Clean up assorted failures under clang's -fsanitize=undefined checks.

Most of these are cases where we could call memcpy() or other libc
functions with a NULL pointer and a zero count, which is forbidden
by POSIX even though every production version of libc allows it.
We've fixed such things before in a piecemeal way, but apparently
never made an effort to try to get them all.  I don't claim that
this patch does so either, but it gets every failure I observe in
check-world, using clang 12.0.1 on current RHEL8.

numeric.c has a different issue that the sanitizer doesn't like:
"ln(-1.0)" will compute log10(0) and then try to assign the
resulting -Inf to an integer variable.  We don't actually use the
result in such a case, so there's no live bug.

Back-patch to all supported branches, with the idea that we might
start running a buildfarm member that tests this case.  This includes
back-patching c1132aae3 (Check the size in COPY_POINTER_FIELD),
which previously silenced some of these issues in copyfuncs.c.

Discussion: https://postgr.es/m/CALNJ-vT9r0DSsAOw9OXVJFxLENoVS_68kJ5x0p44atoYH+H4dg@mail.gmail.com

contrib/pgcrypto/px.c

@@ -198,7 +198,7 @@ combo_init(PX_Combo *cx, const uint8 *key, unsigned klen,
 		ivbuf = palloc0(ivs);
 		if (ivlen > ivs)
 			memcpy(ivbuf, iv, ivs);
-		else
+		else if (ivlen > 0)
 			memcpy(ivbuf, iv, ivlen);
 	}
 

src/backend/access/heap/heapam.c

@@ -328,7 +328,7 @@ initscan(HeapScanDesc scan, ScanKey key, bool keep_startblock)
 	/*
 	 * copy the scan key, if appropriate
 	 */
-	if (key != NULL)
+	if (key != NULL && scan->rs_base.rs_nkeys > 0)
 		memcpy(scan->rs_base.rs_key, key, scan->rs_base.rs_nkeys * sizeof(ScanKeyData));
 
 	/*

src/backend/access/heap/heapam_visibility.c

@@ -1564,8 +1564,8 @@ HeapTupleHeaderIsOnlyLocked(HeapTupleHeader tuple)
 static bool
 TransactionIdInArray(TransactionId xid, TransactionId *xip, Size num)
 {
-	return bsearch(&xid, xip, num,
-				   sizeof(TransactionId), xidComparator) != NULL;
+	return num > 0 &&
+		bsearch(&xid, xip, num, sizeof(TransactionId), xidComparator) != NULL;
 }
 
 /*

src/backend/access/transam/clog.c

@@ -297,8 +297,9 @@ TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
 	if (all_xact_same_page && xid == MyProc->xid &&
 		nsubxids <= THRESHOLD_SUBTRANS_CLOG_OPT &&
 		nsubxids == MyProc->subxidStatus.count &&
-		memcmp(subxids, MyProc->subxids.xids,
-			   nsubxids * sizeof(TransactionId)) == 0)
+		(nsubxids == 0 ||
+		 memcmp(subxids, MyProc->subxids.xids,
+				nsubxids * sizeof(TransactionId)) == 0))
 	{
 		/*
 		 * If we can immediately acquire XactSLRULock, we update the status of

src/backend/access/transam/xact.c

@@ -5353,8 +5353,9 @@ SerializeTransactionState(Size maxsize, char *start_address)
 	{
 		if (FullTransactionIdIsValid(s->fullTransactionId))
 			workspace[i++] = XidFromFullTransactionId(s->fullTransactionId);
-		memcpy(&workspace[i], s->childXids,
-			   s->nChildXids * sizeof(TransactionId));
+		if (s->nChildXids > 0)
+			memcpy(&workspace[i], s->childXids,
+				   s->nChildXids * sizeof(TransactionId));
 		i += s->nChildXids;
 	}
 	Assert(i == nxids);

src/backend/storage/ipc/shm_mq.c

@@ -773,8 +773,11 @@ shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
 
 		/* Copy as much as we can. */
 		Assert(mqh->mqh_partial_bytes + rb <= nbytes);
-		memcpy(&mqh->mqh_buffer[mqh->mqh_partial_bytes], rawdata, rb);
-		mqh->mqh_partial_bytes += rb;
+		if (rb > 0)
+		{
+			memcpy(&mqh->mqh_buffer[mqh->mqh_partial_bytes], rawdata, rb);
+			mqh->mqh_partial_bytes += rb;
+		}
 
 		/*
 		 * Update count of bytes that can be consumed, accounting for

src/backend/utils/adt/numeric.c

@@ -10048,12 +10048,20 @@ exp_var(const NumericVar *arg, NumericVar *result, int rscale)
  *
  * Essentially, we're approximating log10(abs(ln(var))).  This is used to
  * determine the appropriate rscale when computing natural logarithms.
+ *
+ * Note: many callers call this before range-checking the input.  Therefore,
+ * we must be robust against values that are invalid to apply ln() to.
+ * We don't wish to throw an error here, so just return zero in such cases.
  */
 static int
 estimate_ln_dweight(const NumericVar *var)
 {
 	int			ln_dweight;
 
+	/* Caller should fail on ln(negative), but for the moment return zero */
+	if (var->sign != NUMERIC_POS)
+		return 0;
+
 	if (cmp_var(var, &const_zero_point_nine) >= 0 &&
 		cmp_var(var, &const_one_point_one) <= 0)
 	{

src/backend/utils/time/snapmgr.c

@@ -536,12 +536,14 @@ SetTransactionSnapshot(Snapshot sourcesnap, VirtualTransactionId *sourcevxid,
 	CurrentSnapshot->xmax = sourcesnap->xmax;
 	CurrentSnapshot->xcnt = sourcesnap->xcnt;
 	Assert(sourcesnap->xcnt <= GetMaxSnapshotXidCount());
-	memcpy(CurrentSnapshot->xip, sourcesnap->xip,
-		   sourcesnap->xcnt * sizeof(TransactionId));
+	if (sourcesnap->xcnt > 0)
+		memcpy(CurrentSnapshot->xip, sourcesnap->xip,
+			   sourcesnap->xcnt * sizeof(TransactionId));
 	CurrentSnapshot->subxcnt = sourcesnap->subxcnt;
 	Assert(sourcesnap->subxcnt <= GetMaxSnapshotSubxidCount());
-	memcpy(CurrentSnapshot->subxip, sourcesnap->subxip,
-		   sourcesnap->subxcnt * sizeof(TransactionId));
+	if (sourcesnap->subxcnt > 0)
+		memcpy(CurrentSnapshot->subxip, sourcesnap->subxip,
+			   sourcesnap->subxcnt * sizeof(TransactionId));
 	CurrentSnapshot->suboverflowed = sourcesnap->suboverflowed;
 	CurrentSnapshot->takenDuringRecovery = sourcesnap->takenDuringRecovery;
 	/* NB: curcid should NOT be copied, it's a local matter */

src/fe_utils/print.c

@@ -966,7 +966,8 @@ print_aligned_text(const printTableContent *cont, FILE *fout, bool is_pager)
 
 			more_col_wrapping = col_count;
 			curr_nl_line = 0;
-			memset(header_done, false, col_count * sizeof(bool));
+			if (col_count > 0)
+				memset(header_done, false, col_count * sizeof(bool));
 			while (more_col_wrapping)
 			{
 				if (opt_border == 2)