Re: [PATCH] commit-graph: ignore duplicates when merging layers

[Derrick Stolee <stolee@gmail.com>]

On 10/8/2020 10:15 AM, Taylor Blau wrote:
> On Thu, Oct 08, 2020 at 01:56:52PM +0000, Derrick Stolee via GitGitGadget wrote:
>> Thus, this die() is too aggignoring the duplicates.
> 
> s/aggignoring/aggressively ignoring ?
> 
>>
>> This leads to some additional complication that we did no have before:
> 
> s/no/not, but I am more wondering about what "This" is. I think what
> you're saying is: "Suppose we didn't die on duplicates, what would
> happen? Well, there'd be some additional problems, but here's a way that
> we can fix them (storing the de-duplicated OIDs separately)".

Thanks. The message will be edited to fix these brain farts.

>>     I still don't have a grasp on how this happened in the first place, but
>>     will keep looking.
> 
> I'm looking as well, but I haven't found any smoking guns yet. I could
> imagine that this is a problem that existed before 0bd52e27e3
> (commit-graph.h: store an odb in 'struct write_commit_graph_context',
> 2020-02-03), and simply couldn't be tickled because of how brittle
> comparing ODB paths is. I could equally imagine that 0bd52e27e3 did
> introduce this problem.

Thanks.

>> +	ALLOC_ARRAY(deduped_commits.list, deduped_commits.alloc);
> 
> I'm not sure that this deduped_commits list is actually necessary.
> 
> It would be nice for this caller if ctx->commits were a linked list
> since it would make deleting duplicates easy, but I think that it would
> be a burden for other callers. So, that's a dead end.
> 
> But what about marking the duplicate positions by NULL-ing them out, and
> then taking another pass over the list to (1) compact it (i.e., push
> everything down so that all of the NULLs occur at the end), and then (2)
> truncate the length to the number of unique commits.
> 
> I could imagine that something like that is a little trickier, but it
> seems worth it to avoid doubling the memory cost of this function.

You are correct that we can just re-use the commits.list by "collapsing"
the list on top of duplicate entries. I'll send a new version that does
exactly that.

Thanks,
-Stolee