config_file: avoid re-reading files on write (2ba7020f) · Commits · lvzhengyang / git2

Commit 2ba7020f authored Jun 27, 2019 by Patrick Steinhardt

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:

And now with the optimization of `config_refresh_from_buffer`:

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

parent a0dc3027

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View file @ 2ba7020f

@@ -200,9 +200,27 @@ out:
@@ -200,9 +200,27 @@ out:

	return error;

	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)

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;

	git_config_entries *entries = NULL;

	int error, modified;

	int error, modified;

@@ -1230,7 +1248,7 @@ static int config_write(diskfile_backend *cfg, const char *orig_key, const char 
@@ -1230,7 +1248,7 @@ static int config_write(diskfile_backend *cfg, const char *orig_key, const char 

		if ((result = git_filebuf_commit(&file)) < 0)

		if ((result = git_filebuf_commit(&file)) < 0)

			goto done;

			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;

			goto done;

	}

	}

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