Improve out-of-memory error reports by including memory context name.

Add the target context's name to the errdetail field of "out of memory"
errors in mcxt.c.  Per discussion, this seems likely to be useful to
help narrow down the cause of a reported failure, and it costs little.
Also, now that context names are required to be compile-time constants
in all cases, there's little reason to be concerned about security
issues from exposing these names to users.  (Because of such concerns,
we are *not* including the context "ident" field.)

In passing, add unlikely() markers to the allocation-failed tests,
just to be sure the compiler is on the right page about that.
Also, in palloc and friends, copy CurrentMemoryContext into a local
variable, as that's almost surely cheaper to reference than a global.

Discussion: https://postgr.es/m/1099.1522285628@sss.pgh.pa.us

src/backend/utils/mmgr/mcxt.c

@@ -781,13 +781,21 @@ MemoryContextAlloc(MemoryContext context, Size size)
 	context->isReset = false;
 
 	ret = context->methods->alloc(context, size);
-	if (ret == NULL)
+	if (unlikely(ret == NULL))
 	{
 		MemoryContextStats(TopMemoryContext);
+
+		/*
+		 * Here, and elsewhere in this module, we show the target context's
+		 * "name" but not its "ident" (if any) in user-visible error messages.
+		 * The "ident" string might contain security-sensitive data, such as
+		 * values in SQL commands.
+		 */
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
 				 errmsg("out of memory"),
-				 errdetail("Failed on request of size %zu.", size)));
+				 errdetail("Failed on request of size %zu in memory context \"%s\".",
+						   size, context->name)));
 	}
 
 	VALGRIND_MEMPOOL_ALLOC(context, ret, size);
@@ -816,13 +824,14 @@ MemoryContextAllocZero(MemoryContext context, Size size)
 	context->isReset = false;
 
 	ret = context->methods->alloc(context, size);
-	if (ret == NULL)
+	if (unlikely(ret == NULL))
 	{
 		MemoryContextStats(TopMemoryContext);
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
 				 errmsg("out of memory"),
-				 errdetail("Failed on request of size %zu.", size)));
+				 errdetail("Failed on request of size %zu in memory context \"%s\".",
+						   size, context->name)));
 	}
 
 	VALGRIND_MEMPOOL_ALLOC(context, ret, size);
@@ -853,13 +862,14 @@ MemoryContextAllocZeroAligned(MemoryContext context, Size size)
 	context->isReset = false;
 
 	ret = context->methods->alloc(context, size);
-	if (ret == NULL)
+	if (unlikely(ret == NULL))
 	{
 		MemoryContextStats(TopMemoryContext);
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
 				 errmsg("out of memory"),
-				 errdetail("Failed on request of size %zu.", size)));
+				 errdetail("Failed on request of size %zu in memory context \"%s\".",
+						   size, context->name)));
 	}
 
 	VALGRIND_MEMPOOL_ALLOC(context, ret, size);
@@ -888,7 +898,7 @@ MemoryContextAllocExtended(MemoryContext context, Size size, int flags)
 	context->isReset = false;
 
 	ret = context->methods->alloc(context, size);
-	if (ret == NULL)
+	if (unlikely(ret == NULL))
 	{
 		if ((flags & MCXT_ALLOC_NO_OOM) == 0)
 		{
@@ -896,7 +906,8 @@ MemoryContextAllocExtended(MemoryContext context, Size size, int flags)
 			ereport(ERROR,
 					(errcode(ERRCODE_OUT_OF_MEMORY),
 					 errmsg("out of memory"),
-					 errdetail("Failed on request of size %zu.", size)));
+					 errdetail("Failed on request of size %zu in memory context \"%s\".",
+							   size, context->name)));
 		}
 		return NULL;
 	}
@@ -914,26 +925,28 @@ palloc(Size size)
 {
 	/* duplicates MemoryContextAlloc to avoid increased overhead */
 	void	   *ret;
+	MemoryContext context = CurrentMemoryContext;
 
-	AssertArg(MemoryContextIsValid(CurrentMemoryContext));
+	AssertArg(MemoryContextIsValid(context));
-	AssertNotInCriticalSection(CurrentMemoryContext);
+	AssertNotInCriticalSection(context);
 
 	if (!AllocSizeIsValid(size))
 		elog(ERROR, "invalid memory alloc request size %zu", size);
 
-	CurrentMemoryContext->isReset = false;
+	context->isReset = false;
 
-	ret = CurrentMemoryContext->methods->alloc(CurrentMemoryContext, size);
+	ret = context->methods->alloc(context, size);
-	if (ret == NULL)
+	if (unlikely(ret == NULL))
 	{
 		MemoryContextStats(TopMemoryContext);
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
 				 errmsg("out of memory"),
-				 errdetail("Failed on request of size %zu.", size)));
+				 errdetail("Failed on request of size %zu in memory context \"%s\".",
+						   size, context->name)));
 	}
 
-	VALGRIND_MEMPOOL_ALLOC(CurrentMemoryContext, ret, size);
+	VALGRIND_MEMPOOL_ALLOC(context, ret, size);
 
 	return ret;
 }
@@ -943,26 +956,28 @@ palloc0(Size size)
 {
 	/* duplicates MemoryContextAllocZero to avoid increased overhead */
 	void	   *ret;
+	MemoryContext context = CurrentMemoryContext;
 
