pglogical_output_plugin.c

/*-------------------------------------------------------------------------
 *
 * pglogical_output.c
 *		  Logical Replication output plugin
 *
 * Copyright (c) 2012-2015, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *		  pglogical_output.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
#include "pglogical_output_plugin.h"

#include "mb/pg_wchar.h"
#include "replication/logical.h"
#ifdef HAVE_REPLICATION_ORIGINS
#include "replication/origin.h"
#endif

#include "access/xact.h"
#include "executor/executor.h"
#include "catalog/namespace.h"
#include "utils/inval.h"
#include "utils/memutils.h"
#include "utils/rel.h"

#include "pglogical_output_config.h"
#include "pglogical_executor.h"
#include "pglogical_node.h"
#include "pglogical_output_proto.h"
#include "pglogical_queue.h"
#include "pglogical_repset.h"

extern void		_PG_output_plugin_init(OutputPluginCallbacks *cb);

static void pg_decode_startup(LogicalDecodingContext * ctx,
							  OutputPluginOptions *opt, bool is_init);
static void pg_decode_shutdown(LogicalDecodingContext * ctx);
static void pg_decode_begin_txn(LogicalDecodingContext *ctx,
					ReorderBufferTXN *txn);
static void pg_decode_commit_txn(LogicalDecodingContext *ctx,
					 ReorderBufferTXN *txn, XLogRecPtr commit_lsn);
static void pg_decode_change(LogicalDecodingContext *ctx,
				 ReorderBufferTXN *txn, Relation rel,
				 ReorderBufferChange *change);

#ifdef HAVE_REPLICATION_ORIGINS
static bool pg_decode_origin_filter(LogicalDecodingContext *ctx,
						RepOriginId origin_id);
#endif

static void send_startup_message(LogicalDecodingContext *ctx,
		PGLogicalOutputData *data, bool last_message);

static bool startup_message_sent = false;

typedef struct PGLRelMetaCacheEntry
{
	Oid relid;
	/* Does the client have this relation cached? */
	bool is_cached;
	/* Entry is valid and not due to be purged */
	bool is_valid;
} PGLRelMetaCacheEntry;

#define RELMETACACHE_INITIAL_SIZE 128
static HTAB *RelMetaCache = NULL;
static int InvalidRelMetaCacheCnt = 0;

static void relmetacache_init(MemoryContext decoding_context);
static PGLRelMetaCacheEntry *relmetacache_get_relation(PGLogicalOutputData *data,
													   Relation rel);
static void relmetacache_destroy(void);
static void relmetacache_prune(void);


/* specify output plugin callbacks */
void
_PG_output_plugin_init(OutputPluginCallbacks *cb)
{
	AssertVariableIsOfType(&_PG_output_plugin_init, LogicalOutputPluginInit);

	cb->startup_cb = pg_decode_startup;
	cb->begin_cb = pg_decode_begin_txn;
	cb->change_cb = pg_decode_change;
	cb->commit_cb = pg_decode_commit_txn;
#ifdef HAVE_REPLICATION_ORIGINS
	cb->filter_by_origin_cb = pg_decode_origin_filter;
#endif
	cb->shutdown_cb = pg_decode_shutdown;
}

static bool
check_binary_compatibility(PGLogicalOutputData *data)
{
	if (data->client_binary_basetypes_major_version != PG_VERSION_NUM / 100)
		return false;

	if (data->client_binary_bigendian_set
		&& data->client_binary_bigendian != server_bigendian())
	{
		elog(DEBUG1, "Binary mode rejected: Server and client endian mismatch");
		return false;
	}

	if (data->client_binary_sizeofdatum != 0
		&& data->client_binary_sizeofdatum != sizeof(Datum))
	{
		elog(DEBUG1, "Binary mode rejected: Server and client sizeof(Datum) mismatch");
		return false;
	}

	if (data->client_binary_sizeofint != 0
		&& data->client_binary_sizeofint != sizeof(int))
	{
		elog(DEBUG1, "Binary mode rejected: Server and client sizeof(int) mismatch");
		return false;
	}

