Add pg_shmem_allocations view.

This tells you about allocations that have been made from the main
shared memory segment. The original patch also tried to show information
about dynamic shared memory allocation as well, but I decided to
leave that problem for another time.

Andres Freund and Robert Haas, reviewed by Michael Paquier, Marti
Raudsepp, Tom Lane, Álvaro Herrera, and Kyotaro Horiguchi.

Discussion: http://postgr.es/m/20140504114417.GM12715@awork2.anarazel.de

src/backend/storage/ipc/shmem.c

@@ -66,12 +66,16 @@
 #include "postgres.h"
 
 #include "access/transam.h"
+#include "fmgr.h"
+#include "funcapi.h"
 #include "miscadmin.h"
 #include "storage/lwlock.h"
 #include "storage/pg_shmem.h"
 #include "storage/shmem.h"
 #include "storage/spin.h"
+#include "utils/builtins.h"
 
+static void *ShmemAllocRaw(Size size, Size *allocated_size);
 
 /* shared memory global variables */
 
@@ -157,8 +161,9 @@ void *
 ShmemAlloc(Size size)
 {
 	void	   *newSpace;
+	Size		allocated_size;
 
-	newSpace = ShmemAllocNoError(size);
+	newSpace = ShmemAllocRaw(size, &allocated_size);
 	if (!newSpace)
 		ereport(ERROR,
 				(errcode(ERRCODE_OUT_OF_MEMORY),
@@ -174,6 +179,20 @@ ShmemAlloc(Size size)
  */
 void *
 ShmemAllocNoError(Size size)
+{
+	Size		allocated_size;
+
+	return ShmemAllocRaw(size, &allocated_size);
+}
+
+/*
+ * ShmemAllocRaw -- allocate align chunk and return allocated size
+ *
+ * Also sets *allocated_size to the number of bytes allocated, which will
+ * be equal to the number requested plus any padding we choose to add.
+ */
+static void *
+ShmemAllocRaw(Size size, Size *allocated_size)
 {
 	Size		newStart;
 	Size		newFree;
@@ -191,6 +210,7 @@ ShmemAllocNoError(Size size)
 	 * won't be sufficient.
 	 */
 	size = CACHELINEALIGN(size);
+	*allocated_size = size;
 
 	Assert(ShmemSegHdr != NULL);
 
@@ -441,8 +461,10 @@ ShmemInitStruct(const char *name, Size size, bool *foundPtr)
 	}
 	else
 	{
+		Size	allocated_size;
+
 		/* It isn't in the table yet. allocate and initialize it */
-		structPtr = ShmemAllocNoError(size);
+		structPtr = ShmemAllocRaw(size, &allocated_size);
 		if (structPtr == NULL)
 		{
 			/* out of memory; remove the failed ShmemIndex entry */
@@ -455,6 +477,7 @@ ShmemInitStruct(const char *name, Size size, bool *foundPtr)
 							name, size)));
 		}
 		result->size = size;
+		result->allocated_size = allocated_size;
 		result->location = structPtr;
 	}
 
@@ -503,3 +526,82 @@ mul_size(Size s1, Size s2)
 				 errmsg("requested shared memory size overflows size_t")));
 	return result;
 }
+
+/* SQL SRF showing allocated shared memory */
+Datum
+pg_get_shmem_allocations(PG_FUNCTION_ARGS)
+{
+#define PG_GET_SHMEM_SIZES_COLS 4
+	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
+	TupleDesc	tupdesc;
+	Tuplestorestate *tupstore;
+	MemoryContext per_query_ctx;
+	MemoryContext oldcontext;
+	HASH_SEQ_STATUS hstat;
+	ShmemIndexEnt *ent;
+	Size	named_allocated = 0;
+	Datum		values[PG_GET_SHMEM_SIZES_COLS];
+	bool		nulls[PG_GET_SHMEM_SIZES_COLS];
+
+	/* check to see if caller supports us returning a tuplestore */
+	if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("set-valued function called in context that cannot accept a set")));
+	if (!(rsinfo->allowedModes & SFRM_Materialize))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("materialize mode required, but it is not allowed in this context")));
+
+	/* Build a tuple descriptor for our result type */
+	if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
+		elog(ERROR, "return type must be a row type");
+
+	per_query_ctx = rsinfo->econtext->ecxt_per_query_memory;
+	oldcontext = MemoryContextSwitchTo(per_query_ctx);
+
+	tupstore = tuplestore_begin_heap(true, false, work_mem);
+	rsinfo->returnMode = SFRM_Materialize;
+	rsinfo->setResult = tupstore;
+	rsinfo->setDesc = tupdesc;
+
+	MemoryContextSwitchTo(oldcontext);
+
+	LWLockAcquire(ShmemIndexLock, LW_SHARED);
+
+	hash_seq_init(&hstat, ShmemIndex);
+
+	/* output all allocated entries */
+	memset(nulls, 0, sizeof(nulls));
+	while ((ent = (ShmemIndexEnt *) hash_seq_search(&hstat)) != NULL)
+	{
+		values[0] = CStringGetTextDatum(ent->key);
+		values[1] = Int64GetDatum((char *) ent->location - (char *) ShmemSegHdr);
+		values[2] = Int64GetDatum(ent->size);
+		values[3] = Int64GetDatum(ent->allocated_size);
+		named_allocated += ent->allocated_size;
+
+		tuplestore_putvalues(tupstore, tupdesc, values, nulls);
+	}
+
+	/* output shared memory allocated but not counted via the shmem index */
+	values[0] = CStringGetTextDatum("<anonymous>");
+	nulls[1] = true;
+	values[2] = Int64GetDatum(ShmemSegHdr->freeoffset - named_allocated);
+	values[3] = values[2];
+	tuplestore_putvalues(tupstore, tupdesc, values, nulls);
+
+	/* output as-of-yet unused shared memory */
+	nulls[0] = true;
+	values[1] = Int64GetDatum(ShmemSegHdr->freeoffset);
+	nulls[1] = false;
+	values[2] = Int64GetDatum(ShmemSegHdr->totalsize - ShmemSegHdr->freeoffset);
+	values[3] = values[2];
+	tuplestore_putvalues(tupstore, tupdesc, values, nulls);
+
+	LWLockRelease(ShmemIndexLock);
+
+	tuplestore_donestoring(tupstore);
+
+	return (Datum) 0;
+}