Report waiting via PS while recovery is waiting for buffer pin in hot standby.

Previously while the startup process was waiting for the recovery conflict
with snapshot, tablespace or lock to be resolved, waiting was reported in
PS display, but not in the case of recovery conflict with buffer pin.
This commit makes the startup process in hot standby report waiting via PS
while waiting for the conflicts with other backends holding buffer pins to
be resolved.

Author: Masahiko Sawada
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/CA+fd4k4mXWTwfQLS3RPwGr4xnfAEs1ysFfgYHvmmoUgv6Zxvmg@mail.gmail.com

src/backend/storage/buffer/bufmgr.c

@@ -49,6 +49,7 @@
 #include "storage/proc.h"
 #include "storage/smgr.h"
 #include "storage/standby.h"
+#include "utils/ps_status.h"
 #include "utils/rel.h"
 #include "utils/resowner_private.h"
 #include "utils/timestamp.h"
@@ -3616,6 +3617,7 @@ void
 LockBufferForCleanup(Buffer buffer)
 {
 	BufferDesc *bufHdr;
+	char	   *new_status = NULL;
 
 	Assert(BufferIsValid(buffer));
 	Assert(PinCountWaitBuf == NULL);
@@ -3650,6 +3652,13 @@ LockBufferForCleanup(Buffer buffer)
 		{
 			/* Successfully acquired exclusive lock with pincount 1 */
 			UnlockBufHdr(bufHdr, buf_state);
+
+			/* Report change to non-waiting status */
+			if (new_status)
+			{
+				set_ps_display(new_status);
+				pfree(new_status);
+			}
 			return;
 		}
 		/* Failed, so mark myself as waiting for pincount 1 */
@@ -3668,6 +3677,20 @@ LockBufferForCleanup(Buffer buffer)
 		/* Wait to be signaled by UnpinBuffer() */
 		if (InHotStandby)
 		{
+			/* Report change to waiting status */
+			if (update_process_title && new_status == NULL)
+			{
+				const char *old_status;
+				int			len;
+
+				old_status = get_ps_display(&len);
+				new_status = (char *) palloc(len + 8 + 1);
+				memcpy(new_status, old_status, len);
+				strcpy(new_status + len, " waiting");
+				set_ps_display(new_status);
+				new_status[len] = '\0'; /* truncate off " waiting" */
+			}
+
 			/* Publish the bufid that Startup process waits on */
 			SetStartupBufferPinWaitBufId(buffer - 1);
 			/* Set alarm and then wait to be signaled by UnpinBuffer() */