Re: proposal - psql - use pager for \watch command - Mailing list pgsql-hackers

From Pavel Stehule
Subject Re: proposal - psql - use pager for \watch command
Date
Msg-id CAFj8pRB1K1RG=MeJzLhad2hVoeQgug-MPcRmb0o5d_MHzqd2AQ@mail.gmail.com
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In response to Re: proposal - psql - use pager for \watch command  (Thomas Munro <thomas.munro@gmail.com>)
List pgsql-hackers
Hi

út 16. 2. 2021 v 2:49 odesílatel Thomas Munro <thomas.munro@gmail.com> napsal:
On Fri, Jan 8, 2021 at 10:36 PM Pavel Stehule <pavel.stehule@gmail.com> wrote:
> ne 19. 4. 2020 v 19:27 odesílatel Pavel Stehule <pavel.stehule@gmail.com> napsal:
>> last week I finished pspg 3.0 https://github.com/okbob/pspg . pspg now supports pipes, named pipes very well. Today the pspg can be used as pager for output of \watch command. Sure, psql needs attached patch.
>>
>> I propose new psql environment variable PSQL_WATCH_PAGER. When this variable is not empty, then \watch command starts specified pager, and redirect output to related pipe. When pipe is closed - by pager, then \watch cycle is leaved.
>>
>> If you want to test proposed feature, you need a pspg with cb4114f98318344d162a84b895a3b7f8badec241 commit.
>>
>> Then you can set your env
>>
>> export PSQL_WATCH_PAGER="pspg --stream"
>> psql
>>
>> SELECT * FROM pg_stat_database;
>> \watch 1
>>
>> Comments, notes?

I tried this out with pspg 4.1 from my package manager.  It seems
really useful, especially for demos.  I like it!

Thank you :)


         * Set up rendering options, in particular, disable the pager, because
         * nobody wants to be prompted while watching the output of 'watch'.
         */
-       myopt.topt.pager = 0;
+       if (!pagerpipe)
+               myopt.topt.pager = 0;

Obsolete comment.

+static bool sigpipe_received = false;

This should be "static volatile sig_atomic_t", and I suppose our
convention name for that variable would be got_SIGPIPE.  Would it be
possible to ignore SIGPIPE instead, and then rely on another way of
knowing that the pager has quit?  But... hmm:

-                       long            s = Min(i, 1000L);
+                       long            s = Min(i, pagerpipe ? 100L : 1000L);

I haven't studied this (preexisting) polling loop, but I don't like
it.  I understand that it's there because on some systems, pg_usleep()
won't wake up for SIGINT (^C), but now it's being used for a secondary
purpose, that I haven't fully understood.  After I quit pspg (by
pressing q) while running \watch 10, I have to wait until the end of a
10 second cycle before it tries to write to the pipe again, unless I
also press ^C.  I feel like it has to be possible to achieve "precise"
behaviour somehow when you quit; maybe something like waiting for
readiness on the pager's stderr, or something like that -- I haven't
thought hard about this and I admit that I have no idea how this works
on Windows.

I'll look there.


Sometimes I see a message like this after I quit pspg:

postgres=# \watch 10
input stream was closed

This is a pspg's message. It's a little bit strange, because this message comes from event reading, and in the end, the pspg doesn't read events. So it looks like the pspg issue, and I have to check it.

I have one question - now, the pspg has to do complex heuristics to detect an start and an end of data in an stream. Can we, in this case (when PSQL_WATCH_PAGER is active), use invisible chars STX and ETX or maybe ETB? It can be a special \pset option. Surely, the detection of these chars should be much more robust than current pspg's heuristics.

Regards

Pavel
 

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