/*
 * This file and its contents are licensed under the Apache License 2.0.
 * Please see the included NOTICE for copyright information and
 * LICENSE-APACHE for a copy of the license.
 */
#include <postgres.h>

/* BGW includes below */
/* These are always necessary for a bgworker */
#include <miscadmin.h>
#include <postmaster/bgworker.h>
#include <storage/ipc.h>
#include <storage/latch.h>
#include <storage/lwlock.h>
#include <storage/proc.h>
#include <storage/shmem.h>

/* for setting our wait event during waitlatch*/
#include <pgstat.h>

/* needed for getting database list*/
#include <access/heapam.h>
#include <access/htup_details.h>
#include <access/xact.h>
#include <catalog/pg_database.h>
#include <utils/snapmgr.h>

/* and checking db list for whether we're in a template*/
#include <utils/syscache.h>

/* for calling external function*/
#include <fmgr.h>

/* for signal handling (specifically die() function) */
#include <tcop/tcopprot.h>

/* for looking up sending proc information for message handling */
#include <storage/procarray.h>

/* for allocating the htab storage */
#include <utils/memutils.h>

/* for getting settings correct before loading the versioned scheduler */
#include "catalog/pg_db_role_setting.h"

#include "../compat/compat.h"
#include "../extension_constants.h"
#include "bgw_counter.h"
#include "bgw_launcher.h"
#include "bgw_message_queue.h"
#include "loader.h"

#define BGW_DB_SCHEDULER_FUNCNAME "ts_bgw_scheduler_main"
#define BGW_ENTRYPOINT_FUNCNAME "ts_bgw_db_scheduler_entrypoint"

typedef enum AckResult
{
	ACK_FAILURE = 0,
	ACK_SUCCESS,
} AckResult;

/* See state machine in README.md */
typedef enum SchedulerState
{
	/* Scheduler should be started but has not been allocated or started */
	ENABLED = 0,
	/* The scheduler has been allocated a spot in timescaleDB's worker counter */
	ALLOCATED,
	/* Scheduler has been started */
	STARTED,

	/*
	 * Scheduler is stopped and should not be started automatically. START and
	 * RESTART messages can re-enable the scheduler.
	 */
	DISABLED
} SchedulerState;

#ifdef TS_DEBUG
#define BGW_LAUNCHER_RESTART_TIME_S 0
#else
#define BGW_LAUNCHER_RESTART_TIME_S 60
#endif

static volatile sig_atomic_t got_SIGHUP = false;

int ts_guc_bgw_scheduler_restart_time_sec = BGW_DEFAULT_RESTART_INTERVAL;

static void
launcher_sighup(SIGNAL_ARGS)
{
	/* based on av_sighup_handler */
	int save_errno = errno;

	got_SIGHUP = true;
	SetLatch(MyLatch);

	errno = save_errno;
}

/*
 * Main bgw launcher for the cluster.
 *
 * Run through the TimescaleDB loader, so needs to have a small footprint as
 * any interactions it has will need to remain backwards compatible for the
 * foreseeable future.
 *
 * Notes: multiple databases in an instance (PG cluster) can have TimescaleDB
 * installed. They are not necessarily the same version of TimescaleDB (though
 * they could be) Shared memory is allocated and background workers are
 * registered at shared_preload_libraries time We do not know what databases
 * exist, nor which databases TimescaleDB is installed in (if any) at
 * shared_preload_libraries time.
 */

TS_FUNCTION_INFO_V1(ts_bgw_cluster_launcher_main);
TS_FUNCTION_INFO_V1(ts_bgw_db_scheduler_entrypoint);
typedef struct DbHashEntry
{
	Oid db_oid;									 /* key for the hash table, must be first */
	BackgroundWorkerHandle *db_scheduler_handle; /* needed to shut down
												  * properly */
	SchedulerState state;
	VirtualTransactionId vxid;
	int state_transition_failures;
} DbHashEntry;

static void scheduler_state_trans_enabled_to_allocated(DbHashEntry *entry);

static void
bgw_on_postmaster_death(void)
{
	on_exit_reset(); /* don't call exit hooks cause we want to bail
					  * out quickly */
	ereport(FATAL,
			(errcode(ERRCODE_ADMIN_SHUTDOWN),
			 errmsg("postmaster exited while TimescaleDB background worker launcher was working")));
}

static void
report_bgw_limit_exceeded(DbHashEntry *entry)
{
	if (entry->state_transition_failures == 0)
		ereport(LOG,
				(errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
				 errmsg("TimescaleDB background worker limit of %d exceeded",
						ts_guc_max_background_workers),
				 errhint("Consider increasing timescaledb.max_background_workers.")));
	entry->state_transition_failures++;
}

static void
report_error_on_worker_register_failure(DbHashEntry *entry)
{
	if (entry->state_transition_failures == 0)
		ereport(LOG,
				(errcode(ERRCODE_INSUFFICIENT_RESOURCES),
				 errmsg("no available background worker slots"),
				 errhint("Consider increasing max_worker_processes in tandem with "
						 "timescaledb.max_background_workers.")));
	entry->state_transition_failures++;
}

