Print WAL logical message contents in pg_waldump

This helps debuggability when looking at WAL streams containing logical
messages.

Author: Ashutosh Bapat <ashutosh.bapat@2ndquadrant.com>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/CAExHW5sWx49rKmXbg5H1Xc1t+nRv9PaYKQmgw82HPt6vWDVmDg@mail.gmail.com

Author: alvherre

src/backend/access/rmgrdesc/logicalmsgdesc.c

@@ -24,10 +24,21 @@ logicalmsg_desc(StringInfo buf, XLogReaderState *record)
 	if (info == XLOG_LOGICAL_MESSAGE)
 	{
 		xl_logical_message *xlrec = (xl_logical_message *) rec;
+		char	   *prefix = xlrec->message;
+		char	   *message = xlrec->message + xlrec->prefix_size;
+		char	   *sep = "";
 
-		appendStringInfo(buf, "%s message size %zu bytes",
-						 xlrec->transactional ? "transactional" : "nontransactional",
-						 xlrec->message_size);
+		Assert(prefix[xlrec->prefix_size] != '\0');
+
+		appendStringInfo(buf, "%s, prefix \"%s\"; payload (%zu bytes): ",
+						 xlrec->transactional ? "transactional" : "non-transactional",
+						 prefix, xlrec->message_size);
+		/* Write message payload as a series of hex bytes */
+		for (int cnt = 0; cnt < xlrec->message_size; cnt++)
+		{
+			appendStringInfo(buf, "%s%02X", sep, (unsigned char) message[cnt]);
+			sep = " ";
+		}
 	}
 }
 

src/backend/replication/logical/message.c

@@ -59,6 +59,7 @@ LogLogicalMessage(const char *prefix, const char *message, size_t size,
 
 	xlrec.dbId = MyDatabaseId;
 	xlrec.transactional = transactional;
+	/* trailing zero is critical; see logicalmsg_desc */
 	xlrec.prefix_size = strlen(prefix) + 1;
 	xlrec.message_size = size;
 

src/include/replication/message.h

@@ -23,9 +23,8 @@ typedef struct xl_logical_message
 	bool		transactional;	/* is message transactional? */
 	Size		prefix_size;	/* length of prefix */
 	Size		message_size;	/* size of the message */
-	char		message[FLEXIBLE_ARRAY_MEMBER]; /* message including the null
-												 * terminated prefix of length
-												 * prefix_size */
+	/* payload, including null-terminated prefix of length prefix_size */
+	char		message[FLEXIBLE_ARRAY_MEMBER];
 } xl_logical_message;
 
 #define SizeOfLogicalMessage	(offsetof(xl_logical_message, message))