Re: Bug in "git rev-parse --verify"

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 30, 2013, 04:09 UTC
Message-ID: <7vk3opr0ke.fsf@alter.siamese.dyndns.org>
In-Reply-To: <51565F96.1020203@alum.mit.edu>

Michael Haggerty <mhagger@alum.mit.edu> writes:

Show 10 quoted lines

> 1. An SHA1 is a canonical representation of the argument, useful for
> example as the key in a hash map for for looking for the presence of a
> commit in a rev-list output.
>
> 2. An SHA1 is persistent.  For example, I use them when caching
> benchmark results across versions.  Moreover, they are safe for use in
> filenames.  The persistence also makes scripts more robust against
> simultaneous changes to the repo by other processes, whereas if I use a
> string like "branch^" multiple times, there is no guarantee that it
> always refers to the same commit.

These two are half-irrelevant; they only call for use of the current "rev-parse --verify" that always gives you 40-hex. The more important one is the next one.

> 3. Verifying arguments at one spot centralizes error-checking at the
> start of a script and eliminates one reason for random git commands to
> fail later (when error recovery is perhaps more difficult).

Not necessarily, unless your script is a very narrow corner case that is satisfied with "any kind of object goes". When a parameter has to be commit for some purpose and another parameter can be any tree-ish, you would want to validate them _with_ type requirement.

If you are using the "object name persistency" to create a cache of "how many times the word 'gogopuff' appears in the entire tree?", you want to cache the result keyed with the object name of the tree, whether your user gives you v1.8.0 (tag), v1.8.0^0 (commit), or v1.8.0^{tree} (tree), and you want to unwrap the user input down to tree object name to look up the pre-computed result for the cache to be any useful.

> 4. Converting once avoids the overhead of repeated conversion from a
> free-form committish into an object name if the argument needs to be
> passed to multiple git commands (though presumably the overhead is
> insignificant in most cases).

True.

So why not verify arguments while making sure of their type early with 'rev-parse --verify "$userinput^{desiredtype}"?