Allow memory contexts to have both fixed and variable ident strings.

Originally, we treated memory context names as potentially variable in
all cases, and therefore always copied them into the context header.
Commit 9fa6f00b1 rethought this a little bit and invented a distinction
between fixed and variable names, skipping the copy step for the former.
But we can make things both simpler and more useful by instead allowing
there to be two parts to a context's identification, a fixed "name" and
an optional, variable "ident".  The name supplied in the context create
call is now required to be a compile-time-constant string in all cases,
as it is never copied but just pointed to.  The "ident" string, if
wanted, is supplied later.  This is needed because typically we want
the ident to be stored inside the context so that it's cleaned up
automatically on context deletion; that means it has to be copied into
the context before we can set the pointer.

The cost of this approach is basically just an additional pointer field
in struct MemoryContextData, which isn't much overhead, and is bought
back entirely in the AllocSet case by not needing a headerSize field
anymore, since we no longer have to cope with variable header length.
In addition, we can simplify the internal interfaces for memory context
creation still further, saving a few cycles there.  And it's no longer
true that a custom identifier disqualifies a context from participating
in aset.c's freelist scheme, so possibly there's some win on that end.

All the places that were using non-compile-time-constant context names
are adjusted to put the variable info into the "ident" instead.  This
allows more effective identification of those contexts in many cases;
for example, subsidary contexts of relcache entries are now identified
by both type (e.g. "index info") and relname, where before you got only
one or the other.  Contexts associated with PL function cache entries
are now identified more fully and uniformly, too.

I also arranged for plancache contexts to use the query source string
as their identifier.  This is basically free for CachedPlanSources, as
they contained a copy of that string already.  We pay an extra pstrdup
to do it for CachedPlans.  That could perhaps be avoided, but it would
make things more fragile (since the CachedPlanSource is sometimes
destroyed first).  I suspect future improvements in error reporting will
require CachedPlans to have a copy of that string anyway, so it's not
clear that it's worth moving mountains to avoid it now.

This also changes the APIs for context statistics routines so that the
context-specific routines no longer assume that output goes straight
to stderr, nor do they know all details of the output format.  This
is useful immediately to reduce code duplication, and it also allows
for external code to do something with stats output that's different
from printing to stderr.

The reason for pushing this now rather than waiting for v12 is that
it rethinks some of the API changes made by commit 9fa6f00b1.  Seems
better for extension authors to endure just one round of API changes
not two.

Discussion: https://postgr.es/m/CAB=Je-FdtmFZ9y9REHD7VsSrnCkiBhsA4mdsLKSPauwXtQBeNA@mail.gmail.com

src/backend/utils/mmgr/aset.c

@@ -130,7 +130,6 @@ typedef struct AllocSetContext
 	Size		initBlockSize;	/* initial block size */
 	Size		maxBlockSize;	/* maximum block size */
 	Size		nextBlockSize;	/* next block size to allocate */
-	Size		headerSize;		/* allocated size of context header */
 	Size		allocChunkLimit;	/* effective chunk size limit */
 	AllocBlock	keeper;			/* keep this block over resets */
 	/* freelist this context could be put in, or -1 if not a candidate: */
@@ -228,10 +227,7 @@ typedef struct AllocChunkData
  * only its initial malloc chunk and no others.  To be a candidate for a
  * freelist, a context must have the same minContextSize/initBlockSize as
  * other contexts in the list; but its maxBlockSize is irrelevant since that
- * doesn't affect the size of the initial chunk.  Also, candidate contexts
- * *must not* use MEMCONTEXT_COPY_NAME since that would make their header size
- * variable.  (We currently insist that all flags be zero, since other flags
- * would likely make the contexts less interchangeable, too.)
+ * doesn't affect the size of the initial chunk.
  *
  * We currently provide one freelist for ALLOCSET_DEFAULT_SIZES contexts
  * and one for ALLOCSET_SMALL_SIZES contexts; the latter works for
@@ -276,7 +272,8 @@ static void AllocSetReset(MemoryContext context);
 static void AllocSetDelete(MemoryContext context);
 static Size AllocSetGetChunkSpace(MemoryContext context, void *pointer);
 static bool AllocSetIsEmpty(MemoryContext context);
-static void AllocSetStats(MemoryContext context, int level, bool print,
+static void AllocSetStats(MemoryContext context,
+			  MemoryStatsPrintFunc printfunc, void *passthru,
 			  MemoryContextCounters *totals);
 
