patchrevision.c: Remove unneeded check for NULL

The function is called only from one place, which makes sure to have `interesting_cache` not NULL. Additionally it is a dereferenced a few lines before unconditionally, which would result in a segmentation fault.

Signed-off-by: Stefan Beller <sbeller@google.com>
---

Notes:
    > So I think the right solution is just to drop the conditional entirely.
    > The current code is not wrong (it is always a noop). What you have here
    > actually misbehaves; it does not update the cache slot when it has
    > become UNINTERESTING. That does not produce wrong results, but it loses
    > the benefit of the cache in some cases.
    
    After reading the code a bit more, I agree.
    
    > I'm having trouble parsing this sentence.  Do you mean that limit_list()
    > only calls still_interesting() (and thus, indirectly,
    > everybody_uninteresting()), with the second parameter equal to the
    > address of the local interesting_cache variable, so it can never be
    > NULL?
    
    I completely reworded the commit message.
    
    > Should there be
    >
    >        if (!interesting_cache)
    >                die("BUG: &interesting_cache == NULL");
    >
    > checks at the top of still_interesting and everybody_uninteresting to
    > futureproof this?
    
    I don't think this is necessary as these functions are local functions
    so when somebody wants to use them they will be aware of the limitations.
    
    > This code seems to be underdocumented.
    
    I am not a expert in this area of the code, so I hoped Peff
    would document it if he feels like so.

 revision.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

On Fri, Jun 26, 2015 at 12:40:19PM -0700, Stefan Beller wrote:

Yeah, I think this is the right thing to do.

Acked-by: Jeff King <peff@peff.net>

Agreed. We do not assert non-NULL for every parameter in our functions, and I do not think this is any more special than the others.

I kind of thought that the explanation in b6e8a3b covered this code. Does it not, or did people not read it?

The whole of revision.c is not well documented by comments, but I don't think that is a new thing.

-Peff

Jeff King wrote:

I know that "tl;dr" is the last thing anyone who has written a clear description of something wants to read, but I fear it applies here. I tried to skim that commit message to get a gist of what the still_interesting variable is supposed to hold and I failed.

I think part of the problem was that that commit message doesn't give a specific example early on to motivate the problem and fix[*].

More to the point, someone interested in that specific variable doesn't need to necessarily understand the optimization that motivated it. Instead, they'd want to know what invariants to expect and preserve: what value does it start with, what does its value mean, are there some forbidden values, etc.

Is the idea that it represents a commit from the queue which is still interesting, and that it saves us from looping through the queue to find a still-interesting commit as long as mark_parents_uninteresting has not marked this one uninteresting yet? What does it mean when it is NULL?

Thanks, Jonathan

[*] I.e., what command do I run to get quadratic behavior?

The message starts with a diagnosis --- "When we are limiting a rev-list traversal due to UNINTERESTING refs, we have to walk down the tips" --- without introducing what problem is being diganosed.

The problem being solved might have been something like "When we call 'git rev-list $commits --not --all' in check_everything_connected after a fetch, if we fetched something much older than most of our refs, and if we have a large number of refs, the operation is slow --- quadratic in the number of refs. This hasn't been a problem in the past because people did not use so many refs, but now as the number of refs in a typical repository grows, it is becoming more noticeable." Even after re-reading the message more carefully, I'm not sure. I assume there was a report motivating the change, which might have been useful for putting the explanation in context for the reader. Alas, git://repo.or.cz/git/trast.git branch notes/gmane doesn't have any annotations for that commit to find the context.

The commit message then goes on to explain how the patch solves that problem, but without an example to put that explanation in context, it is hard to follow. What linear search is the explanation talking about? What is the interesting commit we find? I couldn't tell without looking at the code.