Fix hash_update_hash_key() to handle same-bucket case correctly.

Original coding would corrupt the hashtable if the item being updated was
at the end of its bucket chain and the new hash key hashed to that same
bucket.  Diagnosis and fix by Heikki Linnakangas.

src/backend/utils/hash/dynahash.c

@@ -1022,6 +1022,7 @@ hash_update_hash_key(HTAB *hashp,
 	uint32		newhashvalue;
 	Size		keysize;
 	uint32		bucket;
+	uint32		newbucket;
 	long		segment_num;
 	long		segment_ndx;
 	HASHSEGMENT segp;
@@ -1078,10 +1079,10 @@ hash_update_hash_key(HTAB *hashp,
 	 */
 	newhashvalue = hashp->hash(newKeyPtr, hashp->keysize);
 
-	bucket = calc_bucket(hctl, newhashvalue);
+	newbucket = calc_bucket(hctl, newhashvalue);
 
-	segment_num = bucket >> hashp->sshift;
-	segment_ndx = MOD(bucket, hashp->ssize);
+	segment_num = newbucket >> hashp->sshift;
+	segment_ndx = MOD(newbucket, hashp->ssize);
 
 	segp = hashp->dir[segment_num];
 
@@ -1115,12 +1116,22 @@ hash_update_hash_key(HTAB *hashp,
 
 	currBucket = existingElement;
 
+	/*
+	 * If old and new hash values belong to the same bucket, we need not
+	 * change any chain links, and indeed should not since this simplistic
+	 * update will corrupt the list if currBucket is the last element.  (We
+	 * cannot fall out earlier, however, since we need to scan the bucket to
+	 * check for duplicate keys.)
+	 */
+	if (bucket != newbucket)
+	{
 		/* OK to remove record from old hash bucket's chain. */
 		*oldPrevPtr = currBucket->link;
 
 		/* link into new hashbucket chain */
 		*prevBucketPtr = currBucket;
 		currBucket->link = NULL;
+	}
 
 	/* copy new key into record */
 	currBucket->hashvalue = newhashvalue;