 #ifdef MEMORY_CONTEXT_CHECKING
@@ -378,14 +375,11 @@ AllocSetFreeIndex(Size size)
  *		Create a new AllocSet context.
  *
  * parent: parent context, or NULL if top-level context
- * name: name of context (for debugging only, need not be unique)
- * flags: bitmask of MEMCONTEXT_XXX option flags
+ * name: name of context (must be statically allocated)
  * minContextSize: minimum context size
  * initBlockSize: initial allocation block size
  * maxBlockSize: maximum allocation block size
  *
- * Notes: if flags & MEMCONTEXT_COPY_NAME, the name string will be copied into
- * context-lifespan storage; otherwise, it had better be statically allocated.
  * Most callers should abstract the context size parameters using a macro
  * such as ALLOCSET_DEFAULT_SIZES.  (This is now *required* when going
  * through the AllocSetContextCreate macro.)
@@ -393,13 +387,11 @@ AllocSetFreeIndex(Size size)
 MemoryContext
 AllocSetContextCreateExtended(MemoryContext parent,
 							  const char *name,
-							  int flags,
 							  Size minContextSize,
 							  Size initBlockSize,
 							  Size maxBlockSize)
 {
 	int			freeListIndex;
-	Size		headerSize;
 	Size		firstBlockSize;
 	AllocSet	set;
 	AllocBlock	block;
@@ -431,12 +423,10 @@ AllocSetContextCreateExtended(MemoryContext parent,
 	 * Check whether the parameters match either available freelist.  We do
 	 * not need to demand a match of maxBlockSize.
 	 */
-	if (flags == 0 &&
-		minContextSize == ALLOCSET_DEFAULT_MINSIZE &&
+	if (minContextSize == ALLOCSET_DEFAULT_MINSIZE &&
 		initBlockSize == ALLOCSET_DEFAULT_INITSIZE)
 		freeListIndex = 0;
-	else if (flags == 0 &&
-			 minContextSize == ALLOCSET_SMALL_MINSIZE &&
+	else if (minContextSize == ALLOCSET_SMALL_MINSIZE &&
 			 initBlockSize == ALLOCSET_SMALL_INITSIZE)
 		freeListIndex = 1;
 	else
@@ -462,25 +452,17 @@ AllocSetContextCreateExtended(MemoryContext parent,
 			/* Reinitialize its header, installing correct name and parent */
 			MemoryContextCreate((MemoryContext) set,
 								T_AllocSetContext,
-								set->headerSize,
-								sizeof(AllocSetContext),
 								&AllocSetMethods,
 								parent,
-								name,
-								flags);
+								name);
 
 			return (MemoryContext) set;
 		}
 	}
 
-	/* Size of the memory context header, including name storage if needed */
-	if (flags & MEMCONTEXT_COPY_NAME)
-		headerSize = MAXALIGN(sizeof(AllocSetContext) + strlen(name) + 1);
-	else
-		headerSize = MAXALIGN(sizeof(AllocSetContext));
-
 	/* Determine size of initial block */
-	firstBlockSize = headerSize + ALLOC_BLOCKHDRSZ + ALLOC_CHUNKHDRSZ;
+	firstBlockSize = MAXALIGN(sizeof(AllocSetContext)) +
+		ALLOC_BLOCKHDRSZ + ALLOC_CHUNKHDRSZ;
 	if (minContextSize != 0)
 		firstBlockSize = Max(firstBlockSize, minContextSize);
 	else
@@ -508,7 +490,7 @@ AllocSetContextCreateExtended(MemoryContext parent,
 	 */
 
 	/* Fill in the initial block's block header */
-	block = (AllocBlock) (((char *) set) + headerSize);
+	block = (AllocBlock) (((char *) set) + MAXALIGN(sizeof(AllocSetContext)));
 	block->aset = set;
 	block->freeptr = ((char *) block) + ALLOC_BLOCKHDRSZ;
 	block->endptr = ((char *) set) + firstBlockSize;
@@ -529,7 +511,6 @@ AllocSetContextCreateExtended(MemoryContext parent,
 	set->initBlockSize = initBlockSize;
 	set->maxBlockSize = maxBlockSize;
 	set->nextBlockSize = initBlockSize;
-	set->headerSize = headerSize;
 	set->freeListIndex = freeListIndex;
 
 	/*
@@ -559,12 +540,9 @@ AllocSetContextCreateExtended(MemoryContext parent,
 	/* Finally, do the type-independent part of context creation */
 	MemoryContextCreate((MemoryContext) set,
 						T_AllocSetContext,
-						headerSize,
-						sizeof(AllocSetContext),
 						&AllocSetMethods,
 						parent,
-						name,
-						flags);
+						name);
 
 	return (MemoryContext) set;
 }
@@ -1327,12 +1305,13 @@ AllocSetIsEmpty(MemoryContext context)
  * AllocSetStats
  *		Compute stats about memory consumption of an allocset.
  *
- * level: recursion level (0 at top level); used for print indentation.
- * print: true to print stats to stderr.
- * totals: if not NULL, add stats about this allocset into *totals.
+ * printfunc: if not NULL, pass a human-readable stats string to this.
+ * passthru: pass this pointer through to printfunc.
+ * totals: if not NULL, add stats about this context into *totals.
  */
 static void
-AllocSetStats(MemoryContext context, int level, bool print,
+AllocSetStats(MemoryContext context,
+			  MemoryStatsPrintFunc printfunc, void *passthru,
 			  MemoryContextCounters *totals)
 {
 	AllocSet	set = (AllocSet) context;
@@ -1344,7 +1323,7 @@ AllocSetStats(MemoryContext context, int level, bool print,
 	int			fidx;
 
 	/* Include context header in totalspace */
-	totalspace = set->headerSize;
+	totalspace = MAXALIGN(sizeof(AllocSetContext));
 
 	for (block = set->blocks; block != NULL; block = block->next)
 	{
@@ -1364,16 +1343,15 @@ AllocSetStats(MemoryContext context, int level, bool print,
 		}
 	}
 
-	if (print)
+	if (printfunc)
 	{
-		int			i;
+		char		stats_string[200];
 
-		for (i = 0; i < level; i++)
-			fprintf(stderr, "  ");
-		fprintf(stderr,
-				"%s: %zu total in %zd blocks; %zu free (%zd chunks); %zu used\n",
-				set->header.name, totalspace, nblocks, freespace, freechunks,
-				totalspace - freespace);
+		snprintf(stats_string, sizeof(stats_string),
+				 "%zu total in %zd blocks; %zu free (%zd chunks); %zu used",
+				 totalspace, nblocks, freespace, freechunks,
+				 totalspace - freespace);
+		printfunc(context, passthru, stats_string);
 	}
 
 	if (totals)