	if (data->client_binary_sizeoflong != 0
		&& data->client_binary_sizeoflong != sizeof(long))
	{
		elog(DEBUG1, "Binary mode rejected: Server and client sizeof(long) mismatch");
		return false;
	}

	if (data->client_binary_float4byval_set
		&& data->client_binary_float4byval != server_float4_byval())
	{
		elog(DEBUG1, "Binary mode rejected: Server and client float4byval mismatch");
		return false;
	}

	if (data->client_binary_float8byval_set
		&& data->client_binary_float8byval != server_float8_byval())
	{
		elog(DEBUG1, "Binary mode rejected: Server and client float8byval mismatch");
		return false;
	}

	if (data->client_binary_intdatetimes_set
		&& data->client_binary_intdatetimes != server_integer_datetimes())
	{
		elog(DEBUG1, "Binary mode rejected: Server and client integer datetimes mismatch");
		return false;
	}

	return true;
}

/* initialize this plugin */
static void
pg_decode_startup(LogicalDecodingContext * ctx, OutputPluginOptions *opt,
				  bool is_init)
{
	PGLogicalOutputData	   *data = palloc0(sizeof(PGLogicalOutputData));

	data->context = AllocSetContextCreate(ctx->context,
										  "pglogical conversion context",
										  ALLOCSET_DEFAULT_MINSIZE,
										  ALLOCSET_DEFAULT_INITSIZE,
										  ALLOCSET_DEFAULT_MAXSIZE);
	data->allow_internal_basetypes = false;
	data->allow_binary_basetypes = false;


	ctx->output_plugin_private = data;

	/*
	 * This is replication start and not slot initialization.
	 *
	 * Parse and validate options passed by the client.
	 */
	if (!is_init)
	{
		int		params_format;
		bool	started_tx = false;
		PGLogicalLocalNode *node;
		MemoryContext oldctx;

		if (!IsTransactionState())
		{
			StartTransactionCommand();
			started_tx = true;
		}
		node = get_local_node(false, false);
		data->local_node_id	= node->node->id;

		 /*
		 * Ideally we'd send the startup message immediately. That way
		 * it'd arrive before any error we emit if we see incompatible
		 * options sent by the client here. That way the client could
		 * possibly adjust its options and reconnect. It'd also make
		 * sure the client gets the startup message in a timely way if
		 * the server is idle, since otherwise it could be a while
		 * before the next callback.
		 *
		 * The decoding plugin API doesn't let us write to the stream
		 * from here, though, so we have to delay the startup message
		 * until the first change processed on the stream, in a begin
		 * callback.
		 *
		 * If we ERROR there, the startup message is buffered but not
		 * sent since the callback didn't finish. So we'd have to send
		 * the startup message, finish the callback and check in the
		 * next callback if we need to ERROR.
		 *
		 * That's a bit much hoop jumping, so for now ERRORs are
		 * immediate. A way to emit a message from the startup callback
		 * is really needed to change that.
		 */
		startup_message_sent = false;

		/* Now parse the rest of the params and ERROR if we see any we don't recognise */
		oldctx = MemoryContextSwitchTo(ctx->context);
		params_format = process_parameters(ctx->output_plugin_options, data);
		MemoryContextSwitchTo(oldctx);

		if (params_format != 1)
			ereport(ERROR,
				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
				 errmsg("client sent startup parameters in format %d but we only support format 1",
					params_format)));

		if (data->client_min_proto_version > PGLOGICAL_PROTO_VERSION_NUM)
			ereport(ERROR,
				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
				 errmsg("client sent min_proto_version=%d but we only support protocol %d or lower",
					 data->client_min_proto_version, PGLOGICAL_PROTO_VERSION_NUM)));

		if (data->client_max_proto_version < PGLOGICAL_PROTO_MIN_VERSION_NUM)
			ereport(ERROR,
				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
				 errmsg("client sent max_proto_version=%d but we only support protocol %d or higher",
				 	data->client_max_proto_version, PGLOGICAL_PROTO_MIN_VERSION_NUM)));

