Re: [PATCH v2] diff-lib.c: adjust position of i-t-a entries in diff
From: Junio C Hamano <hidden>
Date: 2016-06-15 23:04:11
Duy Nguyen [off-list ref] writes:
On Mon, Mar 16, 2015 at 09:05:45AM -0700, Junio C Hamano wrote:quoted
The offending one came from eec3e7e4 (cache-tree: invalidate i-t-a paths after generating trees, 2012-12-16), which was a fix to an earlier bug where a cache-tree written out of an index with i-t-a entries had incorrect information and still claimed it is fully valid after write-tree rebuilt it. The test probably should add another path without i-t-a bit, run the same "diff --cached" with updated expectation before write-tre, and run the "diff --cached" again to make sure it produces a result that match the updated expectation.Would adding another non-i-t-a entry help? Before this patch "diff --cached" after write-tree shows the i-t-a entry only when eec3e7e4 is applied. But with this patch we don't show i-t-a entry any more, before or after write-tree, eec3e7e4 makes no visible difference. We could even revert eec3e7e4 and the outcome of "diff --cached" would be the same because we just sort of move the "invalidation" part from cache-tree to do_oneway_diff(). Not invalidating would speed up "diff --cached" when i-t-a entries are present. Still it may be a good idea to invalidate i-t-a paths to be on the safe side. Perhaps a patch like this to resurrect the test?
My unerstanding of what eec3e7e4 (cache-tree: invalidate i-t-a paths
after generating trees, 2012-12-16) fixed was that in this sequence:
- You prepare an index.
- You write-tree out of the index, which involves:
- updating the cache-tree to match the shape of the resulting
from writing the index out.
- create tree objects matching all levels of the cache-tree as
needed on disk.
- report the top-level tree object name
- run "diff-index --cached", which can and will take advantage of
the fact that everything in a subtree below a known-to-be-valid
cache-tree entry does not have to be checked one-by-one. If a
cache-tree says "everything under D/ in the index would hash to
tree object T" and the HEAD has tree object T at D/, then the
diff machinery will bypass the entire section in the index
under D/, which is a valid optimization.
However, when there is an i-t-a entry, we excluded that entry
from the tree object computation, its presence did not
contribute to the tree object name, but still marked the
cache-tree entries that contain it as valid by mistake. This
old bug was what the commit fixed, so an invocation of "diff
--cached" after a write-tree, even if the index contains an
i-t-a entry, will not see cache-tree entries that are marked
valid when they are not. Instead, "diff --cached" will bypass
the optimization and makes comparison one-by-one for the index
entries.
So reverting the fix obviously is not the right thing to do. If the
tests show different results from two invocations of "diff --cached"
with your patch applied, there is something that is broken by your
patch, because the index and the HEAD does not change across
write-tree in that test.
If on the other hand the tests show the same result from these two
"diff --cached" and the result is different from what the test
expects, that means your patch changed the world order, i.e. an
i-t-a entry used to be treated as if it were adding an empty blob to
the index but it is now treated as non-existent, then that is a good
thing and the only thing we need to update is what the test expects.
I am guessing that instead of expecting dir/bar to be shown, it now
should expect no output?
Does adding an non-i-t-a entry help? It does not hurt, and it makes
the test uses a non-empty output, making its effect more visible,
which may or may not count as helping.