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

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

1 files changed, 9 insertions, 1 deletions

diff --git a/src/backend/storage/ipc/shm_toc.c b/src/backend/storage/ipc/shm_toc.c
index 9110ffa4a0b..5e290df3366 100644
--- a/src/backend/storage/ipc/shm_toc.c
+++ b/src/backend/storage/ipc/shm_toc.c
@@ -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;
 }