		/*
		 * Set correct protocol format.
		 *
		 * This is the output plugin protocol format, this is different
		 * from the individual fields binary vs textual format.
		 */
		if (data->client_protocol_format != NULL
				&& strcmp(data->client_protocol_format, "json") == 0)
		{
			oldctx = MemoryContextSwitchTo(ctx->context);
			data->api = pglogical_init_api(PGLogicalProtoJson);
			opt->output_type = OUTPUT_PLUGIN_TEXTUAL_OUTPUT;
			MemoryContextSwitchTo(oldctx);
		}
		else if ((data->client_protocol_format != NULL
			     && strcmp(data->client_protocol_format, "native") == 0)
				 || data->client_protocol_format == NULL)
		{
			oldctx = MemoryContextSwitchTo(ctx->context);
			data->api = pglogical_init_api(PGLogicalProtoNative);
			opt->output_type = OUTPUT_PLUGIN_BINARY_OUTPUT;

			if (data->client_no_txinfo)
			{
				elog(WARNING, "no_txinfo option ignored for protocols other than json");
				data->client_no_txinfo = false;
			}
			MemoryContextSwitchTo(oldctx);
		}
		else
		{
			ereport(ERROR,
				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
				 errmsg("client requested protocol %s but only \"json\" or \"native\" are supported",
				 	data->client_protocol_format)));
		}

		/* check for encoding match if specific encoding demanded by client */
		if (data->client_expected_encoding != NULL
				&& strlen(data->client_expected_encoding) != 0)
		{
			int wanted_encoding = pg_char_to_encoding(data->client_expected_encoding);

			if (wanted_encoding == -1)
				ereport(ERROR,
						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
						 errmsg("unrecognised encoding name %s passed to expected_encoding",
								data->client_expected_encoding)));

			if (opt->output_type == OUTPUT_PLUGIN_TEXTUAL_OUTPUT)
			{
				/*
				 * datum encoding must match assigned client_encoding in text
				 * proto, since everything is subject to client_encoding
				 * conversion.
				 */
				if (wanted_encoding != pg_get_client_encoding())
					ereport(ERROR,
							(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
							 errmsg("expected_encoding must be unset or match client_encoding in text protocols")));
			}
			else
			{
				/*
				 * currently in the binary protocol we can only emit encoded
				 * datums in the server encoding. There's no support for encoding
				 * conversion.
				 */
				if (wanted_encoding != GetDatabaseEncoding())
					ereport(ERROR,
							(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
							 errmsg("encoding conversion for binary datum not supported yet"),
							 errdetail("expected_encoding %s must be unset or match server_encoding %s",
								 data->client_expected_encoding, GetDatabaseEncodingName())));
			}

			data->field_datum_encoding = wanted_encoding;
		}

		/*
		 * It's obviously not possible to send binary representation of data
		 * unless we use the binary output.
		 */
		if (opt->output_type == OUTPUT_PLUGIN_BINARY_OUTPUT &&
			data->client_want_internal_basetypes)
		{
			data->allow_internal_basetypes =
				check_binary_compatibility(data);
		}

		if (opt->output_type == OUTPUT_PLUGIN_BINARY_OUTPUT &&
			data->client_want_binary_basetypes &&
			data->client_binary_basetypes_major_version == PG_VERSION_NUM / 100)
		{
			data->allow_binary_basetypes = true;
		}

		/*
		 * 9.4 lacks origins info so don't forward it.
		 *
		 * There's currently no knob for clients to use to suppress
		 * this info and it's sent if it's supported and available.
		 */
		if (PG_VERSION_NUM/100 == 904)
			data->forward_changeset_origins = false;
		else
			data->forward_changeset_origins = true;

		if (started_tx)
			CommitTransactionCommand();

		relmetacache_init(ctx->context);
	}
}

/*
 * BEGIN callback
 */
static void
pg_decode_begin_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn)
{
	PGLogicalOutputData* data = (PGLogicalOutputData*)ctx->output_plugin_private;
	bool send_replication_origin = data->forward_changeset_origins;

	if (!startup_message_sent)
		send_startup_message(ctx, data, false /* can't be last message */);

#ifdef HAVE_REPLICATION_ORIGINS
	/* If the record didn't originate locally, send origin info */
	send_replication_origin &= txn->origin_id != InvalidRepOriginId;
#endif

	OutputPluginPrepareWrite(ctx, !send_replication_origin);
	data->api->write_begin(ctx->out, data, txn);

