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

#include "access/htup_details.h"
#include "catalog/pg_type.h"
#include "funcapi.h"
#include "storage/buf_internals.h"
#include "storage/bufmgr.h"


#define NUM_BUFFERCACHE_PAGES_MIN_ELEM	8
#define NUM_BUFFERCACHE_PAGES_ELEM	9
#define NUM_BUFFERCACHE_SUMMARY_ELEM 5
#define NUM_BUFFERCACHE_USAGE_COUNTS_ELEM 4

PG_MODULE_MAGIC;

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

	/*
	 * An int32 is sufficiently large, as MAX_BACKENDS prevents a buffer from
	 * being pinned by too many backends and each backend will only pin once
	 * because of bufmgr.c's PrivateRefCount infrastructure.
	 */
	int32		pinning_backends;
} 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);
PG_FUNCTION_INFO_V1(pg_buffercache_summary);
PG_FUNCTION_INFO_V1(pg_buffercache_usage_counts);
PG_FUNCTION_INFO_V1(pg_buffercache_evict);

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

	if (SRF_IS_FIRSTCALL())
	{
		int			i;

		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));

		/*
		 * To smoothly support upgrades from version 1.0 of this extension
		 * transparently handle the (non-)existence of the pinning_backends
		 * column. We unfortunately have to get the result type for that... -
		 * we can't use the result type determined by the function definition
		 * without potentially crashing when somebody uses the old (or even
		 * wrong) function definition though.
		 */
		if (get_call_result_type(fcinfo, NULL, &expected_tupledesc) != TYPEFUNC_COMPOSITE)
			elog(ERROR, "return type must be a row type");

		if (expected_tupledesc->natts < NUM_BUFFERCACHE_PAGES_MIN_ELEM ||
			expected_tupledesc->natts > NUM_BUFFERCACHE_PAGES_ELEM)
			elog(ERROR, "incorrect number of output arguments");

		/* Construct a tuple descriptor for the result rows. */
		tupledesc = CreateTemplateTupleDesc(expected_tupledesc->natts);
		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);

		if (expected_tupledesc->natts == NUM_BUFFERCACHE_PAGES_ELEM)
			TupleDescInitEntry(tupledesc, (AttrNumber) 9, "pinning_backends",
							   INT4OID, -1, 0);

		fctx->tupdesc = BlessTupleDesc(tupledesc);

		/* Allocate NBuffers worth of BufferCachePagesRec records. */
		fctx->record = (BufferCachePagesRec *)
			MemoryContextAllocHuge(CurrentMemoryContext,
								   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);

		/*
		 * Scan through all the buffers, saving the relevant fields in the
		 * fctx->record structure.
		 *
		 * We don't hold the partition locks, so we don't get a consistent
		 * snapshot across all buffers, but we do grab the buffer header
		 * locks, so the information of each buffer is self-consistent.
		 */
		for (i = 0; i < NBuffers; i++)
		{
			BufferDesc *bufHdr;
			uint32		buf_state;

			bufHdr = GetBufferDescriptor(i);
			/* Lock each buffer header before inspecting. */
			buf_state = LockBufHdr(bufHdr);

			fctx->record[i].bufferid = BufferDescriptorGetBuffer(bufHdr);
			fctx->record[i].relfilenumber = BufTagGetRelNumber(&bufHdr->tag);
			fctx->record[i].reltablespace = bufHdr->tag.spcOid;
			fctx->record[i].reldatabase = bufHdr->tag.dbOid;
			fctx->record[i].forknum = BufTagGetForkNum(&bufHdr->tag);
			fctx->record[i].blocknum = bufHdr->tag.blockNum;
			fctx->record[i].usagecount = BUF_STATE_GET_USAGECOUNT(buf_state);
			fctx->record[i].pinning_backends = BUF_STATE_GET_REFCOUNT(buf_state);

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

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

			UnlockBufHdr(bufHdr, buf_state);
		}
	}

	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;
			/* unused for v1.0 callers, but the array is always long enough */
			nulls[8] = true;
		}
		else
		{
			values[1] = ObjectIdGetDatum(fctx->record[i].relfilenumber);
			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;
			/* unused for v1.0 callers, but the array is always long enough */
			values[8] = Int32GetDatum(fctx->record[i].pinning_backends);
			nulls[8] = 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);
}