/*
 * Aliasing a few things in bgworker.h so that we exit correctly on postmaster
 * death so we don't have to duplicate code basically telling it we shouldn't
 * call exit hooks cause we want to bail out quickly - similar to how the
 * quickdie function works when we receive a sigquit. This should work
 * similarly because postmaster death is a similar severity of issue.
 * Additionally, we're wrapping these calls to make sure we never have a NULL
 * handle, if we have a null handle, we return normal things.
 */
static BgwHandleStatus
get_background_worker_pid(BackgroundWorkerHandle *handle, pid_t *pidp)
{
	BgwHandleStatus status;
	pid_t pid;

	if (handle == NULL)
		status = BGWH_STOPPED;
	else
	{
		status = GetBackgroundWorkerPid(handle, &pid);
		if (pidp != NULL)
			*pidp = pid;
	}

	if (status == BGWH_POSTMASTER_DIED)
		bgw_on_postmaster_death();
	return status;
}

static void
wait_for_background_worker_startup(BackgroundWorkerHandle *handle, pid_t *pidp)
{
	BgwHandleStatus status;

	if (handle == NULL)
		status = BGWH_STOPPED;
	else
		status = WaitForBackgroundWorkerStartup(handle, pidp);

	/*
	 * We don't care whether we get BGWH_STOPPED or BGWH_STARTED here, because
	 * the worker could have started and stopped very quickly before we read
	 * it. We can't get BGWH_NOT_YET_STARTED as that's what we're waiting for.
	 * We do care if the Postmaster died however.
	 */

	if (status == BGWH_POSTMASTER_DIED)
		bgw_on_postmaster_death();

	Assert(status == BGWH_STOPPED || status == BGWH_STARTED);
}

static void
wait_for_background_worker_shutdown(BackgroundWorkerHandle *handle)
{
	BgwHandleStatus status;

	if (handle == NULL)
		status = BGWH_STOPPED;
	else
		status = WaitForBackgroundWorkerShutdown(handle);

	/* We can only ever get BGWH_STOPPED stopped unless the Postmaster died. */
	if (status == BGWH_POSTMASTER_DIED)
		bgw_on_postmaster_death();

	Assert(status == BGWH_STOPPED);
}

static void
terminate_background_worker(BackgroundWorkerHandle *handle)
{
	if (handle == NULL)
		return;
	else
		TerminateBackgroundWorker(handle);
}

extern void
ts_bgw_cluster_launcher_init(void)
{
	BackgroundWorker worker;

	DefineCustomIntVariable(/* name= */ MAKE_EXTOPTION("bgw_scheduler_restart_time"),
							/* short_desc= */ "Restart time for scheduler in seconds",
							/* long_desc= */
							"The number of seconds until the scheduler restart on failure.",
							/* valueAddr= */ &ts_guc_bgw_scheduler_restart_time_sec,
							/* bootValue= */ BGW_DEFAULT_RESTART_INTERVAL,
							/* minValue= */ 1,
							/* maxValue= */ 3600,
							/* context= */ PGC_SIGHUP,
							/* flags= */ GUC_UNIT_S,
							/* check_hook= */ NULL,
							/* assign_hook= */ NULL,
							/* show_hook= */ NULL);

	memset(&worker, 0, sizeof(worker));
	/* set up worker settings for our main worker */
	snprintf(worker.bgw_name, BGW_MAXLEN, TS_BGW_TYPE_LAUNCHER);
	worker.bgw_flags = BGWORKER_SHMEM_ACCESS | BGWORKER_BACKEND_DATABASE_CONNECTION;
	worker.bgw_restart_time = BGW_LAUNCHER_RESTART_TIME_S;

