Add support for atomic file locking

The struct 'git_filelock' represents an atomically-locked
file, git-style.

Locked files can be modified atomically through the new file lock
interface:

int git_filelock_init(git_filelock *lock, const char *path);
int git_filelock_lock(git_filelock *lock, int append);
void git_filelock_unlock(git_filelock *lock);
int git_filelock_commit(git_filelock *lock);
int git_filelock_write(git_filelock *lock, const char *buffer, size_t length);

Signed-off-by: Vicent Marti <tanoku@gmail.com>

src/filelock.c

+/*
+ * This file is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License, version 2,
+ * as published by the Free Software Foundation.
+ *
+ * In addition to the permissions in the GNU General Public License,
+ * the authors give you unlimited permission to link the compiled
+ * version of this file into combinations with other programs,
+ * and to distribute those combinations without any restriction
+ * coming from the use of this file.  (The General Public License
+ * restrictions do apply in other respects; for example, they cover
+ * modification of the file, and distribution when not linked into
+ * a combined executable.)
+ *
+ * This file is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; see the file COPYING.  If not, write to
+ * the Free Software Foundation, 51 Franklin Street, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#include "common.h"
+#include "filelock.h"
+#include "fileops.h"
+
+static const char *GIT_FILELOCK_EXTENSION = ".lock\0";
+static const size_t GIT_FILELOCK_EXTLENGTH = 6;
+
+#define BUILD_PATH_LOCK(_lock, _path) { \
+	memcpy(_path, _lock->path, _lock->path_length); \
+	memcpy(_path + _lock->path_length, GIT_FILELOCK_EXTENSION,\
+			GIT_FILELOCK_EXTLENGTH);\
+}
+
+int git_filelock_init(git_filelock *lock, const char *path)
+{
+	if (lock == NULL || path == NULL)
+		return GIT_ERROR;
+
+	memset(lock, 0x0, sizeof(git_filelock));
+
+	lock->path_length = strlen(path);
+
+	if (lock->path_length + GIT_FILELOCK_EXTLENGTH >= GIT_PATH_MAX)
+		return GIT_ERROR;
+
+	memcpy(lock->path, path, lock->path_length);
+	return 0;
+}
+
+int git_filelock_lock(git_filelock *lock, int append)
+{
+	char path_lock[GIT_PATH_MAX];
+	BUILD_PATH_LOCK(lock, path_lock);
+
+	/* If file already exists, we cannot create a lock */
+	if (gitfo_exists(path_lock) == 0)
+		return GIT_EOSERR;
+
+	lock->file_lock = gitfo_creat(path_lock, 0666);
+
+	if (lock->file_lock < 0)
+		return GIT_EOSERR;
+
+	lock->is_locked = 1;
+
+	/* TODO: do a flock() in the descriptor file_lock */
+
+	if (append && gitfo_exists(lock->path) == 0) {
+		git_file source;
+		char buffer[2048];
+		size_t read_bytes;
+
+		source = gitfo_open(lock->path, O_RDONLY);
+		if (source < 0)
+			return GIT_EOSERR;
+
+		while ((read_bytes = gitfo_read(source, buffer, 2048)) > 0)
+			gitfo_write(lock->file_lock, buffer, read_bytes);
+
+		gitfo_close(source);
+	}
+
+	return 0;
+}
+
+void git_filelock_unlock(git_filelock *lock)
+{
+	char path_lock[GIT_PATH_MAX];
+	BUILD_PATH_LOCK(lock, path_lock);
+
+	if (lock->is_locked) {
+		/* The flock() in lock->file_lock is removed
+		 * automatically when closing the descriptor */
+		gitfo_close(lock->file_lock);
+		gitfo_unlink(path_lock);
+		lock->is_locked = 0;
+	}
+}
+
+int git_filelock_commit(git_filelock *lock)
+{
+	int error;
+	char path_lock[GIT_PATH_MAX];
+	BUILD_PATH_LOCK(lock, path_lock);
+
+	if (!lock->is_locked || lock->file_lock < 0)
+		return GIT_ERROR;
+
+	/* FIXME: flush the descriptor? */
+	gitfo_close(lock->file_lock);
+
+	error = gitfo_move_file(path_lock, lock->path);
+
+	if (error < 0)
+		gitfo_unlink(path_lock);
+
+	lock->is_locked = 0;
+	return error;
+}
+
+int git_filelock_write(git_filelock *lock, const void *buffer, size_t length)
+{
+	if (!lock->is_locked || lock->file_lock < 0)
+		return GIT_ERROR;
+
+	return gitfo_write(lock->file_lock, (void *)buffer, length);
+}

src/filelock.h

+#ifndef INCLUDE_filelock_h__
+#define INCLUDE_filelock_h__
+
+#include "fileops.h"
+
+struct git_filelock {
+
+	char path[GIT_PATH_MAX];
+	size_t path_length;
+
+	git_file file_lock;
+	int is_locked;
+};
+
+typedef struct git_filelock git_filelock;
+
+int git_filelock_init(git_filelock *lock, const char *path);
+int git_filelock_lock(git_filelock *lock, int append);
+void git_filelock_unlock(git_filelock *lock);
+int git_filelock_commit(git_filelock *lock);
+int git_filelock_write(git_filelock *lock, const void *buffer, size_t length);
+
+#endif