Should zstd's negative compression levels be supported here ?
Here's a POC patch which is enough to play with it.
$ src/bin/pg_basebackup/pg_basebackup --wal-method fetch -Ft -D - -h /tmp --no-sync --compress=zstd |wc -c
12305659
$ src/bin/pg_basebackup/pg_basebackup --wal-method fetch -Ft -D - -h /tmp --no-sync --compress=zstd:1 |wc -c
13827521
$ src/bin/pg_basebackup/pg_basebackup --wal-method fetch -Ft -D - -h /tmp --no-sync --compress=zstd:0 |wc -c
12304018
$ src/bin/pg_basebackup/pg_basebackup --wal-method fetch -Ft -D - -h /tmp --no-sync --compress=zstd:-1 |wc -c
16443893
$ src/bin/pg_basebackup/pg_basebackup --wal-method fetch -Ft -D - -h /tmp --no-sync --compress=zstd:-2 |wc -c
17349563
$ src/bin/pg_basebackup/pg_basebackup --wal-method fetch -Ft -D - -h /tmp --no-sync --compress=zstd:-4 |wc -c
19452631
$ src/bin/pg_basebackup/pg_basebackup --wal-method fetch -Ft -D - -h /tmp --no-sync --compress=zstd:-7 |wc -c
21871505
Also, with a partial regression DB, this crashes when writing to stdout.
$ src/bin/pg_basebackup/pg_basebackup --wal-method fetch -Ft -D - -h /tmp --no-sync --compress=lz4 |wc -c
pg_basebackup: bbstreamer_lz4.c:172: bbstreamer_lz4_compressor_content: Assertion `mystreamer->base.bbs_buffer.maxlen
>=out_bound' failed.
24117248
#4 0x000055555555e8b4 in bbstreamer_lz4_compressor_content (streamer=0x5555555a5260, member=0x7fffffffc760,
data=0x7ffff3068010 "{ \"PostgreSQL-Backup-Manifest-Version\": 1,\n\"Files\": [\n{ \"Path\": \"backup_label\",
\"Size\":227, \"Last-Modified\": \"2022-03-16 02:29:11 GMT\", \"Checksum-Algorithm\": \"CRC32C\", \"Checksum\":
\"46f69d99\"},\n{ \"Pa"..., len=401072, context=BBSTREAMER_MEMBER_CONTENTS) at bbstreamer_lz4.c:172
mystreamer = 0x5555555a5260
next_in = 0x7ffff3068010 "{ \"PostgreSQL-Backup-Manifest-Version\": 1,\n\"Files\": [\n{ \"Path\":
\"backup_label\",\"Size\": 227, \"Last-Modified\": \"2022-03-16 02:29:11 GMT\", \"Checksum-Algorithm\": \"CRC32C\",
\"Checksum\":\"46f69d99\" },\n{ \"Pa"...
...
(gdb) p mystreamer->base.bbs_buffer.maxlen
$1 = 524288
(gdb) p (int) LZ4F_compressBound(len, &mystreamer->prefs)
$4 = 524300
This is with: liblz4-1:amd64 1.9.2-2ubuntu0.20.04.1
