/*-------------------------------------------------------------------------
 *
 * pg_buffercache_pages.c
 *	  display some contents of the buffer cache
 *
 *	  contrib/pg_buffercache/pg_buffercache_pages.c
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/heapam.h"
#include "catalog/pg_type.h"
#include "funcapi.h"
#include "storage/buf_internals.h"
#include "storage/bufmgr.h"
#include "utils/relcache.h"


#define NUM_BUFFERCACHE_PAGES_ELEM	8

PG_MODULE_MAGIC;

Datum		pg_buffercache_pages(PG_FUNCTION_ARGS);


/*
 * Record structure holding the to be exposed cache data.
 */
typedef struct
{
	uint32		bufferid;
	Oid			relfilenode;
	Oid			reltablespace;
	Oid			reldatabase;
	ForkNumber	forknum;
	BlockNumber blocknum;
	bool		isvalid;
	bool		isdirty;
	uint16		usagecount;
} BufferCachePagesRec;


/*
 * Function context for data persisting over repeated calls.
 */
typedef struct
{
	TupleDesc	tupdesc;
	BufferCachePagesRec *record;
} BufferCachePagesContext;


/*
 * Function returning data from the shared buffer cache - buffer number,
 * relation node/tablespace/database/blocknum and dirty indicator.
 */
PG_FUNCTION_INFO_V1(pg_buffercache_pages);

Datum
pg_buffercache_pages(PG_FUNCTION_ARGS)
{
	FuncCallContext *funcctx;
	Datum		result;
	MemoryContext oldcontext;
	BufferCachePagesContext *fctx;		/* User function context. */
	TupleDesc	tupledesc;
	HeapTuple	tuple;

	if (SRF_IS_FIRSTCALL())
	{
		int			i;
		volatile BufferDesc *bufHdr;

		funcctx = SRF_FIRSTCALL_INIT();

		/* Switch context when allocating stuff to be used in later calls */
		oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);

		/* Create a user function context for cross-call persistence */
		fctx = (BufferCachePagesContext *) palloc(sizeof(BufferCachePagesContext));

		/* Construct a tuple descriptor for the result rows. */
		tupledesc = CreateTemplateTupleDesc(NUM_BUFFERCACHE_PAGES_ELEM, false);
		TupleDescInitEntry(tupledesc, (AttrNumber) 1, "bufferid",
						   INT4OID, -1, 0);
		TupleDescInitEntry(tupledesc, (AttrNumber) 2, "relfilenode",
						   OIDOID, -1, 0);
		TupleDescInitEntry(tupledesc, (AttrNumber) 3, "reltablespace",
						   OIDOID, -1, 0);
		TupleDescInitEntry(tupledesc, (AttrNumber) 4, "reldatabase",
						   OIDOID, -1, 0);
		TupleDescInitEntry(tupledesc, (AttrNumber) 5, "relforknumber",
						   INT2OID, -1, 0);
		TupleDescInitEntry(tupledesc, (AttrNumber) 6, "relblocknumber",
						   INT8OID, -1, 0);
		TupleDescInitEntry(tupledesc, (AttrNumber) 7, "isdirty",
						   BOOLOID, -1, 0);
		TupleDescInitEntry(tupledesc, (AttrNumber) 8, "usage_count",
						   INT2OID, -1, 0);

		fctx->tupdesc = BlessTupleDesc(tupledesc);

		/* Allocate NBuffers worth of BufferCachePagesRec records. */
		fctx->record = (BufferCachePagesRec *) palloc(sizeof(BufferCachePagesRec) * NBuffers);

		/* Set max calls and remember the user function context. */
		funcctx->max_calls = NBuffers;
		funcctx->user_fctx = fctx;

		/* Return to original context when allocating transient memory */
		MemoryContextSwitchTo(oldcontext);

