Re: [PATCH 1/3] graph: fix case that hit assert()

> On Tue, Jan 07, 2020 at 02:55:45PM +0000, Derrick Stolee via GitGitGadget wrote:
> 
>> From: Derrick Stolee <dstolee@microsoft.com>
>>
>> A failure was reported in "git log --graph --all" with the new
>> graph-rendering logic. Create a test case that matches the
>> topology of that example and uses an explicit ref ordering instead
>> of the "--all" option. The test would fail with the following error:
>>
>> 	graph.c:1228: graph_output_collapsing_line: Assertion
>> 		      `graph->mapping[i - 3] == target' failed.
>>
>> The situation is a little complicated, so let's break it down.
> 
> First off, thanks for digging into this so promptly. Your solution looks
> correct to me. Everything else I'll mention here are nits. :)
> 
> Your commit message starts off talking about the test, but without
> describing what's interesting about it. I think the answer is that we
> have two "skewed" merge parents for the same merge; maybe it would make
> sense to lead with that. I.e.:
> 
>   Subject: graph: drop assert() for merge with two collapsing parents
> 
>   When "git log --graph" shows a merge commit that has two collapsing
>   lines, like:
> 
>     [your diagram]
> 
>   we trigger an assert():
> 
>     graph.c:1228: graph_output_collapsing_line: Assertion
>                   `graph->mapping[i - 3] == target' failed.
> 
>   ...and so on...

>> The assert was introduced by eaf158f8 ("graph API: Use horizontal
>> lines for more compact graphs", 2009-04-21), which is quite old.
>> This assert is trying to say that when we complete a horizontal
>> line with a single slash, it is because we have reached our target.
> 
> Thanks for this final sentence; writing that out in English made the
> purpose of the assert() much clearer.
> 
> That could perhaps be an argument in favor of writing it as a BUG()
> with a similar human-readable explanation. I guess there was already a
> comment in the code, but it didn't quite click with me as much as what
> you wrote above.
> 
>> It is actually the _second_ collapsing line that hits this assert.
>> The reason we are in this code path is because we are collapsing
>> the first line, and it in that case we are hitting our target now
> 
> s/it//
> 
>> that the horizontal line is complete. However, the second line
>> cannot be a horizontal line, so it will collapse without horizontal
>> lines. In this case, it is inappropriate to assert that we have
>> reached our target, as we need to continue for another column
>> before reaching the target. Dropping the assert is safe here.
> 
> I think that makes sense. My big concern here is that the assert() was
> preventing us from looking out of bounds in the graph->mapping array,
> but I don't think that's the case here.
> 
> Worth mentioning that this was due to 0f0f389f12 (graph: tidy up display
> of left-skewed merges, 2019-10-15), in case somebody has to later dig
> deeper?

>> Second, the horizontal lines in that first line drop their coloring.
>> This is due to a use of graph_line_addch() instead of
>> graph_line_write_column(). Using a ternary operator to pick the
>> character is nice for compact code, but we actually need a column
>> to provide the color.
> 
> It seems like this is a totally separate bug, and could be its own
> commit?
>
> I think it's also caused by 0f0f389f12, which actually introduced the
> seen_parent mechanism that you're correcting here.

>> Helped-by: Jeff King <peff@peff.net>
>> Reported-by: Bradley Smith <brad@brad-smith.co.uk>
> 
> I don't know that I did much, but OK. :)
> 
> Thanks once again Bradley for the reproducible case.
> 
>> diff --git a/t/t4215-log-skewed-merges.sh b/t/t4215-log-skewed-merges.sh
>> index 18709a723e..ddf6f6f5d3 100755
>> --- a/t/t4215-log-skewed-merges.sh
>> +++ b/t/t4215-log-skewed-merges.sh
>> @@ -240,4 +240,46 @@ test_expect_success 'log --graph with octopus merge with column joining its penu
>>  	EOF
>>  '
>>  
>> +test_expect_success 'log --graph with multiple tips' '
> 
> This nicely covers the assert() problem. Could we check the same case
> with "--color" and test_decode_color to check the coloring issue (see
> t4214 for some prior art)?