pgsql: Add missing deflateEnd() for server-side gzip base backups

Add missing deflateEnd() for server-side gzip base backups

The gzip basebackup sink called deflateInit2() in begin_archive() but
never called deflateEnd(), leaking zlib's internal compression state
(~256KB per archive) until the memory context of the base backup is
destroyed.

The code tree has already a matching deflateEnd() call for each
deflateInit[2]() call (pgrypto, etc.), except for the file touched in
this commit, so this brings more consistency for all the compression
methods. The server-side LZ4 and zstd implementations require a
dedicated cleanup callback as they allocate their state outside the
context of a palloc().

As currently used, deflateInit2() is called once per tablespace in a
single backup. Memory would slightly bloat only when dealing with many
tablespaces at once, not across multiple base backups so this is not
worth a backpatch. This change could matter for future uses of this
code.

zlib allows the definition of memory allocation and free callbacks in
the z_stream object given to a deflateInit[2](). The base backup
backend code relies on palloc() for the allocations and deflateEnd()
internally only cleans up memory (no fd allocation for example).

Author: Jianghua Yang <yjhjstz(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAAZLFmQNJ0QNArpWEOZXwv=vbumcWKEHz-b1me5gBqRqG67EwQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ded9754804bcdee60d72cfb0e0aaeda03a2c2f44

Modified Files
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src/backend/backup/basebackup_gzip.c | 27 ++++++++++++++++++++++++++-
1 file changed, 26 insertions(+), 1 deletion(-)