pgsql: Fix locking when fixing an incomplete split of a GIN internal pa

Fix locking when fixing an incomplete split of a GIN internal page

ginFinishSplit() expects the caller to hold an exclusive lock on the
buffer, but when finishing an earlier "leftover" incomplete split of
an internal page, the caller held a shared lock. That caused an
assertion failure in MarkBufferDirty(). Without assertions, it could
lead to corruption if two backends tried to complete the split at the
same time.

On master, add a test case using the new injection point facility.

Report and analysis by Fei Changhong. Backpatch the fix to all
supported versions.

Reviewed-by: Fei Changhong, Michael Paquier
Discussion: https://www.postgresql.org/message-id/tencent_A3CE810F59132D8E230475A5F0F7A08C8307@qq.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e6511fe649c5ae6bb647258b634f3730cee3195b

Modified Files
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src/backend/access/gin/ginbtree.c | 68 ++++++++++++++++++++++++++++-----------
1 file changed, 50 insertions(+), 18 deletions(-)