	/*
	 * Starting at BgWorkerStart_RecoveryFinished means we won't ever get
	 * started on a hot_standby see
	 * https://www.postgresql.org/docs/10/static/bgworker.html as it's not
	 * documented in bgworker.c.
	 */
	worker.bgw_start_time = BgWorkerStart_RecoveryFinished;
	worker.bgw_notify_pid = 0;
	snprintf(worker.bgw_library_name, BGW_MAXLEN, EXTENSION_NAME);
	snprintf(worker.bgw_function_name, BGW_MAXLEN, "ts_bgw_cluster_launcher_main");
	RegisterBackgroundWorker(&worker);
}

/*
 * Register a background worker that calls the main TimescaleDB background
 * worker launcher library (i.e. loader) and uses the scheduler entrypoint
 * function.  The scheduler entrypoint will deal with starting a new worker,
 * and waiting on any txns that it needs to, if we pass along a vxid in the
 * bgw_extra field of the BgWorker.
 */
static bool
register_entrypoint_for_db(Oid db_id, VirtualTransactionId vxid, BackgroundWorkerHandle **handle)
{
	BackgroundWorker worker;

	memset(&worker, 0, sizeof(worker));
	snprintf(worker.bgw_type, BGW_MAXLEN, TS_BGW_TYPE_SCHEDULER);
	snprintf(worker.bgw_name, BGW_MAXLEN, "%s for database %d", TS_BGW_TYPE_SCHEDULER, db_id);
	worker.bgw_flags = BGWORKER_SHMEM_ACCESS | BGWORKER_BACKEND_DATABASE_CONNECTION;
	worker.bgw_restart_time = ts_guc_bgw_scheduler_restart_time_sec,
	worker.bgw_start_time = BgWorkerStart_RecoveryFinished;
	snprintf(worker.bgw_library_name, BGW_MAXLEN, EXTENSION_NAME);
	snprintf(worker.bgw_function_name, BGW_MAXLEN, BGW_ENTRYPOINT_FUNCNAME);
	worker.bgw_notify_pid = MyProcPid;
	worker.bgw_main_arg = ObjectIdGetDatum(db_id);
	memcpy(worker.bgw_extra, &vxid, sizeof(VirtualTransactionId));

	return RegisterDynamicBackgroundWorker(&worker, handle);
}

/* Initializes the launcher's hash table of schedulers.
 * Return value is guaranteed to be not-null, because otherwise the function
 * will have thrown an error.
 */
static HTAB *
init_database_htab(void)
{
	HASHCTL info = { .keysize = sizeof(Oid),
					 .entrysize = sizeof(DbHashEntry),
					 .hcxt = TopMemoryContext };

	return hash_create("launcher_db_htab",
					   ts_guc_max_background_workers,
					   &info,
					   HASH_BLOBS | HASH_CONTEXT | HASH_ELEM);
}

/* Insert a scheduler entry into the hash table. Correctly set entry values. */
static DbHashEntry *
db_hash_entry_create_if_not_exists(HTAB *db_htab, Oid db_oid)
{
	DbHashEntry *db_he;
	bool found;

	db_he = (DbHashEntry *) hash_search(db_htab, &db_oid, HASH_ENTER, &found);
	if (!found)
	{
		db_he->db_scheduler_handle = NULL;
		db_he->state = ENABLED;
		SetInvalidVirtualTransactionId(db_he->vxid);
		db_he->state_transition_failures = 0;

		/*
		 * Try to allocate a spot right away to give schedulers priority over
		 * other bgws. This is especially important on initial server startup
		 * where we want to reserve slots for all schedulers before starting
		 * any. This is done so that background workers started by schedulers
		 * don't race for open slots with other schedulers on startup.
		 */
		scheduler_state_trans_enabled_to_allocated(db_he);
	}

	return db_he;
}

/*
 * Result from signalling a backend.
 *
 * Error codes are non-zero, and success is zero.
 */
enum SignalBackendResult
{
	SIGNAL_BACKEND_SUCCESS = 0,
	SIGNAL_BACKEND_ERROR,
	SIGNAL_BACKEND_NOPERMISSION,
	SIGNAL_BACKEND_NOSUPERUSER,
};

