Further tweaking of nomenclature in checkpointer.c.

Get rid of some more naming choices that only make sense if you know that
this code used to be in the bgwriter, as well as some stray comments
referencing the bgwriter.

src/backend/postmaster/checkpointer.c

@@ -10,13 +10,13 @@
  * fill WAL segments; the checkpointer itself doesn't watch for the
  * condition.)
  *
- * The checkpointer is started by the postmaster as soon as the startup subprocess
- * finishes, or as soon as recovery begins if we are doing archive recovery.
- * It remains alive until the postmaster commands it to terminate.
- * Normal termination is by SIGUSR2, which instructs the checkpointer to execute
- * a shutdown checkpoint and then exit(0).	(All backends must be stopped
- * before SIGUSR2 is issued!)  Emergency termination is by SIGQUIT; like any
- * backend, the checkpointer will simply abort and exit on SIGQUIT.
+ * The checkpointer is started by the postmaster as soon as the startup
+ * subprocess finishes, or as soon as recovery begins if we are doing archive
+ * recovery.  It remains alive until the postmaster commands it to terminate.
+ * Normal termination is by SIGUSR2, which instructs the checkpointer to
+ * execute a shutdown checkpoint and then exit(0).  (All backends must be
+ * stopped before SIGUSR2 is issued!)  Emergency termination is by SIGQUIT;
+ * like any backend, the checkpointer will simply abort and exit on SIGQUIT.
  *
  * If the checkpointer exits unexpectedly, the postmaster treats that the same
  * as a backend crash: shared memory may be corrupted, so remaining backends
@@ -65,8 +65,8 @@
  *
  * The ckpt counters allow backends to watch for completion of a checkpoint
  * request they send.  Here's how it works:
- *	* At start of a checkpoint, checkpointer reads (and clears) the request flags
- *	  and increments ckpt_started, while holding ckpt_lck.
+ *	* At start of a checkpoint, checkpointer reads (and clears) the request
+ *	  flags and increments ckpt_started, while holding ckpt_lck.
  *	* On completion of a checkpoint, checkpointer sets ckpt_done to
  *	  equal ckpt_started.
  *	* On failure of a checkpoint, checkpointer increments ckpt_failed
@@ -95,7 +95,7 @@
  * by user backend processes.  This counter should be wide enough that it
  * can't overflow during a single processing cycle.  num_backend_fsync
  * counts the subset of those writes that also had to do their own fsync,
- * because the background writer failed to absorb their request.
+ * because the checkpointer failed to absorb their request.
  *
  * The requests array holds fsync requests sent by backends and not yet
  * absorbed by the checkpointer.
@@ -200,8 +200,8 @@ CheckpointerMain(void)
 
 	/*
 	 * If possible, make this process a group leader, so that the postmaster
-	 * can signal any child processes too.	(checkpointer probably never has any
-	 * child processes, but for consistency we make all postmaster child
+	 * can signal any child processes too.  (checkpointer probably never has
+	 * any child processes, but for consistency we make all postmaster child
 	 * processes do this.)
 	 */
 #ifdef HAVE_SETSID
@@ -216,9 +216,6 @@ CheckpointerMain(void)
 	 * system shutdown cycle, init will SIGTERM all processes at once.	We
 	 * want to wait for the backends to exit, whereupon the postmaster will
 	 * tell us it's okay to shut down (via SIGUSR2).
-	 *
-	 * SIGUSR1 is presently unused; keep it spare in case someday we want this
-	 * process to participate in ProcSignal signalling.
 	 */
 	pqsignal(SIGHUP, ChkptSigHupHandler);	/* set flag to read config file */
 	pqsignal(SIGINT, ReqCheckpointHandler);	/* request checkpoint */
@@ -302,12 +299,12 @@ CheckpointerMain(void)
 		if (ckpt_active)
 		{
 			/* use volatile pointer to prevent code rearrangement */
-			volatile CheckpointerShmemStruct *bgs = CheckpointerShmem;
+			volatile CheckpointerShmemStruct *cps = CheckpointerShmem;
 
