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Don't be so trusting that shm_toc_lookup() will always succeed.

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Given the possibility of race conditions and so on, it seems entirely
unsafe to just assume that shm_toc_lookup() always finds the key it's
looking for --- but that was exactly what all but one call site were
doing.  To fix, add a "bool noError" argument, similarly to what we
have in many other functions, and throw an error on an unexpected
lookup failure.  Remove now-redundant Asserts that a rather random
subset of call sites had.

I doubt this will throw any light on buildfarm member lorikeet's
recent failures, because if an unnoticed lookup failure were involved,
you'd kind of expect a null-pointer-dereference crash rather than the
observed symptom.  But you never know ... and this is better coding
practice even if it never catches anything.

Discussion: https://postgr.es/m/9697.1496675981@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2017-06-05 12:05:42 -04:00
parent af51fea039
commit d466335064
11 changed files with 39 additions and 38 deletions
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10
src/backend/storage/ipc/shm_toc.c
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@ -208,6 +208,9 @@ shm_toc_insert(shm_toc *toc, uint64 key, void *address)
/*
* Look up a TOC entry.
*
* If the key is not found, returns NULL if noError is true, otherwise
* throws elog(ERROR).
*
* Unlike the other functions in this file, this operation acquires no lock;
* it uses only barriers. It probably wouldn't hurt concurrency very much even
* if it did get a lock, but since it's reasonably likely that a group of
@ -215,7 +218,7 @@ shm_toc_insert(shm_toc *toc, uint64 key, void *address)
* right around the same time, there seems to be some value in avoiding it.
*/
void *
shm_toc_lookup(shm_toc *toc, uint64 key)
shm_toc_lookup(shm_toc *toc, uint64 key, bool noError)
{
uint64 nentry;
uint64 i;
@ -226,10 +229,15 @@ shm_toc_lookup(shm_toc *toc, uint64 key)
/* Now search for a matching entry. */
for (i = 0; i < nentry; ++i)
{
if (toc->toc_entry[i].key == key)
return ((char *) toc) + toc->toc_entry[i].offset;
}
/* No matching entry was found. */
if (!noError)
elog(ERROR, "could not find key " UINT64_FORMAT " in shm TOC at %p",
key, toc);
return NULL;
}