config_file: avoid re-reading files on write

When we rewrite the configuration file due to any of its values
being modified, we call `config_refresh` to update the in-memory
representation of our config file backend. This is needlessly
wasteful though, as `config_refresh` will always open the on-disk
representation to reads the file contents while we already know
the complete file contents at this point in time as we have just
written it to disk.

Implement a new function `config_refresh_from_buffer` that will
refresh the backend's config entries from a buffer instead of
from the config file itself. Note that this will thus _not_
update the backend's timestamp, which will cause us to re-read
the buffer when performing a read operation on it. But this is
still an improvement as we now lazily re-read the contents, and
most importantly we will avoid constantly re-reading the contents
if we perform multiple write operations.

The following strace demonstrates this if we're re-writing a key
multiple times. It uses our config example with `config_set`
changed to update the file 10 times with different keys:

	$ strace lg2 config x.x z |& grep '^open.*config'
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/home/pks/.config/git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3

And now with the optimization of `config_refresh_from_buffer`:

	$ strace lg2 config x.x z |& grep '^open.*config'
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/home/pks/.config/git/config", O_RDONLY|O_CLOEXEC) = 3
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4
	open("/tmp/repo/.git/config.lock", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0666) = 3
	open("/tmp/repo/.git/config", O_RDONLY|O_CLOEXEC) = 4

As can be seen, this is quite a lot of `open` calls less.

1 files changed, 20 insertions, 2 deletions

diff --git a/src/config_file.c b/src/config_file.c
index aee17448d..38653274f 100644
--- a/src/config_file.c
+++ b/src/config_file.c
@@ -200,9 +200,27 @@ out:
 	return error;
 }
 
+static int config_refresh_from_buffer(git_config_backend *cfg, const char *buf, size_t buflen)
+{
+	diskfile_backend *b = GIT_CONTAINER_OF(cfg, diskfile_backend, header.parent);
+	git_config_entries *entries = NULL;
+	int error;
+
+	if ((error = git_config_entries_new(&entries)) < 0 ||
+	    (error = config_read_buffer(entries, b->header.repo, &b->file,
+					b->header.level, 0, buf, buflen)) < 0 ||
+	    (error = config_set_entries(cfg, entries)) < 0)
+		goto out;
+
+	entries = NULL;
+out:
+	git_config_entries_free(entries);
+	return error;
+}
+
 static int config_refresh(git_config_backend *cfg)
 {
-	diskfile_backend *b = (diskfile_backend *)cfg;
+	diskfile_backend *b = GIT_CONTAINER_OF(cfg, diskfile_backend, header.parent);
 	git_config_entries *entries = NULL;
 	int error, modified;
 
@@ -1230,7 +1248,7 @@ static int config_write(diskfile_backend *cfg, const char *orig_key, const char
 		if ((result = git_filebuf_commit(&file)) < 0)
 			goto done;
 
-		if ((result = config_refresh(&cfg->header.parent)) < 0)
+		if ((result = config_refresh_from_buffer(&cfg->header.parent, buf.ptr, buf.size)) < 0)
 			goto done;
 	}