/*
 * Terminate a background worker.
 *
 * This is copied from pg_signal_backend() in
 * src/backend/storage/ipc/signalfuncs.c but tweaked to not require a database
 * connection since the launcher does not have one.
 */
static enum SignalBackendResult
ts_signal_backend(int pid, int sig)
{
	PGPROC *proc = BackendPidGetProc(pid);

	if (unlikely(proc == NULL))
	{
		ereport(WARNING, (errmsg("PID %d is not a PostgreSQL backend process", pid)));
		return SIGNAL_BACKEND_ERROR;
	}

	if (unlikely(kill(pid, sig)))
	{
		/* Again, just a warning to allow loops */
		ereport(WARNING, (errmsg("could not send signal to process %d: %m", pid)));
		return SIGNAL_BACKEND_ERROR;
	}

	return SIGNAL_BACKEND_SUCCESS;
}

/*
 * Terminate backends by backend type.
 *
 * We iterate through all backends and mark those that match the given backend
 * type as terminated.
 *
 * Note that there is potentially a delay between marking backends as
 * terminated and their actual termination, so the backends have to be able to
 * run even if there are multiple instances accessing the same data.
 *
 * Parts of this code is taken from pg_stat_get_activity() in
 * src/backend/utils/adt/pgstatfuncs.c.
 */
static void
terminate_backends_by_backend_type(const char *backend_type)
{
	Assert(backend_type);

	const int num_backends = pgstat_fetch_stat_numbackends();
	for (int curr_backend = 1; curr_backend <= num_backends; ++curr_backend)
	{
		const LocalPgBackendStatus *local_beentry =
			pgstat_get_local_beentry_by_index_compat(curr_backend);
		const PgBackendStatus *beentry = &local_beentry->backendStatus;
		const char *bgw_type = GetBackgroundWorkerTypeByPid(beentry->st_procpid);
		if (bgw_type && strcmp(backend_type, bgw_type) == 0)
		{
			int error = ts_signal_backend(beentry->st_procpid, SIGTERM);
			if (error)
				elog(LOG, "failed to terminate backend with pid %d", beentry->st_procpid);
		}
	}
}

/*
 * Model this on autovacuum.c -> get_database_list.
 *
 * Note that we are not doing all the things around memory context that they
 * do, because the hashtable we're using to store db entries is automatically
 * created in its own memory context (a child of TopMemoryContext) This can
 * get called at two different times 1) when the cluster launcher starts and
 * is looking for dbs and 2) if it restarts due to a postmaster signal.
 */
static void
populate_database_htab(HTAB *db_htab)
{
	Relation rel;
	TableScanDesc scan;
	HeapTuple tup;

	/*
	 * by this time we should already be connected to the db, and only have
	 * access to shared catalogs
	 */
	StartTransactionCommand();
	(void) GetTransactionSnapshot();

	rel = table_open(DatabaseRelationId, AccessShareLock);
	scan = table_beginscan_catalog(rel, 0, NULL);

	while (HeapTupleIsValid(tup = heap_getnext(scan, ForwardScanDirection)))
	{
		Form_pg_database pgdb = (Form_pg_database) GETSTRUCT(tup);

		if (!pgdb->datallowconn || pgdb->datistemplate)
			continue; /* don't bother with dbs that don't allow
					   * connections or are templates */
		db_hash_entry_create_if_not_exists(db_htab, pgdb->oid);
	}
	table_endscan(scan);
	table_close(rel, AccessShareLock);

	CommitTransactionCommand();
}

static void
scheduler_modify_state(DbHashEntry *entry, SchedulerState new_state)
{
	Assert(entry->state != new_state);
	entry->state_transition_failures = 0;
	entry->state = new_state;
}

/* TRANSITION FUNCTIONS */
static void
scheduler_state_trans_disabled_to_enabled(DbHashEntry *entry)
{
	Assert(entry->state == DISABLED);
	Assert(entry->db_scheduler_handle == NULL);
	scheduler_modify_state(entry, ENABLED);
}

static void
scheduler_state_trans_enabled_to_allocated(DbHashEntry *entry)
{
	Assert(entry->state == ENABLED);
	Assert(entry->db_scheduler_handle == NULL);
	/* Reserve a spot for this scheduler with BGW counter */
	if (!ts_bgw_total_workers_increment())
	{
		report_bgw_limit_exceeded(entry);
		return;
	}
	scheduler_modify_state(entry, ALLOCATED);
}

static void
scheduler_state_trans_started_to_allocated(DbHashEntry *entry)
{
	Assert(entry->state == STARTED);
	Assert(get_background_worker_pid(entry->db_scheduler_handle, NULL) == BGWH_STOPPED);
	if (entry->db_scheduler_handle != NULL)
	{
		pfree(entry->db_scheduler_handle);
		entry->db_scheduler_handle = NULL;
	}
	scheduler_modify_state(entry, ALLOCATED);
}

static void
scheduler_state_trans_allocated_to_started(DbHashEntry *entry)
{
	pid_t worker_pid;
	bool worker_registered;

