Add an Assertion that you don't palloc within a critical section.

This caught a bunch of cases doing that already, which I just fixed in
previous commit. This is the assertion itself.

Per Tom Lane's idea.

src/backend/utils/mmgr/mcxt.c

@@ -24,6 +24,7 @@
 
 #include "postgres.h"
 
+#include "miscadmin.h"
 #include "utils/memdebug.h"
 #include "utils/memutils.h"
 
@@ -55,6 +56,19 @@ MemoryContext PortalContext = NULL;
 
 static void MemoryContextStatsInternal(MemoryContext context, int level);
 
+/*
+ * You should not do memory allocations within a critical section, because
+ * an out-of-memory error will be escalated to a PANIC. To enforce that
+ * rule, the allocation functions Assert that.
+ *
+ * There are a two exceptions: 1) error recovery uses ErrorContext, which
+ * has some memory set aside so that you don't run out. And 2) checkpointer
+ * currently just hopes for the best, which is wrong and ought to be fixed,
+ * but it's a known issue so let's not complain about in the meanwhile.
+ */
+#define AssertNotInCriticalSection(context) \
+	Assert(CritSectionCount == 0 || (context) == ErrorContext || \
+		   AmCheckpointerProcess())
 
 /*****************************************************************************
  *	  EXPORTED ROUTINES														 *
@@ -519,6 +533,8 @@ MemoryContextCreate(NodeTag tag, Size size,
 	MemoryContext node;
 	Size		needed = size + strlen(name) + 1;
 
+	Assert(CritSectionCount == 0);
+
 	/* Get space for node and name */
 	if (TopMemoryContext != NULL)
 	{
@@ -575,6 +591,7 @@ MemoryContextAlloc(MemoryContext context, Size size)
 	void	   *ret;
 
 	AssertArg(MemoryContextIsValid(context));
+	AssertNotInCriticalSection(context);
 
 	if (!AllocSizeIsValid(size))
 		elog(ERROR, "invalid memory alloc request size %zu", size);
@@ -600,6 +617,7 @@ MemoryContextAllocZero(MemoryContext context, Size size)
 	void	   *ret;
 
 	AssertArg(MemoryContextIsValid(context));
+	AssertNotInCriticalSection(context);
 
 	if (!AllocSizeIsValid(size))
 		elog(ERROR, "invalid memory alloc request size %zu", size);
@@ -627,6 +645,7 @@ MemoryContextAllocZeroAligned(MemoryContext context, Size size)
 	void	   *ret;
 
 	AssertArg(MemoryContextIsValid(context));
+	AssertNotInCriticalSection(context);
 
 	if (!AllocSizeIsValid(size))
 		elog(ERROR, "invalid memory alloc request size %zu", size);
@@ -648,6 +667,7 @@ palloc(Size size)
 	void	   *ret;
 
 	AssertArg(MemoryContextIsValid(CurrentMemoryContext));
+	AssertNotInCriticalSection(CurrentMemoryContext);
 
 	if (!AllocSizeIsValid(size))
 		elog(ERROR, "invalid memory alloc request size %zu", size);
@@ -667,6 +687,7 @@ palloc0(Size size)
 	void	   *ret;
 
 	AssertArg(MemoryContextIsValid(CurrentMemoryContext));
+	AssertNotInCriticalSection(CurrentMemoryContext);
 
 	if (!AllocSizeIsValid(size))
 		elog(ERROR, "invalid memory alloc request size %zu", size);
@@ -738,6 +759,7 @@ repalloc(void *pointer, Size size)
 			   ((char *) pointer - STANDARDCHUNKHEADERSIZE))->context;
 
 	AssertArg(MemoryContextIsValid(context));
+	AssertNotInCriticalSection(context);
 
 	/* isReset must be false already */
 	Assert(!context->isReset);
@@ -760,6 +782,7 @@ MemoryContextAllocHuge(MemoryContext context, Size size)
 	void	   *ret;
 
 	AssertArg(MemoryContextIsValid(context));
+	AssertNotInCriticalSection(context);
 
 	if (!AllocHugeSizeIsValid(size))
 		elog(ERROR, "invalid memory alloc request size %zu", size);
@@ -801,6 +824,7 @@ repalloc_huge(void *pointer, Size size)
 			   ((char *) pointer - STANDARDCHUNKHEADERSIZE))->context;
 
 	AssertArg(MemoryContextIsValid(context));
+	AssertNotInCriticalSection(context);
 
 	/* isReset must be false already */
 	Assert(!context->isReset);