-	AssertArg(MemoryContextIsValid(CurrentMemoryContext));
+	AssertArg(MemoryContextIsValid(context));
-	AssertNotInCriticalSection(CurrentMemoryContext);
+	AssertNotInCriticalSection(context);
 
 	if (!AllocSizeIsValid(size))
 		elog(ERROR, "invalid memory alloc request size %zu", size);
 
-	CurrentMemoryContext->isReset = false;
+	context->isReset = false;
 
-	ret = CurrentMemoryContext->methods->alloc(CurrentMemoryContext, size);
+	ret = context->methods->alloc(context, size);
-	if (ret == NULL)
+	if (unlikely(ret == NULL))
 	{
 		MemoryContextStats(TopMemoryContext);
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
 				 errmsg("out of memory"),
-				 errdetail("Failed on request of size %zu.", size)));
+				 errdetail("Failed on request of size %zu in memory context \"%s\".",
+						   size, context->name)));
 	}
 
-	VALGRIND_MEMPOOL_ALLOC(CurrentMemoryContext, ret, size);
+	VALGRIND_MEMPOOL_ALLOC(context, ret, size);
 
 	MemSetAligned(ret, 0, size);
 
@@ -974,18 +989,19 @@ palloc_extended(Size size, int flags)
 {
 	/* duplicates MemoryContextAllocExtended to avoid increased overhead */
 	void	   *ret;
+	MemoryContext context = CurrentMemoryContext;
 
-	AssertArg(MemoryContextIsValid(CurrentMemoryContext));
+	AssertArg(MemoryContextIsValid(context));
-	AssertNotInCriticalSection(CurrentMemoryContext);
+	AssertNotInCriticalSection(context);
 
 	if (((flags & MCXT_ALLOC_HUGE) != 0 && !AllocHugeSizeIsValid(size)) ||
 		((flags & MCXT_ALLOC_HUGE) == 0 && !AllocSizeIsValid(size)))
 		elog(ERROR, "invalid memory alloc request size %zu", size);
 
-	CurrentMemoryContext->isReset = false;
+	context->isReset = false;
 
-	ret = CurrentMemoryContext->methods->alloc(CurrentMemoryContext, size);
+	ret = context->methods->alloc(context, size);
-	if (ret == NULL)
+	if (unlikely(ret == NULL))
 	{
 		if ((flags & MCXT_ALLOC_NO_OOM) == 0)
 		{
@@ -993,12 +1009,13 @@ palloc_extended(Size size, int flags)
 			ereport(ERROR,
 					(errcode(ERRCODE_OUT_OF_MEMORY),
 					 errmsg("out of memory"),
-					 errdetail("Failed on request of size %zu.", size)));
+					 errdetail("Failed on request of size %zu in memory context \"%s\".",
+							   size, context->name)));
 		}
 		return NULL;
 	}
 
-	VALGRIND_MEMPOOL_ALLOC(CurrentMemoryContext, ret, size);
+	VALGRIND_MEMPOOL_ALLOC(context, ret, size);
 
 	if ((flags & MCXT_ALLOC_ZERO) != 0)
 		MemSetAligned(ret, 0, size);
@@ -1038,13 +1055,14 @@ repalloc(void *pointer, Size size)
 	Assert(!context->isReset);
 
 	ret = context->methods->realloc(context, pointer, size);
-	if (ret == NULL)
+	if (unlikely(ret == NULL))
 	{
 		MemoryContextStats(TopMemoryContext);
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
 				 errmsg("out of memory"),
-				 errdetail("Failed on request of size %zu.", size)));
+				 errdetail("Failed on request of size %zu in memory context \"%s\".",
+						   size, context->name)));
 	}
 
 	VALGRIND_MEMPOOL_CHANGE(context, pointer, ret, size);
@@ -1072,13 +1090,14 @@ MemoryContextAllocHuge(MemoryContext context, Size size)
 	context->isReset = false;
 
 	ret = context->methods->alloc(context, size);
-	if (ret == NULL)
+	if (unlikely(ret == NULL))
 	{
 		MemoryContextStats(TopMemoryContext);
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
 				 errmsg("out of memory"),
-				 errdetail("Failed on request of size %zu.", size)));
+				 errdetail("Failed on request of size %zu in memory context \"%s\".",
+						   size, context->name)));
 	}
 
 	VALGRIND_MEMPOOL_ALLOC(context, ret, size);
@@ -1106,13 +1125,14 @@ repalloc_huge(void *pointer, Size size)
 	Assert(!context->isReset);
 
 	ret = context->methods->realloc(context, pointer, size);
-	if (ret == NULL)
+	if (unlikely(ret == NULL))
 	{
 		MemoryContextStats(TopMemoryContext);
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
 				 errmsg("out of memory"),
-				 errdetail("Failed on request of size %zu.", size)));
+				 errdetail("Failed on request of size %zu in memory context \"%s\".",
+						   size, context->name)));
 	}
 
 	VALGRIND_MEMPOOL_CHANGE(context, pointer, ret, size);