	Assert(entry->state == ALLOCATED);
	Assert(entry->db_scheduler_handle == NULL);

	worker_registered =
		register_entrypoint_for_db(entry->db_oid, entry->vxid, &entry->db_scheduler_handle);

	if (!worker_registered)
	{
		report_error_on_worker_register_failure(entry);
		return;
	}
	wait_for_background_worker_startup(entry->db_scheduler_handle, &worker_pid);
	SetInvalidVirtualTransactionId(entry->vxid);
	scheduler_modify_state(entry, STARTED);
}

static void
scheduler_state_trans_enabled_to_disabled(DbHashEntry *entry)
{
	Assert(entry->state == ENABLED);
	Assert(entry->db_scheduler_handle == NULL);
	scheduler_modify_state(entry, DISABLED);
}

static void
scheduler_state_trans_allocated_to_disabled(DbHashEntry *entry)
{
	Assert(entry->state == ALLOCATED);
	Assert(entry->db_scheduler_handle == NULL);

	ts_bgw_total_workers_decrement();
	scheduler_modify_state(entry, DISABLED);
}

static void
scheduler_state_trans_started_to_disabled(DbHashEntry *entry)
{
	Assert(entry->state == STARTED);
	Assert(get_background_worker_pid(entry->db_scheduler_handle, NULL) == BGWH_STOPPED);

	ts_bgw_total_workers_decrement();
	if (entry->db_scheduler_handle != NULL)
	{
		pfree(entry->db_scheduler_handle);
		entry->db_scheduler_handle = NULL;
	}
	scheduler_modify_state(entry, DISABLED);
}

static void
scheduler_state_trans_automatic(DbHashEntry *entry)
{
	switch (entry->state)
	{
		case ENABLED:
			scheduler_state_trans_enabled_to_allocated(entry);
			if (entry->state == ALLOCATED)
				scheduler_state_trans_allocated_to_started(entry);
			break;
		case ALLOCATED:
			scheduler_state_trans_allocated_to_started(entry);
			break;
		case STARTED:
			if (get_background_worker_pid(entry->db_scheduler_handle, NULL) == BGWH_STOPPED)
				scheduler_state_trans_started_to_disabled(entry);
			break;
		case DISABLED:
			break;
	}
}

static void
scheduler_state_trans_automatic_all(HTAB *db_htab)
{
	HASH_SEQ_STATUS hash_seq;
	DbHashEntry *current_entry;

	hash_seq_init(&hash_seq, db_htab);
	while ((current_entry = hash_seq_search(&hash_seq)) != NULL)
		scheduler_state_trans_automatic(current_entry);
}

/* This is called when we're going to shut down so we don't leave things messy*/
static void
launcher_pre_shmem_cleanup(int code, Datum arg)
{
	HTAB *db_htab = *(HTAB **) DatumGetPointer(arg);
	HASH_SEQ_STATUS hash_seq;
	DbHashEntry *current_entry;

	/* db_htab will be NULL if we fail during init_database_htab */
	if (db_htab != NULL)
	{
		hash_seq_init(&hash_seq, db_htab);

		/*
		 * Stop everyone (or at least tell the Postmaster we don't care about
		 * them anymore)
		 */
		while ((current_entry = hash_seq_search(&hash_seq)) != NULL)
		{
			if (current_entry->db_scheduler_handle != NULL)
			{
				terminate_background_worker(current_entry->db_scheduler_handle);
				pfree(current_entry->db_scheduler_handle);
			}
		}

		hash_destroy(db_htab);
	}

	/*
	 * Reset our pid in the queue so that others know we've died and don't
	 * wait forever
	 */
	ts_bgw_message_queue_shmem_cleanup();
}

/*
 *************
 * Actions for message types we could receive off of the bgw_message_queue.
 *************
 */

/*
 * This should be idempotent. If we find the background worker and it's not
 * stopped, do nothing. In order to maintain idempotency, a scheduler in the
 * ENABLED, ALLOCATED or STARTED state cannot get a new vxid to wait on. (We
 * cannot pass in a new vxid to wait on for an already-started scheduler in any
 * case). This means that actions like restart, which are not idempotent, will
 * not have their effects changed by subsequent start actions, no matter the
 * state they are in when the start action is received.
 */
static AckResult
message_start_action(HTAB *db_htab, BgwMessage *message)
{
	DbHashEntry *entry;

	entry = db_hash_entry_create_if_not_exists(db_htab, message->db_oid);

	if (entry->state == DISABLED)
		scheduler_state_trans_disabled_to_enabled(entry);

	scheduler_state_trans_automatic(entry);

	return (entry->state == STARTED ? ACK_SUCCESS : ACK_FAILURE);
}

static AckResult
message_stop_action(HTAB *db_htab, BgwMessage *message)
{
	DbHashEntry *entry;

