Improve error message when standby does accept connections.

Even after reaching the minimum recovery point, if there are long-lived
write transactions with 64 subtransactions on the primary, the recovery
snapshot may not yet be ready for hot standby, delaying read-only
connections on the standby. Previously, when read-only connections were
not accepted due to this condition, the following error message was logged:

    FATAL:  the database system is not yet accepting connections
    DETAIL:  Consistent recovery state has not been yet reached.

This DETAIL message was misleading because the following message was
already logged in this case:

    LOG:  consistent recovery state reached

This contradiction, i.e., indicating that the recovery state was consistent
while also stating it wasn’t, caused confusion.

This commit improves the error message to better reflect the actual state:

    FATAL: the database system is not yet accepting connections
    DETAIL: Recovery snapshot is not yet ready for hot standby.
    HINT: To enable hot standby, close write transactions with more than 64 subtransactions on the primary server.

To implement this, the commit introduces a new postmaster signal,
PMSIGNAL_RECOVERY_CONSISTENT. When the startup process reaches
a consistent recovery state, it sends this signal to the postmaster,
allowing it to correctly recognize that state.

Since this is not a clear bug, the change is applied only to the master
branch and is not back-patched.

Author: Atsushi Torikoshi <torikoshia@oss.nttdata.com>
Co-authored-by: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Yugo Nagata <nagata@sraoss.co.jp>
Discussion: https://postgr.es/m/02db8cd8e1f527a8b999b94a4bee3165@oss.nttdata.com

doc/src/sgml/high-availability.sgml

@@ -1535,7 +1535,8 @@ synchronous_standby_names = 'ANY 2 (s1, s2, s3)'
    <para>
     When the <xref linkend="guc-hot-standby"/> parameter is set to true on a
     standby server, it will begin accepting connections once the recovery has
-    brought the system to a consistent state.  All such connections are
+    brought the system to a consistent state and be ready for hot standby.
+    All such connections are
     strictly read-only; not even temporary tables may be written.
    </para>
 
@@ -1974,9 +1975,12 @@ LOG:  database system is ready to accept read-only connections
     Consistency information is recorded once per checkpoint on the primary.
     It is not possible to enable hot standby when reading WAL
     written during a period when <varname>wal_level</varname> was not set to
-    <literal>replica</literal> or <literal>logical</literal> on the primary.  Reaching
-    a consistent state can also be delayed in the presence of both of these
-    conditions:
+    <literal>replica</literal> or <literal>logical</literal> on the primary.
+    Even after reaching a consistent state, the recovery snapshot may not
+    be ready for hot standby if both of the following conditions are met,
+    delaying accepting read-only connections.  To enable hot standby,
+    long-lived write transactions with more than 64 subtransactions
+    need to be closed on the primary.
 
       <itemizedlist>
        <listitem>

src/backend/access/transam/xlogrecovery.c

@@ -291,6 +291,11 @@ static bool backupEndRequired = false;
  * Consistent state means that the system is internally consistent, all
  * the WAL has been replayed up to a certain point, and importantly, there
  * is no trace of later actions on disk.
+ *
+ * This flag is used only by the startup process and postmaster. When
+ * minRecoveryPoint is reached, the startup process sets it to true and
+ * sends a PMSIGNAL_RECOVERY_CONSISTENT signal to the postmaster,
+ * which then sets it to true upon receiving the signal.
  */
 bool		reachedConsistency = false;
 
@@ -2248,6 +2253,7 @@ CheckRecoveryConsistency(void)
 		CheckTablespaceDirectory();
 
 		reachedConsistency = true;
+		SendPostmasterSignal(PMSIGNAL_RECOVERY_CONSISTENT);
 		ereport(LOG,
 				(errmsg("consistent recovery state reached at %X/%X",
 						LSN_FORMAT_ARGS(lastReplayedEndRecPtr))));

src/backend/postmaster/postmaster.c

@@ -1825,8 +1825,7 @@ canAcceptConnections(BackendType backend_type)
 		else if (!FatalError && pmState == PM_STARTUP)
 			return CAC_STARTUP; /* normal startup */
 		else if (!FatalError && pmState == PM_RECOVERY)
-			return CAC_NOTCONSISTENT;	/* not yet at consistent recovery
-										 * state */
+			return CAC_NOTHOTSTANDBY;	/* not yet ready for hot standby */
 		else
 			return CAC_RECOVERY;	/* else must be crash recovery */
 	}
@@ -3699,6 +3698,7 @@ process_pm_pmsignal(void)
 		/* WAL redo has started. We're out of reinitialization. */
 		FatalError = false;
 		AbortStartTime = 0;
+		reachedConsistency = false;
 