#ifdef HAVE_REPLICATION_ORIGINS
	if (send_replication_origin)
	{
		char *origin;

		/* Message boundary */
		OutputPluginWrite(ctx, false);
		OutputPluginPrepareWrite(ctx, true);

		/*
		 * XXX: which behaviour we want here?
		 *
		 * Alternatives:
		 *  - don't send origin message if origin name not found
		 *    (that's what we do now)
		 *  - throw error - that will break replication, not good
		 *  - send some special "unknown" origin
		 */
		if (data->api->write_origin &&
			replorigin_by_oid(txn->origin_id, true, &origin))
			data->api->write_origin(ctx->out, origin, txn->origin_lsn);
	}
#endif

	OutputPluginWrite(ctx, true);
}

/*
 * COMMIT callback
 */
static void
pg_decode_commit_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
					 XLogRecPtr commit_lsn)
{
	PGLogicalOutputData* data = (PGLogicalOutputData*)ctx->output_plugin_private;

	OutputPluginPrepareWrite(ctx, true);
	data->api->write_commit(ctx->out, data, txn, commit_lsn);
	OutputPluginWrite(ctx, true);

	/*
	 * Now is a good time to get rid of invalidated relation
	 * metadata entries since nothing will be referencing them
	 * at the moment.
	 */
	relmetacache_prune();
}

static bool
pglogical_change_filter(PGLogicalOutputData *data, Relation relation,
						ReorderBufferChange *change, Bitmapset **att_list)
{
	PGLogicalTableRepInfo *tblinfo;
	ListCell	   *lc;

	if (data->replicate_only_table)
	{
		/*
		 * Special case - we are catching up just one table.
		 * TODO: performance
		 */
		return strcmp(RelationGetRelationName(relation),
					  data->replicate_only_table->relname) == 0 &&
			RelationGetNamespace(relation) ==
				get_namespace_oid(data->replicate_only_table->schemaname, true);
	}
	else if (RelationGetRelid(relation) == get_queue_table_oid())
	{
		/* Special case - queue table */
		if (change->action == REORDER_BUFFER_CHANGE_INSERT)
		{
			HeapTuple		tup = &change->data.tp.newtuple->tuple;
			QueuedMessage  *q = queued_message_from_tuple(tup);
			ListCell	   *qlc;

			/*
			 * No replication set means global message, those are always
			 * replicated.
			 */
			if (q->replication_sets == NULL)
				return true;

			foreach (qlc, q->replication_sets)
			{
				char	   *queue_set = (char *) lfirst(qlc);
				ListCell   *plc;

				foreach (plc, data->replication_sets)
				{
					PGLogicalRepSet	   *rs = lfirst(plc);

					/* TODO: this is somewhat ugly. */
					if (strcmp(queue_set, rs->name) == 0 &&
						(q->message_type != QUEUE_COMMAND_TYPE_TRUNCATE ||
						 rs->replicate_truncate))
						return true;
				}
			}
		}

		return false;
	}
	else if (RelationGetRelid(relation) == get_replication_set_rel_oid())
	{
		/*
		 * Special case - replication set table.
		 *
		 * We can use this to update our cached replication set info, without
		 * having to deal with cache invalidation callbacks.
		 */
		HeapTuple			tup;
		PGLogicalRepSet	   *replicated_set;
		ListCell		   *plc;

		if (change->action == REORDER_BUFFER_CHANGE_UPDATE)
			 tup = &change->data.tp.newtuple->tuple;
		else if (change->action == REORDER_BUFFER_CHANGE_DELETE)
			 tup = &change->data.tp.oldtuple->tuple;
		else
			return false;

		replicated_set = replication_set_from_tuple(tup);
		foreach (plc, data->replication_sets)
		{
			PGLogicalRepSet	   *rs = lfirst(plc);

			/* Check if the changed repset is used by us. */
			if (rs->id == replicated_set->id)
			{
				/*
				 * In case this was delete, somebody deleted one of our
				 * rep sets, bail here and let reconnect logic handle any
				 * potential issues.
				 */
				if (change->action == REORDER_BUFFER_CHANGE_DELETE)
					elog(ERROR, "replication set \"%s\" used by this connection was deleted, existing",
						 rs->name);

