Print logical WAL message content

On 2020-Aug-18, Ashutosh Bapat wrote:

Right now pg_waldump just prints whether the message is transactional
or not and its size. That doesn't help much to understand the message
itself. If it prints the contents of a logical WAL message, it helps
debugging logical replication related problems. Prefix is a
null-terminated ASCII string, so no problem printing that. Even the
contents can be printed as a series of hex bytes. Here's a patch to do
that.

Looks like a good idea.

I didn't like that you're documenting the message format in the new
function:

xl_logical_message *xlrec = (xl_logical_message *) rec;
+		/*
+		 * Per LogLogicalMessage() actual logical message follows a null-terminated prefix of length
+		 * prefix_size.

I would prefer to remove this comment, and instead add a comment atop
xl_logical_message's struct definition in message.h to say that the
message has a valid C-string as prefix, whose length is prefix_size, and
please see logicalmesg_desc() if you change this.
This way, you don't need to blame LogLogicalMessage for this
restriction, but it's actually part of the definition of the WAL
message.

+		/*
+		 * Per LogLogicalMessage() actual logical message follows a null-terminated prefix of length
+		 * prefix_size.
+		 */
+		char   *prefix = xlrec->message;
+		char   *message = xlrec->message + xlrec->prefix_size;
+		int		cnt;
+		char   *sep = "";

This would cause a crash if the message actually fails to follow the
rule. Let's test that prefix[xlrec->prefix_size] is a trailing zero,
and if not, avoid printing it. Although, just Assert()'ing that it's a
trailing zero would seem to suffice.

+		appendStringInfo(buf, "%s message size %zu bytes, prefix %s; mesage: ",
xlrec->transactional ? "transactional" : "nontransactional",
-						 xlrec->message_size);
+						 xlrec->message_size, prefix);

Misspelled "message", but also the line looks a bit repetitive -- the
word "message" would appear three times:

lsn: 0/01570608, prev 0/015705D0, desc: MESSAGE nontransactional message size 12 bytes, prefix some_prefix; mesage: 73 6F 6D 65 20 6D 65 73 73 61 67 65

I would reduce it to

lsn: 0/01570608, prev 0/015705D0, desc: MESSAGE nontransactional, prefix "some_prefix"; payload (12 bytes): 73 6F 6D 65 20 6D 65 73 73 61 67 65

--
�lvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Thanks Alvaro for review.

On Wed, Aug 19, 2020 at 3:21 AM Alvaro Herrera <alvherre@2ndquadrant.com> wrote:

I didn't like that you're documenting the message format in the new
function:

xl_logical_message *xlrec = (xl_logical_message *) rec;
+             /*
+              * Per LogLogicalMessage() actual logical message follows a null-terminated prefix of length
+              * prefix_size.

I would prefer to remove this comment, and instead add a comment atop
xl_logical_message's struct definition in message.h to say that the
message has a valid C-string as prefix, whose length is prefix_size, and
please see logicalmesg_desc() if you change this.

It's documented in the struct definition. Added a note about logicalmesg_desc().

This way, you don't need to blame LogLogicalMessage for this
restriction, but it's actually part of the definition of the WAL
message.

+             /*
+              * Per LogLogicalMessage() actual logical message follows a null-terminated prefix of length
+              * prefix_size.
+              */
+             char   *prefix = xlrec->message;
+             char   *message = xlrec->message + xlrec->prefix_size;
+             int             cnt;
+             char   *sep = "";

This would cause a crash if the message actually fails to follow the
rule. Let's test that prefix[xlrec->prefix_size] is a trailing zero,
and if not, avoid printing it. Although, just Assert()'ing that it's a
trailing zero would seem to suffice.

Added an Assert.

+             appendStringInfo(buf, "%s message size %zu bytes, prefix %s; mesage: ",
xlrec->transactional ? "transactional" : "nontransactional",
-                                              xlrec->message_size);
+                                              xlrec->message_size, prefix);

Misspelled "message", but also the line looks a bit repetitive -- the
word "message" would appear three times:

lsn: 0/01570608, prev 0/015705D0, desc: MESSAGE nontransactional message size 12 bytes, prefix some_prefix; mesage: 73 6F 6D 65 20 6D 65 73 73 61 67 65

I would reduce it to

lsn: 0/01570608, prev 0/015705D0, desc: MESSAGE nontransactional, prefix "some_prefix"; payload (12 bytes): 73 6F 6D 65 20 6D 65 73 73 61 67 65

I like this format as well. Done.

PFA the patch attached with your comments addressed.

Thanks for your review.

--
Best Wishes,
Ashutosh Bapat

On 2020-Aug-19, Ashutosh Bapat wrote:

I like this format as well. Done.

PFA the patch attached with your comments addressed.

Pushed, thanks!

--
�lvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

On Fri, 11 Sep 2020 at 04:08, Alvaro Herrera <alvherre@2ndquadrant.com>
wrote:

Pushed, thanks!

Thanks Alvaro.

--
Best Wishes,
Ashutosh