Display old and new values in pg_resetxlog -n output.

For extra clarity.

Rajeev Rastogi, reviewed by Amit Kapila

doc/src/sgml/ref/pg_resetxlog.sgml

@@ -177,9 +177,10 @@ PostgreSQL documentation
   <para>
    The <option>-n</> (no operation) option instructs
    <command>pg_resetxlog</command> to print the values reconstructed from
-   <filename>pg_control</> and then exit without modifying anything.
+   <filename>pg_control</> and values about to be changed, and then exit
-   This is mainly a debugging tool, but can be useful as a sanity check
+   without modifying anything. This is mainly a debugging tool, but can be
-   before allowing <command>pg_resetxlog</command> to proceed for real.
+   useful as a sanity check before allowing <command>pg_resetxlog</command>
+   to proceed for real.
   </para>
 
   <para>

src/bin/pg_resetxlog/pg_resetxlog.c

@@ -64,10 +64,18 @@ static ControlFileData ControlFile;		/* pg_control values */
 static XLogSegNo newXlogSegNo;	/* new XLOG segment # */
 static bool guessed = false;	/* T if we had to guess at any values */
 static const char *progname;
+static uint32 set_xid_epoch = (uint32) -1;
+static TransactionId set_xid = 0;
+static Oid	set_oid = 0;
+static MultiXactId set_mxid = 0;
+static MultiXactOffset set_mxoff = (MultiXactOffset) -1;
+static uint32 minXlogTli = 0;
+static XLogSegNo minXlogSegNo = 0;
 
 static bool ReadControlFile(void);
 static void GuessControlValues(void);
 static void PrintControlValues(bool guessed);
+static void PrintNewControlValues(void);
 static void RewriteControlFile(void);
 static void FindEndOfXLOG(void);
 static void KillExistingXLOG(void);
@@ -82,14 +90,7 @@ main(int argc, char *argv[])
 	int			c;
 	bool		force = false;
 	bool		noupdate = false;
-	uint32		set_xid_epoch = (uint32) -1;
-	TransactionId set_xid = 0;
-	Oid			set_oid = 0;
-	MultiXactId set_mxid = 0;
 	MultiXactId set_oldestmxid = 0;
-	MultiXactOffset set_mxoff = (MultiXactOffset) -1;
-	uint32		minXlogTli = 0;
-	XLogSegNo	minXlogSegNo = 0;
 	char	   *endptr;
 	char	   *endptr2;
 	char	   *DataDir;
@@ -301,6 +302,13 @@ main(int argc, char *argv[])
 	 */
 	FindEndOfXLOG();
 
+	/*
+	 * If we're not going to proceed with the reset, print the current control
+	 * file parameters.
+	 */
+	if ((guessed && !force) || noupdate)
+		PrintControlValues(guessed);
+
 	/*
 	 * Adjust fields if required by switches.  (Do this now so that printout,
 	 * if any, includes these values.)
@@ -356,7 +364,7 @@ main(int argc, char *argv[])
 	 */
 	if ((guessed && !force) || noupdate)
 	{
-		PrintControlValues(guessed);
+		PrintNewControlValues();
 		if (!noupdate)
 		{
 			printf(_("\nIf these values seem acceptable, use -f to force reset.\n"));
@@ -556,12 +564,11 @@ static void
 PrintControlValues(bool guessed)
 {
 	char		sysident_str[32];
-	char		fname[MAXFNAMELEN];
 
 	if (guessed)
 		printf(_("Guessed pg_control values:\n\n"));
 	else
-		printf(_("pg_control values:\n\n"));
+		printf(_("Current pg_control values:\n\n"));
 
 	/*
 	 * Format system_identifier separately to keep platform-dependent format
@@ -570,10 +577,6 @@ PrintControlValues(bool guessed)
 	snprintf(sysident_str, sizeof(sysident_str), UINT64_FORMAT,
 			 ControlFile.system_identifier);
 
-	XLogFileName(fname, ControlFile.checkPointCopy.ThisTimeLineID, newXlogSegNo);
-
-	printf(_("First log segment after reset:        %s\n"),
-		   fname);
 	printf(_("pg_control version number:            %u\n"),
 		   ControlFile.pg_control_version);
 	printf(_("Catalog version number:               %u\n"),
@@ -631,6 +634,60 @@ PrintControlValues(bool guessed)
 }
 
 
+/*
+ * Print the values to be changed.
+ */
+static void
+PrintNewControlValues()
+{
+	char		fname[MAXFNAMELEN];
+
+	/* This will be always printed in order to keep format same. */
+	printf(_("\n\nValues to be changed:\n\n"));
+
+	XLogFileName(fname, ControlFile.checkPointCopy.ThisTimeLineID, newXlogSegNo);
+	printf(_("First log segment after reset:        %s\n"), fname);
+
+	if (set_mxid != 0)
+	{
+		printf(_("NextMultiXactId:                      %u\n"),
+			   ControlFile.checkPointCopy.nextMulti);
+		printf(_("OldestMultiXid:                       %u\n"),
+			   ControlFile.checkPointCopy.oldestMulti);
+		printf(_("OldestMulti's DB:                     %u\n"),
+			   ControlFile.checkPointCopy.oldestMultiDB);
+	}
+
+	if (set_mxoff != -1)
+	{
+		printf(_("NextMultiOffset:                      %u\n"),
+			   ControlFile.checkPointCopy.nextMultiOffset);
+	}
+
+	if (set_oid != 0)
+	{
+		printf(_("NextOID:                              %u\n"),
+			   ControlFile.checkPointCopy.nextOid);
+	}
+
+	if (set_xid != 0)
+	{
+		printf(_("NextXID:                              %u\n"),
+			   ControlFile.checkPointCopy.nextXid);
+		printf(_("OldestXID:                            %u\n"),
+			   ControlFile.checkPointCopy.oldestXid);
+		printf(_("OldestXID's DB:                       %u\n"),
+			   ControlFile.checkPointCopy.oldestXidDB);
+	}
+
+	if (set_xid_epoch != -1)
+	{
+		printf(_("NextXID Epoch:                        %u\n"),
+			   ControlFile.checkPointCopy.nextXidEpoch);
+	}
+}
+
+
 /*
  * Write out the new pg_control file.
  */
@@ -1039,7 +1096,7 @@ usage(void)
 	printf(_("  -f               force update to be done\n"));
 	printf(_("  -l XLOGFILE      force minimum WAL starting location for new transaction log\n"));
 	printf(_("  -m MXID,MXID     set next and oldest multitransaction ID\n"));
-	printf(_("  -n               no update, just show extracted control values (for testing)\n"));
+	printf(_("  -n               no update, just show what would be done (for testing)\n"));
 	printf(_("  -o OID           set next OID\n"));
 	printf(_("  -O OFFSET        set next multitransaction offset\n"));
 	printf(_("  -V, --version    output version information, then exit\n"));