				/* This was update of our repset, update the cache. */
				rs->replicate_insert = replicated_set->replicate_insert;
				rs->replicate_update = replicated_set->replicate_update;
				rs->replicate_delete = replicated_set->replicate_delete;
				rs->replicate_truncate = replicated_set->replicate_truncate;

				return false;
			}
		}

		return false;
	}

	/* Normal case - use replication set membership. */
	tblinfo = get_table_replication_info(data->local_node_id, relation,
										 data->replication_sets);

	/* First try filter out by change type. */
	switch (change->action)
	{
		case REORDER_BUFFER_CHANGE_INSERT:
			if (!tblinfo->replicate_insert)
				return false;
			break;
		case REORDER_BUFFER_CHANGE_UPDATE:
			if (!tblinfo->replicate_update)
				return false;
			break;
		case REORDER_BUFFER_CHANGE_DELETE:
			if (!tblinfo->replicate_delete)
				return false;
			break;
		default:
			elog(ERROR, "Unhandled reorder buffer change type %d",
				 change->action);
			return false; /* shut compiler up */
	}

	/*
	 * Proccess row filters.
	 * XXX: we could probably cache some of the executor stuff.
	 */
	if (list_length(tblinfo->row_filter) > 0)
	{
		EState		   *estate;
		ExprContext	   *econtext;
		TupleDesc		tupdesc = RelationGetDescr(relation);
		HeapTuple		oldtup = change->data.tp.oldtuple ?
			&change->data.tp.oldtuple->tuple : NULL;
		HeapTuple		newtup = change->data.tp.newtuple ?
			&change->data.tp.newtuple->tuple : NULL;

		/* Skip empty changes. */
		if (!newtup && !oldtup)
		{
			elog(DEBUG1, "pglogical output got empty change");
			return false;
		}

		estate = create_estate_for_relation(relation, false);
		econtext = prepare_per_tuple_econtext(estate, tupdesc);

		ExecStoreTuple(newtup ? newtup : oldtup, econtext->ecxt_scantuple,
					   InvalidBuffer, false);

		/* Next try the row_filters if there are any. */
		foreach (lc, tblinfo->row_filter)
		{
			Node	   *row_filter = (Node *) lfirst(lc);
			ExprState  *exprstate = pglogical_prepare_row_filter(row_filter);
			Datum		res;
			bool		isnull;

			res = ExecEvalExpr(exprstate, econtext, &isnull, NULL);

			/* NULL is same as false for our use. */
			if (isnull)
				return false;

			if (!DatumGetBool(res))
				return false;
		}

		ExecDropSingleTupleTableSlot(econtext->ecxt_scantuple);
		FreeExecutorState(estate);
	}

	/* Make sure caller is aware of any attribute filter. */
	*att_list = tblinfo->att_list;

	return true;
}

static void
pg_decode_change(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
				 Relation relation, ReorderBufferChange *change)
{
	PGLogicalOutputData *data = ctx->output_plugin_private;
	MemoryContext	old;
	Bitmapset	   *att_list = NULL;

	/* First check the table filter */
	if (!pglogical_change_filter(data, relation, change, &att_list))
		return;

	/* Avoid leaking memory by using and resetting our own context */
	old = MemoryContextSwitchTo(data->context);

	/*
	 * If the protocol wants to write relation information and the client
	 * isn't known to have metadata cached for this relation already,
	 * send relation metadata.
	 *
	 * TODO: track hit/miss stats
	 */
	if (data->api->write_rel != NULL)
	{
		PGLRelMetaCacheEntry *cached_relmeta;
		cached_relmeta = relmetacache_get_relation(data, relation);

		if (!cached_relmeta->is_cached)
		{
			OutputPluginPrepareWrite(ctx, false);
			data->api->write_rel(ctx->out, data, relation, att_list);
			OutputPluginWrite(ctx, false);
			cached_relmeta->is_cached = true;
		}
	}

