Make some minor improvements in memory-context infrastructure.

We lack a version of repalloc() that supports MCXT_ALLOC_NO_OOM
semantics, so invent repalloc_extended() with the usual set of
flags.  repalloc_huge() becomes a legacy wrapper for that.

Also, fix dynahash.c so that it can support HASH_ENTER_NULL
requests when using the default palloc-based allocator.
The only reason it didn't do that already was the lack of the
MCXT_ALLOC_NO_OOM option when that code was written, ages ago.

While here, simplify a few overcomplicated tests in mcxt.c.

Discussion: https://postgr.es/m/2982579.1662416866@sss.pgh.pa.us

src/backend/utils/hash/dynahash.c

@@ -289,7 +289,8 @@ static void *
 DynaHashAlloc(Size size)
 {
 	Assert(MemoryContextIsValid(CurrentDynaHashCxt));
-	return MemoryContextAlloc(CurrentDynaHashCxt, size);
+	return MemoryContextAllocExtended(CurrentDynaHashCxt, size,
+									  MCXT_ALLOC_NO_OOM);
 }
 
 
@@ -939,9 +940,7 @@ calc_bucket(HASHHDR *hctl, uint32 hash_val)
  *
  * HASH_ENTER will normally ereport a generic "out of memory" error if
  * it is unable to create a new entry.  The HASH_ENTER_NULL operation is
- * the same except it will return NULL if out of memory.  Note that
+ * the same except it will return NULL if out of memory.
- * HASH_ENTER_NULL cannot be used with the default palloc-based allocator,
- * since palloc internally ereports on out-of-memory.
  *
  * If foundPtr isn't NULL, then *foundPtr is set true if we found an
  * existing entry in the table, false otherwise.  This is needed in the
@@ -1084,12 +1083,8 @@ hash_search_with_hash_value(HTAB *hashp,
 			}
 			return NULL;
 
-		case HASH_ENTER_NULL:
-			/* ENTER_NULL does not work with palloc-based allocator */
-			Assert(hashp->alloc != DynaHashAlloc);
-			/* FALL THRU */
-
 		case HASH_ENTER:
+		case HASH_ENTER_NULL:
 			/* Return existing element if found, else create one */
 			if (currBucket != NULL)
 				return (void *) ELEMENTKEY(currBucket);

src/backend/utils/mmgr/mcxt.c

@@ -1114,8 +1114,8 @@ MemoryContextAllocExtended(MemoryContext context, Size size, int flags)
 	AssertArg(MemoryContextIsValid(context));
 	AssertNotInCriticalSection(context);
 
-	if (((flags & MCXT_ALLOC_HUGE) != 0 && !AllocHugeSizeIsValid(size)) ||
+	if (!((flags & MCXT_ALLOC_HUGE) != 0 ? AllocHugeSizeIsValid(size) :
-		((flags & MCXT_ALLOC_HUGE) == 0 && !AllocSizeIsValid(size)))
+		  AllocSizeIsValid(size)))
 		elog(ERROR, "invalid memory alloc request size %zu", size);
 
 	context->isReset = false;
@@ -1269,8 +1269,8 @@ palloc_extended(Size size, int flags)
 	AssertArg(MemoryContextIsValid(context));
 	AssertNotInCriticalSection(context);
 
-	if (((flags & MCXT_ALLOC_HUGE) != 0 && !AllocHugeSizeIsValid(size)) ||
+	if (!((flags & MCXT_ALLOC_HUGE) != 0 ? AllocHugeSizeIsValid(size) :
-		((flags & MCXT_ALLOC_HUGE) == 0 && !AllocSizeIsValid(size)))
+		  AllocSizeIsValid(size)))
 		elog(ERROR, "invalid memory alloc request size %zu", size);
 
 	context->isReset = false;
@@ -1351,6 +1351,50 @@ repalloc(void *pointer, Size size)
 	return ret;
 }
 
+/*
+ * repalloc_extended
+ *		Adjust the size of a previously allocated chunk,
+ *		with HUGE and NO_OOM options.
+ */
+void *
+repalloc_extended(void *pointer, Size size, int flags)
+{
+#if defined(USE_ASSERT_CHECKING) || defined(USE_VALGRIND)
+	MemoryContext context = GetMemoryChunkContext(pointer);
+#endif
+	void	   *ret;
+
+	if (!((flags & MCXT_ALLOC_HUGE) != 0 ? AllocHugeSizeIsValid(size) :
+		  AllocSizeIsValid(size)))
+		elog(ERROR, "invalid memory alloc request size %zu", size);
+
+	AssertNotInCriticalSection(context);
+
+	/* isReset must be false already */
+	Assert(!context->isReset);
+
+	ret = MCXT_METHOD(pointer, realloc) (pointer, size);
+	if (unlikely(ret == NULL))
+	{
+		if ((flags & MCXT_ALLOC_NO_OOM) == 0)
+		{
+			MemoryContext cxt = GetMemoryChunkContext(pointer);
+
+			MemoryContextStats(TopMemoryContext);
+			ereport(ERROR,
+					(errcode(ERRCODE_OUT_OF_MEMORY),
+					 errmsg("out of memory"),
+					 errdetail("Failed on request of size %zu in memory context \"%s\".",
+							   size, cxt->name)));
+		}
+		return NULL;
+	}
+
+	VALGRIND_MEMPOOL_CHANGE(context, pointer, ret, size);
+
+	return ret;
+}
+
 /*
  * MemoryContextAllocHuge
  *		Allocate (possibly-expansive) space within the specified context.
@@ -1394,35 +1438,8 @@ MemoryContextAllocHuge(MemoryContext context, Size size)
 void *
 repalloc_huge(void *pointer, Size size)
 {
-#if defined(USE_ASSERT_CHECKING) || defined(USE_VALGRIND)
+	/* this one seems not worth its own implementation */
-	MemoryContext context = GetMemoryChunkContext(pointer);
+	return repalloc_extended(pointer, size, MCXT_ALLOC_HUGE);
-#endif
-	void	   *ret;
-
-	if (!AllocHugeSizeIsValid(size))
-		elog(ERROR, "invalid memory alloc request size %zu", size);
-
-	AssertNotInCriticalSection(context);
-
-	/* isReset must be false already */
-	Assert(!context->isReset);
-
-	ret = MCXT_METHOD(pointer, realloc) (pointer, size);
-	if (unlikely(ret == NULL))
-	{
-		MemoryContext cxt = GetMemoryChunkContext(pointer);
-
-		MemoryContextStats(TopMemoryContext);
-		ereport(ERROR,
-				(errcode(ERRCODE_OUT_OF_MEMORY),
-				 errmsg("out of memory"),
-				 errdetail("Failed on request of size %zu in memory context \"%s\".",
-						   size, cxt->name)));
-	}
-
-	VALGRIND_MEMPOOL_CHANGE(context, pointer, ret, size);
-
-	return ret;
 }
 
 /*

src/include/utils/palloc.h

@@ -78,6 +78,8 @@ extern void *palloc(Size size);
 extern void *palloc0(Size size);
 extern void *palloc_extended(Size size, int flags);
 extern pg_nodiscard void *repalloc(void *pointer, Size size);
+extern pg_nodiscard void *repalloc_extended(void *pointer,
+											Size size, int flags);
 extern void pfree(void *pointer);
 
 /*