config: implement basic transactional support

When a configuration file is locked, any updates made to it will be done
to the in-memory copy of the file. This allows for multiple updates to
happen while we hold the lock, preventing races during complex
config-file manipulation.

include/git2/sys/config.h

@@ -67,6 +67,20 @@ struct git_config_backend {
 	int (*iterator)(git_config_iterator **, struct git_config_backend *);
 	/** Produce a read-only version of this backend */
 	int (*snapshot)(struct git_config_backend **, struct git_config_backend *);
+	/**
+	 * Lock this backend.
+	 *
+	 * Prevent any writes to the data store backing this
+	 * backend. Any updates must not be visible to any other
+	 * readers.
+	 */
+	int (*lock)(struct git_config_backend *);
+	/**
+	 * Unlock the data store backing this backend. If success is
+	 * true, the changes should be committed, otherwise rolled
+	 * back.
+	 */
+	int (*unlock)(struct git_config_backend *, int success);
 	void (*free)(struct git_config_backend *);
 };
 #define GIT_CONFIG_BACKEND_VERSION 1

src/config_file.c

@@ -105,6 +105,10 @@ typedef struct {
 
 	git_array_t(struct reader) readers;
 
+	bool locked;
+	git_filebuf locked_buf;
+	git_buf locked_content;
+
 	char  *file_path;
 } diskfile_backend;
 
@@ -685,6 +689,42 @@ static int config_snapshot(git_config_backend **out, git_config_backend *in)
 	return git_config_file__snapshot(out, b);
 }
 
+static int config_lock(git_config_backend *_cfg)
+{
+	diskfile_backend *cfg = (diskfile_backend *) _cfg;
+	int error;
+
+	if ((error = git_filebuf_open(&cfg->locked_buf, cfg->file_path, 0, GIT_CONFIG_FILE_MODE)) < 0)
+		return error;
+
+	error = git_futils_readbuffer(&cfg->locked_content, cfg->file_path);
+	if (error < 0 && error != GIT_ENOTFOUND) {
+		git_filebuf_cleanup(&cfg->locked_buf);
+		return error;
+	}
+
+	cfg->locked = true;
+	return 0;
+
+}
+
+static int config_unlock(git_config_backend *_cfg, int success)
+{
+	diskfile_backend *cfg = (diskfile_backend *) _cfg;
+	int error = 0;
+
+	if (success) {
+		git_filebuf_write(&cfg->locked_buf, cfg->locked_content.ptr, cfg->locked_content.size);
+		error = git_filebuf_commit(&cfg->locked_buf);
+	}
+
+	git_filebuf_cleanup(&cfg->locked_buf);
+	git_buf_free(&cfg->locked_content);
+	cfg->locked = false;
+
+	return error;
+}
+
 int git_config_file__ondisk(git_config_backend **out, const char *path)
 {
 	diskfile_backend *backend;
@@ -706,6 +746,8 @@ int git_config_file__ondisk(git_config_backend **out, const char *path)
 	backend->header.parent.del_multivar = config_delete_multivar;
 	backend->header.parent.iterator = config_iterator_new;
 	backend->header.parent.snapshot = config_snapshot;
+	backend->header.parent.lock = config_lock;
+	backend->header.parent.unlock = config_unlock;
 	backend->header.parent.free = backend_free;
 
 	*out = (git_config_backend *)backend;
@@ -750,6 +792,21 @@ static int config_delete_readonly(git_config_backend *cfg, const char *name)
 	return config_error_readonly();
 }
 
+static int config_lock_readonly(git_config_backend *_cfg)
+{
+	GIT_UNUSED(_cfg);
+
+	return config_error_readonly();
+}
+
+static int config_unlock_readonly(git_config_backend *_cfg, int success)
+{
+	GIT_UNUSED(_cfg);
+	GIT_UNUSED(success);
+
+	return config_error_readonly();
+}
+
 static void backend_readonly_free(git_config_backend *_backend)
 {
 	diskfile_backend *backend = (diskfile_backend *)_backend;
@@ -803,6 +860,8 @@ int git_config_file__snapshot(git_config_backend **out, diskfile_backend *in)
 	backend->header.parent.del = config_delete_readonly;
 	backend->header.parent.del_multivar = config_delete_multivar_readonly;
 	backend->header.parent.iterator = config_iterator_new;
+	backend->header.parent.lock = config_lock_readonly;
+	backend->header.parent.unlock = config_unlock_readonly;
 	backend->header.parent.free = backend_readonly_free;
 
 	*out = (git_config_backend *)backend;
@@ -1801,6 +1860,9 @@ static int config_write(diskfile_backend *cfg, const char *key, const regex_t *p
 	struct reader *reader = git_array_get(cfg->readers, 0);
 	struct write_data write_data;
 