	/* Send the data */
	switch (change->action)
	{
		case REORDER_BUFFER_CHANGE_INSERT:
			OutputPluginPrepareWrite(ctx, true);
			data->api->write_insert(ctx->out, data, relation,
									&change->data.tp.newtuple->tuple,
									att_list);
			OutputPluginWrite(ctx, true);
			break;
		case REORDER_BUFFER_CHANGE_UPDATE:
			{
				HeapTuple oldtuple = change->data.tp.oldtuple ?
					&change->data.tp.oldtuple->tuple : NULL;

				OutputPluginPrepareWrite(ctx, true);
				data->api->write_update(ctx->out, data, relation, oldtuple,
										&change->data.tp.newtuple->tuple,
										att_list);
				OutputPluginWrite(ctx, true);
				break;
			}
		case REORDER_BUFFER_CHANGE_DELETE:
			if (change->data.tp.oldtuple)
			{
				OutputPluginPrepareWrite(ctx, true);
				data->api->write_delete(ctx->out, data, relation,
										&change->data.tp.oldtuple->tuple,
										att_list);
				OutputPluginWrite(ctx, true);
			}
			else
				elog(DEBUG1, "didn't send DELETE change because of missing oldtuple");
			break;
		default:
			Assert(false);
	}

	/* Cleanup */
	MemoryContextSwitchTo(old);
	MemoryContextReset(data->context);
}

#ifdef HAVE_REPLICATION_ORIGINS
/*
 * Decide if the whole transaction with specific origin should be filtered out.
 */
static bool
pg_decode_origin_filter(LogicalDecodingContext *ctx,
						RepOriginId origin_id)
{
	PGLogicalOutputData *data = ctx->output_plugin_private;
	bool ret;

	if (origin_id == InvalidRepOriginId)
	    /* Never filter out locally originated tx's */
	    ret = false;

	else
		/*
		 * Otherwise, ignore the origin passed in txnfilter_args->origin_id,
		 * and just forward all or nothing based on the configuration option
		 * 'forward_origins'.
		 */
		ret = list_length(data->forward_origins) == 0;

	return ret;
}
#endif

static void
send_startup_message(LogicalDecodingContext *ctx,
		PGLogicalOutputData *data, bool last_message)
{
	List *msg;

	Assert(!startup_message_sent);

	msg = prepare_startup_message(data);

	/*
	 * We could free the extra_startup_params DefElem list here, but it's
	 * pretty harmless to just ignore it, since it's in the decoding memory
	 * context anyway, and we don't know if it's safe to free the defnames or
	 * not.
	 */

	OutputPluginPrepareWrite(ctx, last_message);
	data->api->write_startup_message(ctx->out, msg);
	OutputPluginWrite(ctx, last_message);

	list_free_deep(msg);

	startup_message_sent = true;
}


/*
 * Shutdown callback.
 */
static void
pg_decode_shutdown(LogicalDecodingContext * ctx)
{
	relmetacache_destroy();

	/*
	 * no need to delete data->context as it's child of ctx->context which
	 * will expire on return.
	 */
}


/*
 * Relation metadata invalidation, for when a relcache invalidation
 * means that we need to resend table metadata to the client.
 */
static void
relmetacache_invalidation_cb(Datum arg, Oid relid)
 {
	struct PGLRelMetaCacheEntry *hentry;
	Assert (RelMetaCache != NULL);

	/*
	 * Nobody keeps pointers to entries in this hash table around outside
	 * logical decoding callback calls - but invalidation events can come in
	 * *during* a callback if we access the relcache in the callback. Because
	 * of that we must mark the cache entry as invalid but not remove it from
	 * the hash while it could still be referenced, then prune it at a later
	 * safe point.
	 *
	 * Getting invalidations for relations that aren't in the table is
	 * entirely normal, since there's no way to unregister for an
	 * invalidation event. So we don't care if it's found or not.
	 */
	hentry = (struct PGLRelMetaCacheEntry *)
		hash_search(RelMetaCache, &relid, HASH_FIND, NULL);

	if (hentry != NULL)
	{
		hentry->is_valid = false;
		InvalidRelMetaCacheCnt++;
	}
}

/*
 * Initialize the relation metadata cache for a decoding session.
 *
 * The hash table is destoyed at the end of a decoding session. While
 * relcache invalidations still exist and will still be invoked, they
 * will just see the null hash table global and take no action.
 */
static void
relmetacache_init(MemoryContext decoding_context)
{
	HASHCTL	ctl;
	int		hash_flags;

