From: Phillip Wood <phillip.wood123@gmail.com>
Date: Thu, 17 Nov 2022 10:26:07 GMT
Subject: Re: [PATCH] range-diff: support reading mbox files
Message-ID: <50d8767b-cb8c-74c5-b280-43472fa074a2@dunelm.org.uk>
In-Reply-To: <dfe0190c-1d2e-804a-5312-877b7b2f5822@dunelm.org.uk>

Hi Dscho

Having slept on it I think I misunderstood what was happening in the 
diff parsing yesterday

On 16/11/2022 14:40, Phillip Wood wrote:
>> +        } else if (state == MBOX_IN_DIFF) {
>> +            switch (line[0]) {
>> +            case '\0':
>> +                continue; /* ignore empty lines after diff */
>> +            case '+':
>> +            case '-':
>> +            case ' ':
>> +                if (!old_count && !new_count)
>> +                    break;
> 
> This shouldn't happen in a well formed diff. Below we happily accept bad 
> counts, is there a reason to reject them here?

I think this might be picking up the "--" at the end of the patch as we 
don't want to break here at the end of a hunk. If so then a comment 
would be helpful.

>> +                if (old_count && line[0] != '+')
>> +                    old_count--;
>> +                if (new_count && line[0] != '-')
>> +                    new_count--;
> 
> The diff is malformed if old_count == 0 and we see '-' or ' ' or 
> new_count == 0 and we see '+' or ' '. The code is careful not to 
> decrement the count in that case so I think it is harmless to accept 
> diffs with bad line counts in the hunk header.
>> +                /* fallthrough */
>> +            case '\\':
>> +                strbuf_addstr(&buf, line);
>> +                strbuf_addch(&buf, '\n');
>> +                util->diffsize++;
> 
> I think this might be a better place to break if old_count and new_count 
> are both zero.

It would be the right place to break at the end of each hunk, but I 
don't think we want to do that.

>> +                continue;
>> +            case '@':
>> +                if (parse_hunk_header(line, &old_count,
>> +                              &new_count, &p))
>> +                    break;
>> +
>> +                strbuf_addstr(&buf, "@@");
>> +                if (current_filename && *p)
>> +                    strbuf_addf(&buf, " %s:",
>> +                            current_filename);
>> +                strbuf_addstr(&buf, p);
>> +                strbuf_addch(&buf, '\n');
>> +                util->diffsize++;
>> +                continue;
>> +            }
> 
> This is effectively the `default:` clause as it is executed when we 
> don't handle the line above. We ignore the contents of this line which 
> makes me wonder what happens if it is the start of another diff.

We'll pick that up earlier with "if (starts_with(line, "diff --git"))"

We only get here at the end of a patch (assuming it has the "--" line 
from format-patch)

Best Wishes

Phillip

> Do we 
> have tests that alter more than one file in a single commit?
> 
> I think this is a useful addition, it could perhaps benefit from more 
> tests though. Having tests for bad input, "\r\n" line endings and 
> getting the author from a From: header as well as an in-body From: line 
> would give a bit more reassurance about how robust the parser is.
> 
> Best Wishes
> 
> Phillip