 		/*
 		 * Start the archiver if we're responsible for (re-)archiving received
@@ -3724,8 +3724,14 @@ process_pm_pmsignal(void)
 		UpdatePMState(PM_RECOVERY);
 	}
 
-	if (CheckPostmasterSignal(PMSIGNAL_BEGIN_HOT_STANDBY) &&
+	if (CheckPostmasterSignal(PMSIGNAL_RECOVERY_CONSISTENT) &&
 		pmState == PM_RECOVERY && Shutdown == NoShutdown)
+	{
+		reachedConsistency = true;
+	}
+
+	if (CheckPostmasterSignal(PMSIGNAL_BEGIN_HOT_STANDBY) &&
+		(pmState == PM_RECOVERY && Shutdown == NoShutdown))
 	{
 		ereport(LOG,
 				(errmsg("database system is ready to accept read-only connections")));

src/backend/tcop/backend_startup.c

@@ -18,6 +18,7 @@
 #include <unistd.h>
 
 #include "access/xlog.h"
+#include "access/xlogrecovery.h"
 #include "common/ip.h"
 #include "common/string.h"
 #include "libpq/libpq.h"
@@ -306,17 +307,24 @@ BackendInitialize(ClientSocket *client_sock, CAC_state cac)
 						(errcode(ERRCODE_CANNOT_CONNECT_NOW),
 						 errmsg("the database system is starting up")));
 				break;
-			case CAC_NOTCONSISTENT:
-				if (EnableHotStandby)
-					ereport(FATAL,
-							(errcode(ERRCODE_CANNOT_CONNECT_NOW),
-							 errmsg("the database system is not yet accepting connections"),
-							 errdetail("Consistent recovery state has not been yet reached.")));
-				else
+			case CAC_NOTHOTSTANDBY:
+				if (!EnableHotStandby)
 					ereport(FATAL,
 							(errcode(ERRCODE_CANNOT_CONNECT_NOW),
 							 errmsg("the database system is not accepting connections"),
 							 errdetail("Hot standby mode is disabled.")));
+				else if (reachedConsistency)
+					ereport(FATAL,
+							(errcode(ERRCODE_CANNOT_CONNECT_NOW),
+							 errmsg("the database system is not yet accepting connections"),
+							 errdetail("Recovery snapshot is not yet ready for hot standby."),
+							 errhint("To enable hot standby, close write transactions with more than %d subtransactions on the primary server.",
+									 PGPROC_MAX_CACHED_SUBXIDS)));
+				else
+					ereport(FATAL,
+							(errcode(ERRCODE_CANNOT_CONNECT_NOW),
+							 errmsg("the database system is not yet accepting connections"),
+							 errdetail("Consistent recovery state has not been yet reached.")));
 				break;
 			case CAC_SHUTDOWN:
 				ereport(FATAL,

src/include/storage/pmsignal.h

@@ -33,6 +33,7 @@
 typedef enum
 {
 	PMSIGNAL_RECOVERY_STARTED,	/* recovery has started */
+	PMSIGNAL_RECOVERY_CONSISTENT,	/* recovery has reached consistent state */
 	PMSIGNAL_BEGIN_HOT_STANDBY, /* begin Hot Standby */
 	PMSIGNAL_ROTATE_LOGFILE,	/* send SIGUSR1 to syslogger to rotate logfile */
 	PMSIGNAL_START_AUTOVAC_LAUNCHER,	/* start an autovacuum launcher */

src/include/tcop/backend_startup.h

@@ -36,7 +36,7 @@ typedef enum CAC_state
 	CAC_STARTUP,
 	CAC_SHUTDOWN,
 	CAC_RECOVERY,
-	CAC_NOTCONSISTENT,
+	CAC_NOTHOTSTANDBY,
 	CAC_TOOMANY,
 } CAC_state;