	InvalidRelMetaCacheCnt = 0;

	if (RelMetaCache == NULL)
	{
		MemoryContext old_ctxt;

		/* Make a new hash table for the cache */
		hash_flags = HASH_ELEM | HASH_CONTEXT;

		MemSet(&ctl, 0, sizeof(ctl));
		ctl.keysize = sizeof(Oid);
		ctl.entrysize = sizeof(struct PGLRelMetaCacheEntry);
		ctl.hcxt = TopMemoryContext;

#if PG_VERSION_NUM >= 90500
		hash_flags |= HASH_BLOBS;
#else
		ctl.hash = tag_hash;
		hash_flags |= HASH_FUNCTION;
#endif

		old_ctxt = MemoryContextSwitchTo(TopMemoryContext);
		RelMetaCache = hash_create("pglogical relation metadata cache",
								   RELMETACACHE_INITIAL_SIZE,
								   &ctl, hash_flags);
		(void) MemoryContextSwitchTo(old_ctxt);

		Assert(RelMetaCache != NULL);

		CacheRegisterRelcacheCallback(relmetacache_invalidation_cb, (Datum)0);
	}
}


/*
 * Look up an entry, creating it if not found.
 *
 * Newly created entries are returned as is_cached=false. The API
 * hook can set is_cached to skip subsequent updates if it sent a
 * complete response that the client will cache.
 *
 * Returns true on a cache hit, false on a miss.
 */
static PGLRelMetaCacheEntry *
relmetacache_get_relation(struct PGLogicalOutputData *data,
						  Relation rel)
{
	struct PGLRelMetaCacheEntry *hentry;
	bool found;
	MemoryContext old_mctx;

	/* Find cached function info, creating if not found */
	old_mctx = MemoryContextSwitchTo(TopMemoryContext);
	hentry = (struct PGLRelMetaCacheEntry*) hash_search(RelMetaCache,
										 (void *)(&RelationGetRelid(rel)),
										 HASH_ENTER, &found);
	(void) MemoryContextSwitchTo(old_mctx);

	/* If not found or not valid, it can't be cached. */
	if (!found || !hentry->is_valid)
	{
		Assert(hentry->relid = RelationGetRelid(rel));
		hentry->is_cached = false;
		/* Only used for lazy purging of invalidations */
		hentry->is_valid = true;
	}

	Assert(hentry != NULL);

	return hentry;
}


/*
 * Flush the relation metadata cache at the end of a decoding session.
 */
static void
relmetacache_destroy(void)
{
	HASH_SEQ_STATUS status;
	struct PGLRelMetaCacheEntry *hentry;

	if (RelMetaCache != NULL)
	{
		hash_seq_init(&status, RelMetaCache);

		while ((hentry = (struct PGLRelMetaCacheEntry*) hash_seq_search(&status)) != NULL)
		{
			if (hash_search(RelMetaCache,
							(void *) &hentry->relid,
							HASH_REMOVE, NULL) == NULL)
				elog(ERROR, "hash table corrupted");
		}
	}
}

/*
 * Prune !is_valid entries from the relation metadata cache
 *
 * This must only be called when there couldn't be any references to
 * possibly-invalid entries.
 */
static void
relmetacache_prune(void)
{
	HASH_SEQ_STATUS status;
	struct PGLRelMetaCacheEntry *hentry;

	/*
	 * Since the pruning can be expensive, do it only if ig we invalidated
	 * at least half of initial cache size.
	 */
	if (InvalidRelMetaCacheCnt < RELMETACACHE_INITIAL_SIZE/2)
		return;

	hash_seq_init(&status, RelMetaCache);

	while ((hentry = (struct PGLRelMetaCacheEntry*) hash_seq_search(&status)) != NULL)
	{
		if (!hentry->is_valid)
		{
			if (hash_search(RelMetaCache,
							(void *) &hentry->relid,
							HASH_REMOVE, NULL) == NULL)
				elog(ERROR, "hash table corrupted");
		}
	}

	InvalidRelMetaCacheCnt = 0;
}