add "show help" command

[sebastianwebber]

Hey Everyone, I'm new to Rust and this is an attempt to implement the show help command.

Currently, it has a bug in the package that I'm trying to figure out:

❯ PGPASSWORD=postgres psql -h 127.0.0.1 -p 6432 -U postgres pgbouncer -c 'show help';
NOTICE:  Console usage
DETAIL:
	SHOW HELP|CONFIG|DATABASES|POOLS|CLIENTS|SERVERS|USERS|VERSION
	SHOW LISTS
	SHOW STATS
	SET key = arg
	RELOAD
	PAUSE [<db>, <user>]
	RESUME [<db>, <user>]
	SHUTDOWN
message contents do not agree with length in message type "N"
SHOW

I've understood that pgcat is a replacement for pgbouncer, so I copied the help in the same format as pgbouncer does in admin.c.

Currently trying to figure what is the cause of the error below:

message contents do not agree with length in message type "N"

[levkk]

Subtract one (-1). The length of the message does not include the message identifier (N).

[sebastianwebber]

when chaging to +3, like this:

diff --git a/src/messages.rs b/src/messages.rs
index cb3b595..b1cd3f9 100644
--- a/src/messages.rs
+++ b/src/messages.rs
@@ -541,7 +541,7 @@ pub fn notify(message: &str, details: String) -> BytesMut {
 
     let mut res = BytesMut::new();
     res.put_u8(b'N');
-    res.put_i32(notify_cmd.len() as i32 + 4);
+    res.put_i32(notify_cmd.len() as i32 + 3);
     res.put(notify_cmd);
 
     res

it drops the connection:

❯ PGPASSWORD=postgres psql -h 127.0.0.1 -p 6432 -U postgres pgbouncer -c 'show help';
NOTICE:  Console usage
DETAIL:
	SHOW HELP|CONFIG|DATABASES|POOLS|CLIENTS|SERVERS|USERS|VERSION
	SHOW LISTS
	SHOW STATS
	SET key = arg
	RELOAD
	PAUSE [<db>, <user>]
	RESUME [<db>, <user>]
	SHUTDOWN
message contents do not agree with length in message type "N"
lost synchronization with server: got message type "
connection to server was lost

[sebastianwebber]

From the log:

[2023-07-13T20:43:36.328145Z WARN  pgcat] Client disconnected with error SocketError("Error reading message code from socket - Error Kind(UnexpectedEof)")

[levkk]

Ah nevermind, it must be something else. From the looks of it, your implementation seems correct.

[sebastianwebber]

do you have any idea how to troubleshoot this?

[levkk]

Ahh! I think there is a missing NULL byte. This is how we format the error message (same format as notice): https://github.com/postgresml/pgcat/blob/main/src/messages.rs#L357-L358

[sebastianwebber]

It worked! 💪

I've rebased and added a better commit message.

Is there anything else that you need me to change?

[levkk]

looks great!