Datum
pg_buffercache_summary(PG_FUNCTION_ARGS)
{
	Datum		result;
	TupleDesc	tupledesc;
	HeapTuple	tuple;
	Datum		values[NUM_BUFFERCACHE_SUMMARY_ELEM];
	bool		nulls[NUM_BUFFERCACHE_SUMMARY_ELEM];

	int32		buffers_used = 0;
	int32		buffers_unused = 0;
	int32		buffers_dirty = 0;
	int32		buffers_pinned = 0;
	int64		usagecount_total = 0;

	if (get_call_result_type(fcinfo, NULL, &tupledesc) != TYPEFUNC_COMPOSITE)
		elog(ERROR, "return type must be a row type");

	for (int i = 0; i < NBuffers; i++)
	{
		BufferDesc *bufHdr;
		uint32		buf_state;

		/*
		 * This function summarizes the state of all headers. Locking the
		 * buffer headers wouldn't provide an improved result as the state of
		 * the buffer can still change after we release the lock and it'd
		 * noticeably increase the cost of the function.
		 */
		bufHdr = GetBufferDescriptor(i);
		buf_state = pg_atomic_read_u32(&bufHdr->state);

		if (buf_state & BM_VALID)
		{
			buffers_used++;
			usagecount_total += BUF_STATE_GET_USAGECOUNT(buf_state);

			if (buf_state & BM_DIRTY)
				buffers_dirty++;
		}
		else
			buffers_unused++;

		if (BUF_STATE_GET_REFCOUNT(buf_state) > 0)
			buffers_pinned++;
	}

	memset(nulls, 0, sizeof(nulls));
	values[0] = Int32GetDatum(buffers_used);
	values[1] = Int32GetDatum(buffers_unused);
	values[2] = Int32GetDatum(buffers_dirty);
	values[3] = Int32GetDatum(buffers_pinned);

	if (buffers_used != 0)
		values[4] = Float8GetDatum((double) usagecount_total / buffers_used);
	else
		nulls[4] = true;

	/* Build and return the tuple. */
	tuple = heap_form_tuple(tupledesc, values, nulls);
	result = HeapTupleGetDatum(tuple);

	PG_RETURN_DATUM(result);
}

Datum
pg_buffercache_usage_counts(PG_FUNCTION_ARGS)
{
	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
	int			usage_counts[BM_MAX_USAGE_COUNT + 1] = {0};
	int			dirty[BM_MAX_USAGE_COUNT + 1] = {0};
	int			pinned[BM_MAX_USAGE_COUNT + 1] = {0};
	Datum		values[NUM_BUFFERCACHE_USAGE_COUNTS_ELEM];
	bool		nulls[NUM_BUFFERCACHE_USAGE_COUNTS_ELEM] = {0};

	InitMaterializedSRF(fcinfo, 0);

	for (int i = 0; i < NBuffers; i++)
	{
		BufferDesc *bufHdr = GetBufferDescriptor(i);
		uint32		buf_state = pg_atomic_read_u32(&bufHdr->state);
		int			usage_count;

		usage_count = BUF_STATE_GET_USAGECOUNT(buf_state);
		usage_counts[usage_count]++;

		if (buf_state & BM_DIRTY)
			dirty[usage_count]++;

		if (BUF_STATE_GET_REFCOUNT(buf_state) > 0)
			pinned[usage_count]++;
	}

	for (int i = 0; i < BM_MAX_USAGE_COUNT + 1; i++)
	{
		values[0] = Int32GetDatum(i);
		values[1] = Int32GetDatum(usage_counts[i]);
		values[2] = Int32GetDatum(dirty[i]);
		values[3] = Int32GetDatum(pinned[i]);

		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);
	}

	return (Datum) 0;
}

/*
 * Try to evict a shared buffer.
 */
Datum
pg_buffercache_evict(PG_FUNCTION_ARGS)
{
	Buffer		buf = PG_GETARG_INT32(0);

	if (!superuser())
		ereport(ERROR,
				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
				 errmsg("must be superuser to use pg_buffercache_evict function")));

	if (buf < 1 || buf > NBuffers)
		elog(ERROR, "bad buffer ID: %d", buf);

	PG_RETURN_BOOL(EvictUnpinnedBuffer(buf));
}
