Re: [PATCH 2/2] refs: add GIT_REF_URI to specify reference backend and directory

> On 25/11/19 10:48PM, Karthik Nayak wrote:
>> Git allows setting a different object directory via
>> 'GIT_OBJECT_DIRECTORY', but provides no equivalent for references.
>> This asymmetry makes it difficult to test different reference backends
>> or use alternative reference storage locations without modifying the
>> repository structure.
>>
>> Add a new environment variable 'GIT_REF_URI' that specifies both the
>> reference backend and directory path using a URI format:
>>
>>     <ref_backend>://<path>
>
> Ok, we include the reference format as part of the URI here since it is
> possible that the alternative reference store could be using a different
> backend that what the repository is currently configured to use. Makes
> sense.
>
>> When set, this variable is used to obtain the main reference store for
>> all Git commands. The variable is checked in `get_main_ref_store()`
>> when lazily assigning `repo->refs_private`. We cannot initialize this
>> earlier in `repo_set_gitdir()` because the repository's hash algorithm
>> isn't known at that point, and the reftable backend requires this
>> information during initialization.
>>
>> When used with worktrees, the specified directory is treated as the
>> reference directory for all worktree operations.
>>
>> Add a new test file 't1423-ref-backend.sh' to test this environment
>> variable.
>>
>> Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
>> ---
>>  Documentation/git.adoc |   8 ++++
>>  environment.h          |   1 +
>>  refs.c                 |  53 +++++++++++++++++++++++-
>>  t/meson.build          |   1 +
>>  t/t1423-ref-backend.sh | 109 +++++++++++++++++++++++++++++++++++++++++++++++++
>>  5 files changed, 171 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/git.adoc b/Documentation/git.adoc
>> index ce099e78b8..a1d1078f42 100644
>> --- a/Documentation/git.adoc
>> +++ b/Documentation/git.adoc
>> @@ -584,6 +584,14 @@ double-quotes and respecting backslash escapes. E.g., the value
>>  	repositories will be set to this value. The default is "files".
>>  	See `--ref-format` in linkgit:git-init[1].
>>
>> +`GIT_REF_URI`::
>> +    Specify which reference backend and path to be used, if not specified the
>> +    backend is inferred from the configuration and $GIT_DIR is used as the
>> +    path.
>> ++
>> +Expects the format '<ref_backend>://<path>', where the 'backend' specifies the
>> +reference backend and the 'path' specifies the directory used by the backend.
>
> I think some users may assume that the path to the reference backend
> would be something like ".git/refs" similar to how
> `GIT_OBJECT_DIRECTORY` is usually ".git/objects". It might be worth
> clarifying this in the docs here.
>

>> +
>>  Git Commits
>>  ~~~~~~~~~~~
>>  `GIT_AUTHOR_NAME`::
>> diff --git a/environment.h b/environment.h
>> index 51898c99cd..9bc380bba4 100644
>> --- a/environment.h
>> +++ b/environment.h
>> @@ -42,6 +42,7 @@
>>  #define GIT_OPTIONAL_LOCKS_ENVIRONMENT "GIT_OPTIONAL_LOCKS"
>>  #define GIT_TEXT_DOMAIN_DIR_ENVIRONMENT "GIT_TEXTDOMAINDIR"
>>  #define GIT_ATTR_SOURCE_ENVIRONMENT "GIT_ATTR_SOURCE"
>> +#define GIT_REF_URI_ENVIRONMENT "GIT_REF_URI"
>>
>>  /*
>>   * Environment variable used to propagate the --no-advice global option to the
>> diff --git a/refs.c b/refs.c
>> index 23f46867f2..0922f08c9f 100644
>> --- a/refs.c
>> +++ b/refs.c
>> @@ -2186,15 +2186,66 @@ static struct ref_store *get_ref_store_for_dir(struct repository *r,
>>  	return maybe_debug_wrap_ref_store(dir, ref_store);
>>  }
>>
>> +static struct ref_store *get_ref_store_from_uri(struct repository *repo,
>> +						const char *uri)
>> +{
>> +	struct string_list ref_backend_info = STRING_LIST_INIT_DUP;
>> +	enum ref_storage_format format;
>> +	struct ref_store *store = NULL;
>> +	char *format_string;
>> +	char *dir;
>> +
>> +	if (!uri || !uri[0]) {
>> +		error("reference backend uri is empty");
>> +		goto cleanup;
>> +	}
>> +
>> +	if (string_list_split(&ref_backend_info, uri, ":", 2) != 2) {
>> +		error("invalid reference backend uri format '%s'", uri);
>> +		goto cleanup;
>> +	}
>> +
>> +	format_string = ref_backend_info.items[0].string;
>> +	dir = ref_backend_info.items[1].string + 2;
>> +
>> +	if (!dir || !dir[0]) {
>> +		error("invalid path in uri '%s'", uri);
>> +		goto cleanup;
>> +	}
>> +
>> +	format = ref_storage_format_by_name(format_string);
>> +	if (format == REF_STORAGE_FORMAT_UNKNOWN) {
>> +		error("unknown reference backend '%s'", format_string);
>> +		goto cleanup;
>> +	}
>> +
>> +	store = get_ref_store_for_dir(repo, dir, format);
>
> Since we don't update the reference format stored in repo, if we were to
> run:
>
>   $ GIT_REF_URI="reftable://<path> git repo info references.format
>
> it would still report what ever the repository was originally configured
> with. Since only a single reference backend can be used at time, I
> wonder if we should go a bit further and update `r->ref_storage_format`
> to be inline with how the repository reference backend is configured via
> `GIT_REF_URI`.
>