		/*
		 * To get a consistent picture of the buffer state, we must lock all
		 * partitions of the buffer map.  Needless to say, this is horrible
		 * for concurrency.  Must grab locks in increasing order to avoid
		 * possible deadlocks.
		 */
		for (i = 0; i < NUM_BUFFER_PARTITIONS; i++)
			LWLockAcquire(FirstBufMappingLock + i, LW_SHARED);

		/*
		 * Scan though all the buffers, saving the relevant fields in the
		 * fctx->record structure.
		 */
		for (i = 0, bufHdr = BufferDescriptors; i < NBuffers; i++, bufHdr++)
		{
			/* Lock each buffer header before inspecting. */
			LockBufHdr(bufHdr);

			fctx->record[i].bufferid = BufferDescriptorGetBuffer(bufHdr);
			fctx->record[i].relfilenode = bufHdr->tag.rnode.relNode;
			fctx->record[i].reltablespace = bufHdr->tag.rnode.spcNode;
			fctx->record[i].reldatabase = bufHdr->tag.rnode.dbNode;
			fctx->record[i].forknum = bufHdr->tag.forkNum;
			fctx->record[i].blocknum = bufHdr->tag.blockNum;
			fctx->record[i].usagecount = bufHdr->usage_count;

			if (bufHdr->flags & BM_DIRTY)
				fctx->record[i].isdirty = true;
			else
				fctx->record[i].isdirty = false;

			/* Note if the buffer is valid, and has storage created */
			if ((bufHdr->flags & BM_VALID) && (bufHdr->flags & BM_TAG_VALID))
				fctx->record[i].isvalid = true;
			else
				fctx->record[i].isvalid = false;

			UnlockBufHdr(bufHdr);
		}

		/*
		 * And release locks.  We do this in reverse order for two reasons:
		 * (1) Anyone else who needs more than one of the locks will be trying
		 * to lock them in increasing order; we don't want to release the
		 * other process until it can get all the locks it needs. (2) This
		 * avoids O(N^2) behavior inside LWLockRelease.
		 */
		for (i = NUM_BUFFER_PARTITIONS; --i >= 0;)
			LWLockRelease(FirstBufMappingLock + i);
	}

	funcctx = SRF_PERCALL_SETUP();

	/* Get the saved state */
	fctx = funcctx->user_fctx;

	if (funcctx->call_cntr < funcctx->max_calls)
	{
		uint32		i = funcctx->call_cntr;
		Datum		values[NUM_BUFFERCACHE_PAGES_ELEM];
		bool		nulls[NUM_BUFFERCACHE_PAGES_ELEM];

		values[0] = Int32GetDatum(fctx->record[i].bufferid);
		nulls[0] = false;

		/*
		 * Set all fields except the bufferid to null if the buffer is unused
		 * or not valid.
		 */
		if (fctx->record[i].blocknum == InvalidBlockNumber ||
			fctx->record[i].isvalid == false)
		{
			nulls[1] = true;
			nulls[2] = true;
			nulls[3] = true;
			nulls[4] = true;
			nulls[5] = true;
			nulls[6] = true;
			nulls[7] = true;
		}
		else
		{
			values[1] = ObjectIdGetDatum(fctx->record[i].relfilenode);
			nulls[1] = false;
			values[2] = ObjectIdGetDatum(fctx->record[i].reltablespace);
			nulls[2] = false;
			values[3] = ObjectIdGetDatum(fctx->record[i].reldatabase);
			nulls[3] = false;
			values[4] = ObjectIdGetDatum(fctx->record[i].forknum);
			nulls[4] = false;
			values[5] = Int64GetDatum((int64) fctx->record[i].blocknum);
			nulls[5] = false;
			values[6] = BoolGetDatum(fctx->record[i].isdirty);
			nulls[6] = false;
			values[7] = Int16GetDatum(fctx->record[i].usagecount);
			nulls[7] = false;
		}

		/* Build and return the tuple. */
		tuple = heap_form_tuple(fctx->tupdesc, values, nulls);
		result = HeapTupleGetDatum(tuple);

		SRF_RETURN_NEXT(funcctx, result);
	}
	else
		SRF_RETURN_DONE(funcctx);
}
