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323bd018cd
The prefetch-queue hash table uses a BufferTag struct as the hash key,
and it's hashed using hash_bytes(). It's important that all the padding
bytes in the key are cleared, because hash_bytes() will include them.
I was getting compiler warnings like this on v14 and v15, when compiling
with -Warray-bounds:
In function ‘prfh_lookup_hash_internal’,
inlined from ‘prfh_lookup’ at
pg_install/v14/include/postgresql/server/lib/simplehash.h:821:9,
inlined from ‘neon_read_at_lsnv’ at pgxn/neon/pagestore_smgr.c:2789:11,
inlined from ‘neon_read_at_lsn’ at pgxn/neon/pagestore_smgr.c:2904:2:
pg_install/v14/include/postgresql/server/storage/relfilenode.h:90:43:
warning: array subscript ‘PrefetchRequest[0]’ is partly outside array
bounds of ‘BufferTag[1]’ {aka ‘struct buftag[1]’} [-Warray-bounds]
89 | ((node1).relNode == (node2).relNode && \
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
90 | (node1).dbNode == (node2).dbNode && \
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
91 | (node1).spcNode == (node2).spcNode)
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
pg_install/v14/include/postgresql/server/storage/buf_internals.h:116:9:
note: in expansion of macro ‘RelFileNodeEquals’
116 | RelFileNodeEquals((a).rnode, (b).rnode) && \
| ^~~~~~~~~~~~~~~~~
pgxn/neon/neon_pgversioncompat.h:25:31: note: in expansion of macro
‘BUFFERTAGS_EQUAL’
25 | #define BufferTagsEqual(a, b) BUFFERTAGS_EQUAL(*(a), *(b))
| ^~~~~~~~~~~~~~~~
pgxn/neon/pagestore_smgr.c:220:34: note: in expansion of macro
‘BufferTagsEqual’
220 | #define SH_EQUAL(tb, a, b) (BufferTagsEqual(&(a)->buftag,
&(b)->buftag))
| ^~~~~~~~~~~~~~~
pg_install/v14/include/postgresql/server/lib/simplehash.h:280:77: note:
in expansion of macro ‘SH_EQUAL’
280 | #define SH_COMPARE_KEYS(tb, ahash, akey, b) (ahash ==
SH_GET_HASH(tb, b) && SH_EQUAL(tb, b->SH_KEY, akey))
| ^~~~~~~~
pg_install/v14/include/postgresql/server/lib/simplehash.h:799:21: note:
in expansion of macro ‘SH_COMPARE_KEYS’
799 | if (SH_COMPARE_KEYS(tb, hash, key, entry))
| ^~~~~~~~~~~~~~~
pgxn/neon/pagestore_smgr.c: In function ‘neon_read_at_lsn’:
pgxn/neon/pagestore_smgr.c:2742:25: note: object ‘buftag’ of size 20
2742 | BufferTag buftag = {0};
| ^~~~~~
This commit silences those warnings, although it's not clear to me why
the compiler complained like that in the first place. I found the issue
with padding bytes while looking into those warnings, but that was
coincidental, I don't think the padding bytes explain the warnings as
such.
In v16, the BUFFERTAGS_EQUAL macro was replaced with a static inline
function, and that also silences the compiler warning. Not clear to me
why.