Prevent very-low-probability PANIC during PREPARE TRANSACTION.

The code in PostPrepare_Locks supposed that it could reassign locks to
the prepared transaction's dummy PGPROC by deleting the PROCLOCK table
entries and immediately creating new ones.  This was safe when that code
was written, but since we invented partitioning of the shared lock table,
it's not safe --- another process could steal away the PROCLOCK entry in
the short interval when it's on the freelist.  Then, if we were otherwise
out of shared memory, PostPrepare_Locks would have to PANIC, since it's
too late to back out of the PREPARE at that point.

Fix by inventing a dynahash.c function to atomically update a hashtable
entry's key.  (This might possibly have other uses in future.)

This is an ancient bug that in principle we ought to back-patch, but the
odds of someone hitting it in the field seem really tiny, because (a) the
risk window is small, and (b) nobody runs servers with maxed-out lock
tables for long, because they'll be getting non-PANIC out-of-memory errors
anyway.  So fixing it in HEAD seems sufficient, at least until the new
code has gotten some testing.

src/backend/utils/hash/dynahash.c

@@ -183,6 +183,12 @@ struct HTAB
  */
 #define ELEMENTKEY(helem)  (((char *)(helem)) + MAXALIGN(sizeof(HASHELEMENT)))
 
+/*
+ * Obtain element pointer given pointer to key
+ */
+#define ELEMENT_FROM_KEY(key)  \
+	((HASHELEMENT *) (((char *) (key)) - MAXALIGN(sizeof(HASHELEMENT))))
+
 /*
  * Fast MOD arithmetic, assuming that y is a power of 2 !
  */
@@ -987,6 +993,144 @@ hash_search_with_hash_value(HTAB *hashp,
 	return NULL;				/* keep compiler quiet */
 }
 
+/*
+ * hash_update_hash_key -- change the hash key of an existing table entry
+ *
+ * This is equivalent to removing the entry, making a new entry, and copying
+ * over its data, except that the entry never goes to the table's freelist.
+ * Therefore this cannot suffer an out-of-memory failure, even if there are
+ * other processes operating in other partitions of the hashtable.
+ *
+ * Returns TRUE if successful, FALSE if the requested new hash key is already
+ * present.  Throws error if the specified entry pointer isn't actually a
+ * table member.
+ *
+ * NB: currently, there is no special case for old and new hash keys being
+ * identical, which means we'll report FALSE for that situation.  This is
+ * preferable for existing uses.
+ *
+ * NB: for a partitioned hashtable, caller must hold lock on both relevant
+ * partitions, if the new hash key would belong to a different partition.
+ */
+bool
+hash_update_hash_key(HTAB *hashp,
+					 void *existingEntry,
+					 const void *newKeyPtr)
+{
+	HASHELEMENT *existingElement = ELEMENT_FROM_KEY(existingEntry);
+	HASHHDR    *hctl = hashp->hctl;
+	uint32		newhashvalue;
+	Size		keysize;
+	uint32		bucket;
+	long		segment_num;
+	long		segment_ndx;
+	HASHSEGMENT segp;
+	HASHBUCKET	currBucket;
+	HASHBUCKET *prevBucketPtr;
+	HASHBUCKET *oldPrevPtr;
+	HashCompareFunc match;
+
+#if HASH_STATISTICS
+	hash_accesses++;
+	hctl->accesses++;
+#endif
+
+	/* disallow updates if frozen */
+	if (hashp->frozen)
+		elog(ERROR, "cannot update in frozen hashtable \"%s\"",
+			 hashp->tabname);
+
+	/*
+	 * Lookup the existing element using its saved hash value.  We need to
+	 * do this to be able to unlink it from its hash chain, but as a side
+	 * benefit we can verify the validity of the passed existingEntry pointer.
+	 */
+	bucket = calc_bucket(hctl, existingElement->hashvalue);
+
+	segment_num = bucket >> hashp->sshift;
+	segment_ndx = MOD(bucket, hashp->ssize);
+
+	segp = hashp->dir[segment_num];
+
+	if (segp == NULL)
+		hash_corrupted(hashp);
+
+	prevBucketPtr = &segp[segment_ndx];
+	currBucket = *prevBucketPtr;
+
+	while (currBucket != NULL)
+	{
+		if (currBucket == existingElement)
+			break;
+		prevBucketPtr = &(currBucket->link);
+		currBucket = *prevBucketPtr;
+	}
+
+	if (currBucket == NULL)
+		elog(ERROR, "hash_update_hash_key argument is not in hashtable \"%s\"",
+			 hashp->tabname);
+
+	oldPrevPtr = prevBucketPtr;
+
+	/*
+	 * Now perform the equivalent of a HASH_ENTER operation to locate the
+	 * hash chain we want to put the entry into.
+	 */
+	newhashvalue = hashp->hash(newKeyPtr, hashp->keysize);
+
+	bucket = calc_bucket(hctl, newhashvalue);
+
+	segment_num = bucket >> hashp->sshift;
+	segment_ndx = MOD(bucket, hashp->ssize);
+
+	segp = hashp->dir[segment_num];
+
+	if (segp == NULL)
+		hash_corrupted(hashp);
+
+	prevBucketPtr = &segp[segment_ndx];
+	currBucket = *prevBucketPtr;
+
+	/*
+	 * Follow collision chain looking for matching key
+	 */
+	match = hashp->match;		/* save one fetch in inner loop */
+	keysize = hashp->keysize;	/* ditto */
+
+	while (currBucket != NULL)
+	{
+		if (currBucket->hashvalue == newhashvalue &&
+			match(ELEMENTKEY(currBucket), newKeyPtr, keysize) == 0)
+			break;
+		prevBucketPtr = &(currBucket->link);
+		currBucket = *prevBucketPtr;
+#if HASH_STATISTICS
+		hash_collisions++;
+		hctl->collisions++;
+#endif
+	}
+
+	if (currBucket != NULL)
+		return false;			/* collision with an existing entry */
+
+	currBucket = existingElement;
+
+	/* 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;
+	hashp->keycopy(ELEMENTKEY(currBucket), newKeyPtr, keysize);
+
+	/* rest of record is untouched */
+
+	return true;
+}
+
 /*
  * create a new entry if possible
  */