	/*
	 * If the entry does not exist try to create it so we can put it in the
	 * DISABLED state. Otherwise, it will be created during the next poll and
	 * then will end up in the ENABLED state and proceed to being STARTED. But
	 * this is not the behavior we want.
	 */
	entry = db_hash_entry_create_if_not_exists(db_htab, message->db_oid);

	switch (entry->state)
	{
		case ENABLED:
			scheduler_state_trans_enabled_to_disabled(entry);
			break;
		case ALLOCATED:
			scheduler_state_trans_allocated_to_disabled(entry);
			break;
		case STARTED:
			terminate_background_worker(entry->db_scheduler_handle);
			wait_for_background_worker_shutdown(entry->db_scheduler_handle);
			scheduler_state_trans_started_to_disabled(entry);
			break;
		case DISABLED:
			break;
	}
	return entry->state == DISABLED ? ACK_SUCCESS : ACK_FAILURE;
}

/*
 * This function will stop and restart a scheduler in the STARTED state,  ENABLE
 * a scheduler if it does not exist or is in the DISABLED state and set the vxid
 * to wait on for a scheduler in any state. It is not idempotent. Additionally,
 * one might think that this function would simply be a combination of stop and
 * start above, but it is not as we maintain the worker's "slot" by never
 * releasing the worker from our "pool" of background workers as stopping and
 * starting would.  We don't want a race condition where some other db steals
 * the scheduler of the other by requesting a worker at the wrong time. (This is
 * accomplished by moving from STARTED to ALLOCATED after shutting down the
 * worker, never releasing the entry and transitioning all the way back to
 * ENABLED).
 */
static AckResult
message_restart_action(HTAB *db_htab, BgwMessage *message, VirtualTransactionId vxid)
{
	DbHashEntry *entry;

	entry = db_hash_entry_create_if_not_exists(db_htab, message->db_oid);

	entry->vxid = vxid;

	switch (entry->state)
	{
		case ENABLED:
			break;
		case ALLOCATED:
			break;
		case STARTED:
			terminate_background_worker(entry->db_scheduler_handle);
			wait_for_background_worker_shutdown(entry->db_scheduler_handle);
			scheduler_state_trans_started_to_allocated(entry);
			break;
		case DISABLED:
			scheduler_state_trans_disabled_to_enabled(entry);
	}

	scheduler_state_trans_automatic(entry);
	return entry->state == STARTED ? ACK_SUCCESS : ACK_FAILURE;
}

/*
 * Handle 1 message.
 */
static bool
launcher_handle_message(HTAB *db_htab)
{
	BgwMessage *message = ts_bgw_message_receive();
	PGPROC *sender;
	VirtualTransactionId vxid;
	AckResult action_result = ACK_FAILURE;

	if (message == NULL)
		return false;

	sender = BackendPidGetProc(message->sender_pid);
	if (sender == NULL)
	{
		ereport(LOG,
				(errmsg("TimescaleDB background worker launcher received message from non-existent "
						"backend")));
		return true;
	}

	GET_VXID_FROM_PGPROC(vxid, *sender);

	switch (message->message_type)
	{
		case START:
			action_result = message_start_action(db_htab, message);
			break;
		case STOP:
			action_result = message_stop_action(db_htab, message);
			break;
		case RESTART:
			action_result = message_restart_action(db_htab, message, vxid);
			break;
	}

	ts_bgw_message_send_ack(message, action_result);
	return true;
}

/*
 * The default, `bgworker_die()`, can't be used due to the fact that it
 * handles signals synchronously, rather than waiting for a
 * CHECK_FOR_INTERRUPTS(). `die()` (which is arguably misnamed) sets flags
 * that will cause the backend to exit on the next call to
 * CHECK_FOR_INTERRUPTS(), which can happen either in our code or in functions
 * within the Postgres codebase that we call. This means that we don't need to
 * wait for the next time control is returned to our loop to exit, which would
 * be necessary if we set our own flag and checked it in a loop
 * condition. However, because it cannot exit 0, the launcher will be
 * restarted by the postmaster, even when it has received a SIGTERM, which we
 * decided was the proper behavior. If users want to disable the launcher,
 * they can set `timescaledb.max_background_workers = 0` and then we will
 * `proc_exit(0)` before doing anything else.
 */

extern Datum
ts_bgw_cluster_launcher_main(PG_FUNCTION_ARGS)
{
	HTAB **htab_storage;