-			SpinLockAcquire(&bgs->ckpt_lck);
-			bgs->ckpt_failed++;
-			bgs->ckpt_done = bgs->ckpt_started;
-			SpinLockRelease(&bgs->ckpt_lck);
+			SpinLockAcquire(&cps->ckpt_lck);
+			cps->ckpt_failed++;
+			cps->ckpt_done = cps->ckpt_started;
+			SpinLockRelease(&cps->ckpt_lck);
 
 			ckpt_active = false;
 		}
@@ -455,7 +452,7 @@ CheckpointerMain(void)
 			bool		do_restartpoint;
 
 			/* use volatile pointer to prevent code rearrangement */
-			volatile CheckpointerShmemStruct *bgs = CheckpointerShmem;
+			volatile CheckpointerShmemStruct *cps = CheckpointerShmem;
 
 			/*
 			 * Check if we should perform a checkpoint or a restartpoint. As a
@@ -469,11 +466,11 @@ CheckpointerMain(void)
 			 * checkpoint we should perform, and increase the started-counter
 			 * to acknowledge that we've started a new checkpoint.
 			 */
-			SpinLockAcquire(&bgs->ckpt_lck);
-			flags |= bgs->ckpt_flags;
-			bgs->ckpt_flags = 0;
-			bgs->ckpt_started++;
-			SpinLockRelease(&bgs->ckpt_lck);
+			SpinLockAcquire(&cps->ckpt_lck);
+			flags |= cps->ckpt_flags;
+			cps->ckpt_flags = 0;
+			cps->ckpt_started++;
+			SpinLockRelease(&cps->ckpt_lck);
 
 			/*
 			 * The end-of-recovery checkpoint is a real checkpoint that's
@@ -528,9 +525,9 @@ CheckpointerMain(void)
 			/*
 			 * Indicate checkpoint completion to any waiting backends.
 			 */
-			SpinLockAcquire(&bgs->ckpt_lck);
-			bgs->ckpt_done = bgs->ckpt_started;
-			SpinLockRelease(&bgs->ckpt_lck);
+			SpinLockAcquire(&cps->ckpt_lck);
+			cps->ckpt_done = cps->ckpt_started;
+			SpinLockRelease(&cps->ckpt_lck);
 
 			if (ckpt_performed)
 			{
@@ -559,7 +556,11 @@ CheckpointerMain(void)
 		CheckArchiveTimeout();
 
 		/*
-		 * Send off activity statistics to the stats collector
+		 * Send off activity statistics to the stats collector.  (The reason
+		 * why we re-use bgwriter-related code for this is that the bgwriter
+		 * and checkpointer used to be just one process.  It's probably not
+		 * worth the trouble to split the stats support into two independent
+		 * stats message types.)
 		 */
 		pgstat_send_bgwriter();
 
@@ -651,13 +652,13 @@ ImmediateCheckpointRequested(void)
 {
 	if (checkpoint_requested)
 	{
-		volatile CheckpointerShmemStruct *bgs = CheckpointerShmem;
+		volatile CheckpointerShmemStruct *cps = CheckpointerShmem;
 
 		/*
 		 * We don't need to acquire the ckpt_lck in this case because we're
 		 * only looking at a single flag bit.
 		 */
-		if (bgs->ckpt_flags & CHECKPOINT_IMMEDIATE)
+		if (cps->ckpt_flags & CHECKPOINT_IMMEDIATE)
 			return true;
 	}
 	return false;
@@ -698,7 +699,7 @@ CheckpointWriteDelay(int flags, double progress)
 		{
 			got_SIGHUP = false;
 			ProcessConfigFile(PGC_SIGHUP);
-			/* update global shmem state for sync rep */
+			/* update shmem copies of config variables */
 			UpdateSharedMemoryConfig();
 		}
 
@@ -708,7 +709,7 @@ CheckpointWriteDelay(int flags, double progress)
 		CheckArchiveTimeout();
 
 		/*
-		 * Checkpoint sleep used to be connected to bgwriter_delay at 200ms.
+		 * This sleep used to be connected to bgwriter_delay, typically 200ms.
 		 * That resulted in more frequent wakeups if not much work to do.
 		 * Checkpointer and bgwriter are no longer related so take the Big Sleep.
 		 */
@@ -894,7 +895,7 @@ ReqShutdownHandler(SIGNAL_ARGS)
 
 /*
  * CheckpointerShmemSize
- *		Compute space needed for bgwriter-related shared memory
+ *		Compute space needed for checkpointer-related shared memory
  */
 Size
 CheckpointerShmemSize(void)
@@ -913,7 +914,7 @@ CheckpointerShmemSize(void)
 
