negative-refspec: fix segfault on : refspec

If remote.origin.push was set to ":", git segfaults during a push operation, due to bad parsing logic in query_matches_negative_refspec. Per bisect, the bug was introduced in: c0192df630 (refspec: add support for negative refspecs, 2020-09-30)

We found this issue when rolling out git 2.29 at Dropbox - as several folks had "push = :" in their configuration. I based my diff off the master branch, but also confirmed that it patches cleanly onto maint - if the maintainers would like to also fix the segfault on 2.29

Update since Patch series V1:

* Handled matching refspec explicitly
* Added testing for "+:" case
* Added comment explaining how the two loops work together

Update since Patch series V2

* style suggestion in remote.c
* Use test_config
* Add test for a case with a matching refspec + negative refspec
* Fix test_config to work with --add
* Updated commit message to describe what git is told to do instead of segfaulting

Update since Patch series V3

* Removed commit modifying test_config
* Remove segfault-related comments in test
* Consolidate the three tests to two tests (1st and 3rd test overlapped in functionality)
* Base the patch series on the maint branch - since the bug affects 2.29.2

Update since Patch series V4

* Squashed in Junio's patch to handle non-master named branches
* Explicitly use test_unconfig

Appreciate the reviews from Junio and Eric! Happy Holidays!

Nipunn Koorapati (2):
negative-refspec: fix segfault on : refspec
negative-refspec: improve comment on query_matches_negative_refspec

 remote.c                          | 16 ++++++++--
 t/t5582-fetch-negative-refspec.sh | 51 +++++++++++++++++++++++++++++++
 2 files changed, 64 insertions(+), 3 deletions(-)

base-commit: 898f80736c75878acc02dc55672317fcc0e0a5a6
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-820%2Fnipunn1313%2Fnk%2Fpush-refspec-segfault-v5
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-820/nipunn1313/nk/push-refspec-segfault-v5
Pull-Request: https://github.com/gitgitgadget/git/pull/820

Range-diff vs v4:

1: e59ff29bdef ! 1: 48c79dc3d84 negative-refspec: fix segfault on : refspec
@@ t/t5582-fetch-negative-refspec.sh: test_expect_success "fetch --prune with negat
'

+test_expect_success "push with matching : and negative refspec" '
-+ test_config -C two remote.one.push : &&
-+ # Fails to push master w/ tip behind counterpart
++ # Manually handle cleanup, since test_config is not
++ # prepared to take arbitrary options like --add
++ test_when_finished "test_unconfig -C two remote.one.push" &&
++
++ # For convenience, we use "master" to refer to the name of
++ # the branch created by default in the following.
++ #
++ # Repositories two and one have branches other than "master"
++ # but they have no overlap---"master" is the only one that
++ # is shared between them. And the master branch at two is
++ # behind the master branch at one by one commit.
++ git -C two config --add remote.one.push : &&
++
++ # A matching push tries to update master, fails due to non-ff
+ test_must_fail git -C two push one &&
+
++ # "master" may actually not be "master"---find it out.
++ current=$(git symbolic-ref HEAD) &&
++
+ # If master is in negative refspec, then the command will not attempt
+ # to push and succeed.
-+ # We do not need test_config here as we are updating remote.one.push
-+ # again. The teardown of the first test_config will do --unset-all
-+ git -C two config --add remote.one.push ^refs/heads/master &&
-+ git -C two push one
++ git -C two config --add remote.one.push "^$current" &&
++
++ # With "master" excluded, this push is a no-op. Nothing gets
++ # pushed and it succeeds.
++ git -C two push -v one
+'
+
+test_expect_success "push with matching +: and negative refspec" '
-+ test_config -C two remote.one.push +: &&
-+ # Fails to push master w/ tip behind counterpart
++ test_when_finished "test_unconfig -C two remote.one.push" &&
++
++ # The same set-up as above, whose side-effect was a no-op.
++ git -C two config --add remote.one.push +: &&
++
++ # The push refuses to update the "master" branch that is checked
++ # out in the "one" repository, even when it is forced with +:
+ test_must_fail git -C two push one &&
+
++ # "master" may actually not be "master"---find it out.
++ current=$(git symbolic-ref HEAD) &&
++
+ # If master is in negative refspec, then the command will not attempt
+ # to push and succeed
-+ git -C two config --add remote.one.push ^refs/heads/master &&
-+ git -C two push one
++ git -C two config --add remote.one.push "^$current" &&
++
++ # With "master" excluded, this push is a no-op. Nothing gets
++ # pushed and it succeeds.
++ git -C two push -v one
+'
+
test_done
2: 20575407cc0 = 2: 1f9af0e991c negative-refspec: improve comment on query_matches_negative_refspec

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