	HTAB *db_htab;

	pqsignal(SIGINT, StatementCancelHandler);
	pqsignal(SIGTERM, die);
	pqsignal(SIGHUP, launcher_sighup);

	/* Some SIGHUPS may already have been dropped, so we must load the file here */
	got_SIGHUP = false;
	ProcessConfigFile(PGC_SIGHUP);
	BackgroundWorkerUnblockSignals();
	ereport(DEBUG1, (errmsg("TimescaleDB background worker launcher started")));

	/* set counter back to zero on restart */
	ts_bgw_counter_reinit();
	if (!ts_bgw_total_workers_increment())
	{
		/*
		 * Should be the first thing happening so if we already exceeded our
		 * limits it means we have a limit of 0 and we should just exit We
		 * have to exit(0) because if we exit in error we get restarted by the
		 * postmaster.
		 */
		ereport(LOG,
				(errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
				 errmsg("TimescaleDB background worker is set to 0"),
				 errhint("TimescaleDB background worker launcher shutting down.")));
		proc_exit(0);
	}
	/* Connect to the db, no db name yet, so can only access shared catalogs */
	BackgroundWorkerInitializeConnection(NULL, NULL, 0);
	pgstat_report_appname(MyBgworkerEntry->bgw_name);
	ereport(LOG, (errmsg("TimescaleDB background worker launcher connected to shared catalogs")));

	htab_storage = (HTAB **) MemoryContextAllocZero(TopMemoryContext, sizeof(void *));

	/*
	 * We must setup the cleanup function _before_ initializing any state it
	 * touches (specifically the bgw_message_queue and db_htab). Failing to do
	 * this can cause cascading failures when the launcher fails in
	 * init_database_htab (eg. due to running out of shared memory) but
	 * doesn't deregister itself from the shared bgw_message_queue.
	 */
	before_shmem_exit(launcher_pre_shmem_cleanup, PointerGetDatum(htab_storage));

	ts_bgw_message_queue_set_reader();

	db_htab = init_database_htab();
	*htab_storage = db_htab;

	/*
	 * If the launcher was restarted and discovers old schedulers, these has
	 * to be terminated to avoid exhausting the worker slots.
	 *
	 * We cannot easily pick up the old schedulers since we do not have access
	 * to the slots array in PostgreSQL, so instead we scan for something that
	 * looks like schedulers for databases, and kill them. New ones will then
	 * be spawned below.
	 */
	terminate_backends_by_backend_type(TS_BGW_TYPE_SCHEDULER);
	populate_database_htab(db_htab);

	while (true)
	{
		int wl_rc;
		bool handled_msgs = false;

		CHECK_FOR_INTERRUPTS();
		populate_database_htab(db_htab);
		handled_msgs = launcher_handle_message(db_htab);
		scheduler_state_trans_automatic_all(db_htab);
		if (handled_msgs)
			continue;

		wl_rc = WaitLatch(MyLatch,
						  WL_LATCH_SET | WL_POSTMASTER_DEATH | WL_TIMEOUT,
						  (long) ts_guc_bgw_launcher_poll_time,
						  PG_WAIT_EXTENSION);
		ResetLatch(MyLatch);
		if (wl_rc & WL_POSTMASTER_DEATH)
			bgw_on_postmaster_death();

		if (got_SIGHUP)
		{
			got_SIGHUP = false;
			ProcessConfigFile(PGC_SIGHUP);
		}
	}
	PG_RETURN_VOID();
}

/*
 * Inside the entrypoint, we must check again if we're in a template db
 * even though we excluded template dbs in populate_database_htab because
 * we can be called on, say, CREATE EXTENSION in a template db and then
 * we'll not stop til next server shutdown so if we hit this point and are
 * in a template db, we throw an error and shut down Check in the syscache
 * rather than searching through the entire database catalog again.
 * Modelled on autovacuum.c -> do_autovacuum.
 */
static void
database_is_template_check(void)
{
	Form_pg_database pgdb;
	HeapTuple tuple;

	tuple = SearchSysCache1(DATABASEOID, ObjectIdGetDatum(MyDatabaseId));
	if (!HeapTupleIsValid(tuple))
		ereport(ERROR,
				(errmsg("TimescaleDB background worker failed to find entry for database in "
						"syscache")));

	pgdb = (Form_pg_database) GETSTRUCT(tuple);
	if (pgdb->datistemplate)
		ereport(ERROR,
				(errmsg("TimescaleDB background worker connected to template database, exiting")));