+	if (cfg->locked) {
+		result = git_buf_puts(&reader->buffer, git_buf_cstr(&cfg->locked_content));
+	} else {
 		/* Lock the file */
 		if ((result = git_filebuf_open(
 			     &file, cfg->file_path, 0, GIT_CONFIG_FILE_MODE)) < 0) {
@@ -1810,6 +1872,7 @@ static int config_write(diskfile_backend *cfg, const char *key, const regex_t *p
 
 		/* We need to read in our own config file */
 		result = git_futils_readbuffer(&reader->buffer, cfg->file_path);
+	}
 
 	/* Initialise the reading position */
 	if (result == GIT_ENOTFOUND) {
@@ -1840,20 +1903,26 @@ static int config_write(diskfile_backend *cfg, const char *key, const regex_t *p
 	git__free(section);
 
 	if (result < 0) {
-		git_buf_free(&buf);
 		git_filebuf_cleanup(&file);
 		goto done;
 	}
 
+	if (cfg->locked) {
+		size_t len = buf.asize;
+		/* Update our copy with the modified contents */
+		git_buf_free(&cfg->locked_content);
+		git_buf_attach(&cfg->locked_content, git_buf_detach(&buf), len);
+	} else {
 		git_filebuf_write(&file, git_buf_cstr(&buf), git_buf_len(&buf));
 
 		/* refresh stats - if this errors, then commit will error too */
 		(void)git_filebuf_stats(&reader->file_mtime, &reader->file_size, &file);
 
 		result = git_filebuf_commit(&file);
-	git_buf_free(&reader->buffer);
+	}
 
 done:
+	git_buf_free(&buf);
 	git_buf_free(&reader->buffer);
 	return result;
 }

src/config_file.h

@@ -55,6 +55,16 @@ GIT_INLINE(int) git_config_file_foreach_match(
 	return git_config_backend_foreach_match(cfg, regexp, fn, data);
 }
 
+GIT_INLINE(int) git_config_file_lock(git_config_backend *cfg)
+{
+	return cfg->lock(cfg);
+}
+
+GIT_INLINE(int) git_config_file_unlock(git_config_backend *cfg, int success)
+{
+	return cfg->unlock(cfg, success);
+}
+
 extern int git_config_file_normalize_section(char *start, char *end);
 
 #endif

tests/config/write.c

@@ -1,6 +1,9 @@
 #include "clar_libgit2.h"
 #include "buffer.h"
 #include "fileops.h"
+#include "git2/sys/config.h"
+#include "config_file.h"
+#include "config.h"
 
 void test_config_write__initialize(void)
 {
@@ -630,3 +633,46 @@ void test_config_write__to_file_with_only_comment(void)
 	git_buf_free(&result);
 }
 
+void test_config_write__locking(void)
+{
+	git_config_backend *cfg, *cfg2;
+	git_config_entry *entry;
+	const char *filename = "locked-file";
+
+	/* Open the config and lock it */
+	cl_git_mkfile(filename, "[section]\n\tname = value\n");
+	cl_git_pass(git_config_file__ondisk(&cfg, filename));
+	cl_git_pass(git_config_file_open(cfg, GIT_CONFIG_LEVEL_APP));
+	cl_git_pass(git_config_file_get_string(&entry, cfg, "section.name"));
+	cl_assert_equal_s("value", entry->value);
+	git_config_entry_free(entry);
+	cl_git_pass(git_config_file_lock(cfg));
+
+	/* Change entries in the locked backend */
+	cl_git_pass(git_config_file_set_string(cfg, "section.name", "other value"));
+	cl_git_pass(git_config_file_set_string(cfg, "section2.name3", "more value"));
+
+	/* We can see that the file we read from hasn't changed */
+	cl_git_pass(git_config_file__ondisk(&cfg2, filename));
+	cl_git_pass(git_config_file_open(cfg2, GIT_CONFIG_LEVEL_APP));
+	cl_git_pass(git_config_file_get_string(&entry, cfg2, "section.name"));
+	cl_assert_equal_s("value", entry->value);
+	git_config_entry_free(entry);
+	cl_git_fail_with(GIT_ENOTFOUND, git_config_file_get_string(&entry, cfg2, "section2.name3"));
+	git_config_file_free(cfg2);
+
+	git_config_file_unlock(cfg, true);
+	git_config_file_free(cfg);
+
+	/* Now that we've unlocked it, we should see both updates */
+	cl_git_pass(git_config_file__ondisk(&cfg, filename));
+	cl_git_pass(git_config_file_open(cfg, GIT_CONFIG_LEVEL_APP));
+	cl_git_pass(git_config_file_get_string(&entry, cfg, "section.name"));
+	cl_assert_equal_s("other value", entry->value);
+	git_config_entry_free(entry);
+	cl_git_pass(git_config_file_get_string(&entry, cfg, "section2.name3"));
+	cl_assert_equal_s("more value", entry->value);
+	git_config_entry_free(entry);
+
+	git_config_file_free(cfg);
+}