))
+					return;
+				unlink(newpath);
+				break;
+			}
+			if (errno != EEXIST)
+				break;
+		}			
 	}
-	return ret;
+	die("unable to write file %s mode %o", path, mode);
 }
 
 static void create_file(struct patch *patch)
@@ -1281,28 +1315,8 @@ static void create_file(struct patch *patch)
 
 	if (!mode)
 		mode = S_IFREG | 0644;
-	if (S_ISREG(mode)) {
-		int fd;
-		mode = (mode & 0100) ? 0777 : 0666;
-		fd = create_regular_file(path, mode);
-		if (fd < 0)
-			die("unable to create file %s (%s)", path, strerror(errno));
-		if (write(fd, buf, size) != size)
-			die("unable to write file %s", path);
-		close(fd);
-		add_index_file(path, mode, buf, size);
-		return;
-	}
-	if (S_ISLNK(mode)) {
-		if (size && buf[size-1] == '\n')
-			size--;
-		buf[size] = 0;
-		if (create_symlink(buf, path) < 0)
-			die("unable to write symlink %s", path);
-		add_index_file(path, mode, buf, size);
-		return;
-	}
-	die("unable to write file mode %o", mode);
+	create_one_file(path, mode, buf, size);	
+	add_index_file(path, mode, buf, size);
 }
 
 static void write_out_one_result(struct patch *patch)