	ReleaseSysCache(tuple);
}

/*
 * Before we morph into the scheduler, we also need to reload configs from their
 * defaults if the database default has changed. Defaults are changed in the
 * post_restore function where we change the db default for the restoring guc
 * wait until the txn commits and then must see if the txn made the change.
 * Checks for changes are normally run at connection startup, but because we
 * have to connect in order to wait on the txn we have to re-run after the wait.
 * This function is based on the postgres function in postinit.c by the same
 * name.
 */

static void
process_settings(Oid databaseid)
{
	Relation relsetting;
	Snapshot snapshot;

	if (!IsUnderPostmaster)
		return;

	relsetting = table_open(DbRoleSettingRelationId, AccessShareLock);

	/* read all the settings under the same snapshot for efficiency */
	snapshot = RegisterSnapshot(GetCatalogSnapshot(DbRoleSettingRelationId));

	/* Later settings are ignored if set earlier. */
	ApplySetting(snapshot, databaseid, InvalidOid, relsetting, PGC_S_DATABASE);
	ApplySetting(snapshot, InvalidOid, InvalidOid, relsetting, PGC_S_GLOBAL);

	UnregisterSnapshot(snapshot);
	table_close(relsetting, AccessShareLock);
}

/*
 * This can be run either from the cluster launcher at db_startup time, or
 * in the case of an install/uninstall/update of the extension, in the
 * first case, we have no vxid that we're waiting on. In the second case,
 * we do, because we have to wait so that we see the effects of said txn.
 * So we wait for it to finish, then we  morph into the new db_scheduler
 * worker using whatever version is now installed (or exit gracefully if
 * no version is now installed).
 */
extern Datum
ts_bgw_db_scheduler_entrypoint(PG_FUNCTION_ARGS)
{
	Oid db_id = DatumGetObjectId(MyBgworkerEntry->bgw_main_arg);
	bool ts_installed = false;
	char version[MAX_VERSION_LEN];
	VirtualTransactionId vxid;

	pqsignal(SIGINT, StatementCancelHandler);
	pqsignal(SIGTERM, die);
	BackgroundWorkerUnblockSignals();
	BackgroundWorkerInitializeConnectionByOid(db_id, InvalidOid, 0);
	pgstat_report_appname(MyBgworkerEntry->bgw_name);

	/*
	 * Wait until whatever vxid that potentially called us finishes before we
	 * happens in a txn so it's cleaned up correctly if we get a sigkill in
	 * the meantime, but we will need stop after and take a new txn so we can
	 * see the correct state after its effects
	 */
	StartTransactionCommand();
	(void) GetTransactionSnapshot();
	memcpy(&vxid, MyBgworkerEntry->bgw_extra, sizeof(VirtualTransactionId));
	if (VirtualTransactionIdIsValid(vxid))
		VirtualXactLock(vxid, true);
	CommitTransactionCommand();

	/*
	 * now we can start our transaction and get the version currently
	 * installed
	 */
	StartTransactionCommand();
	(void) GetTransactionSnapshot();

	/*
	 * Check whether a database is a template database and raise an error if
	 * so, as we don't want to run in template dbs.
	 */
	database_is_template_check();
	/*  Process any config changes caused by an ALTER DATABASE */
	process_settings(MyDatabaseId);
	ts_installed = ts_loader_extension_exists();
	if (ts_installed)
		strlcpy(version, ts_loader_extension_version(), MAX_VERSION_LEN);

	ts_loader_extension_check();
	CommitTransactionCommand();
	if (ts_installed)
	{
		char soname[MAX_SO_NAME_LEN];
		PGFunction versioned_scheduler_main;

		snprintf(soname, MAX_SO_NAME_LEN, "%s-%s", EXTENSION_SO, version);
		versioned_scheduler_main =
			load_external_function(soname, BGW_DB_SCHEDULER_FUNCNAME, false, NULL);
		if (versioned_scheduler_main == NULL)
			ereport(LOG,
					(errmsg("TimescaleDB version %s does not have a background worker, exiting",
							soname)));
		else /* essentially we morph into the versioned
			  * worker here */
			DirectFunctionCall1(versioned_scheduler_main, ObjectIdGetDatum(InvalidOid));
	}
	PG_RETURN_VOID();
}