 /*
  * CheckpointerShmemInit
- *		Allocate and initialize bgwriter-related shared memory
+ *		Allocate and initialize checkpointer-related shared memory
  */
 void
 CheckpointerShmemInit(void)
@@ -921,7 +922,7 @@ CheckpointerShmemInit(void)
 	bool		found;
 
 	CheckpointerShmem = (CheckpointerShmemStruct *)
-		ShmemInitStruct("Background Writer Data",
+		ShmemInitStruct("Checkpointer Data",
 						CheckpointerShmemSize(),
 						&found);
 
@@ -955,7 +956,7 @@ void
 RequestCheckpoint(int flags)
 {
 	/* use volatile pointer to prevent code rearrangement */
-	volatile CheckpointerShmemStruct *bgs = CheckpointerShmem;
+	volatile CheckpointerShmemStruct *cps = CheckpointerShmem;
 	int			ntries;
 	int			old_failed,
 				old_started;
@@ -989,13 +990,13 @@ RequestCheckpoint(int flags)
 	 * a "stronger" request by another backend.  The flag senses must be
 	 * chosen to make this work!
 	 */
-	SpinLockAcquire(&bgs->ckpt_lck);
+	SpinLockAcquire(&cps->ckpt_lck);
 
-	old_failed = bgs->ckpt_failed;
-	old_started = bgs->ckpt_started;
-	bgs->ckpt_flags |= flags;
+	old_failed = cps->ckpt_failed;
+	old_started = cps->ckpt_started;
+	cps->ckpt_flags |= flags;
 
-	SpinLockRelease(&bgs->ckpt_lck);
+	SpinLockRelease(&cps->ckpt_lck);
 
 	/*
 	 * Send signal to request checkpoint.  It's possible that the checkpointer
@@ -1043,9 +1044,9 @@ RequestCheckpoint(int flags)
 		/* Wait for a new checkpoint to start. */
 		for (;;)
 		{
-			SpinLockAcquire(&bgs->ckpt_lck);
-			new_started = bgs->ckpt_started;
-			SpinLockRelease(&bgs->ckpt_lck);
+			SpinLockAcquire(&cps->ckpt_lck);
+			new_started = cps->ckpt_started;
+			SpinLockRelease(&cps->ckpt_lck);
 
 			if (new_started != old_started)
 				break;
@@ -1061,10 +1062,10 @@ RequestCheckpoint(int flags)
 		{
 			int			new_done;
 
-			SpinLockAcquire(&bgs->ckpt_lck);
-			new_done = bgs->ckpt_done;
-			new_failed = bgs->ckpt_failed;
-			SpinLockRelease(&bgs->ckpt_lck);
+			SpinLockAcquire(&cps->ckpt_lck);
+			new_done = cps->ckpt_done;
+			new_failed = cps->ckpt_failed;
+			SpinLockRelease(&cps->ckpt_lck);
 
 			if (new_done - new_started >= 0)
 				break;
@@ -1178,7 +1179,7 @@ ForwardFsyncRequest(RelFileNodeBackend rnode, ForkNumber forknum,
 static bool
 CompactCheckpointerRequestQueue(void)
 {
-	struct BgWriterSlotMapping
+	struct CheckpointerSlotMapping
 	{
 		CheckpointerRequest request;
 		int			slot;
@@ -1197,7 +1198,7 @@ CompactCheckpointerRequestQueue(void)
 	/* Initialize temporary hash table */
 	MemSet(&ctl, 0, sizeof(ctl));
 	ctl.keysize = sizeof(CheckpointerRequest);
-	ctl.entrysize = sizeof(struct BgWriterSlotMapping);
+	ctl.entrysize = sizeof(struct CheckpointerSlotMapping);
 	ctl.hash = tag_hash;
 	htab = hash_create("CompactCheckpointerRequestQueue",
 					   CheckpointerShmem->num_requests,
@@ -1223,7 +1224,7 @@ CompactCheckpointerRequestQueue(void)
 	for (n = 0; n < CheckpointerShmem->num_requests; ++n)
 	{
 		CheckpointerRequest *request;
-		struct BgWriterSlotMapping *slotmap;
+		struct CheckpointerSlotMapping *slotmap;
 		bool		found;
 
 		request = &CheckpointerShmem->requests[n];