Improve hash_create()'s API for some added robustness.

Invent a new flag bit HASH_STRINGS to specify C-string hashing, which
was formerly the default; and add assertions insisting that exactly
one of the bits HASH_STRINGS, HASH_BLOBS, and HASH_FUNCTION be set.
This is in hopes of preventing recurrences of the type of oversight
fixed in commit a1b8aa1e4 (i.e., mistakenly omitting HASH_BLOBS).

Also, when HASH_STRINGS is specified, insist that the keysize be
more than 8 bytes.  This is a heuristic, but it should catch
accidental use of HASH_STRINGS for integer or pointer keys.
(Nearly all existing use-cases set the keysize to NAMEDATALEN or
more, so there's little reason to think this restriction should
be problematic.)

Tweak hash_create() to insist that the HASH_ELEM flag be set, and
remove the defaults it had for keysize and entrysize.  Since those
defaults were undocumented and basically useless, no callers
omitted HASH_ELEM anyway.

Also, remove memset's zeroing the HASHCTL parameter struct from
those callers that had one.  This has never been really necessary,
and while it wasn't a bad coding convention it was confusing that
some callers did it and some did not.  We might as well save a few
cycles by standardizing on "not".

Also improve the documentation for hash_create().

In passing, improve reinit.c's usage of a hash table by storing
the key as a binary Oid rather than a string; and, since that's
a temporary hash table, allocate it in CurrentMemoryContext for
neatness.

Discussion: https://postgr.es/m/590625.1607878171@sss.pgh.pa.us

1 files changed, 2 insertions, 4 deletions

diff --git a/src/backend/utils/adt/jsonfuncs.c b/src/backend/utils/adt/jsonfuncs.c
index 12557ce3af..7a25415078 100644
--- a/src/backend/utils/adt/jsonfuncs.c
+++ b/src/backend/utils/adt/jsonfuncs.c
@@ -3439,14 +3439,13 @@ get_json_object_as_hash(char *json, int len, const char *funcname)
 	JsonLexContext *lex = makeJsonLexContextCstringLen(json, len, GetDatabaseEncoding(), true);
 	JsonSemAction *sem;
 
-	memset(&ctl, 0, sizeof(ctl));
 	ctl.keysize = NAMEDATALEN;
 	ctl.entrysize = sizeof(JsonHashEntry);
 	ctl.hcxt = CurrentMemoryContext;
 	tab = hash_create("json object hashtable",
 					  100,
 					  &ctl,
-					  HASH_ELEM | HASH_CONTEXT);
+					  HASH_ELEM | HASH_STRINGS | HASH_CONTEXT);
 
 	state = palloc0(sizeof(JHashState));
 	sem = palloc0(sizeof(JsonSemAction));
@@ -3831,14 +3830,13 @@ populate_recordset_object_start(void *state)
 		return;
 
 	/* Object at level 1: set up a new hash table for this object */
-	memset(&ctl, 0, sizeof(ctl));
 	ctl.keysize = NAMEDATALEN;
 	ctl.entrysize = sizeof(JsonHashEntry);
 	ctl.hcxt = CurrentMemoryContext;
 	_state->json_hash = hash_create("json object hashtable",
 									100,
 									&ctl,
-									HASH_ELEM | HASH_CONTEXT);
+									HASH_ELEM | HASH_STRINGS | HASH_CONTEXT);
 }
 
 static void