Re: Parsing diff --git lines

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mar 9, 2008, 04:04 UTC
Message-ID: <alpine.LFD.1.00.0803081955110.5896@woody.linux-foundation.org>
In-Reply-To: <F4D3B820-CCF4-4212-BF32-700EFD448143@mac.com>

On Sat, 8 Mar 2008, Simon Fraser wrote:

> 
> However, I don't see a reliable way to identify the two files
> from a "diff --git" line. Here's a (deliberately pathological)
> example:

See how "git-apply" does it.

The rule is:

 - if the filenames are different, you should ignore the filenames on the 
   "diff --git" line, and trust the ones on the "renamed from/to" ones 
   (which are unambiguous because they only have one filename per line)

 - if the filenames aren't different, then you can unambiguously know how 
   to parse it by simply making sure they are the same.

So to take your example:

> diff --git a/a / b/file with spaces.txt b/a / b/file with spaces.txt

Here, you can *know* that the filename is "a / b/file with spaces.txt", because it must match the pattern "a/$filename b/$filename", and no other split at a space would ever do that!

See in particular "git_header_name()" in builtin-apply.c. See the comments both at the top of the function and there in the middle to reflect the above rule:

/*
 * This is to extract the same name that appears on "diff --git"
 * line.  We do not find and return anything if it is a rename
 * patch, and it is OK because we will find the name elsewhere.
 * We need to reliably find name only when it is mode-change only,
 * creation or deletion of an empty file.  In any of these cases,
 * both sides are the same name under a/ and b/ respectively.
 */
...
        /*
         * Accept a name only if it shows up twice, exactly the same
         * form.
         */

> What would make this possible would be either to always quote
> file paths containing spaces, or use a character other than
> a space (e.g. a \t) between the two file names.

Well, git didn't originally ever quote filenames at all, and I actually wanted the "diff --git" line to look as much like a regular "diff -urN" line as possible (which has spaces between the names)

These days, we could quote, but hey, anybody who parses diff lines needs to be able to handle the non-quoted form *anyway*, so quoting doesn't really help anybody in the end.

			Linus