Re: pgbnech: allow to cancel queries during benchmark

> On Wed, 6 Sep 2023 20:13:34 +0900
> Yugo NAGATA <nagata@sraoss.co.jp> wrote:
>  
>> I attached the updated patch v3. The changes since the previous
>> patch includes the following;
>> 
>> I removed the unnecessary condition (&& false) that you
>> pointed out in [1].
>> 
>> The test was rewritten by using IPC::Run signal() and integrated
>> to "001_pgbench_with_server.pl". This test is skipped on Windows
>> because SIGINT causes to terminate the test itself as discussed
>> in [2] about query cancellation test in psql.
>> 
>> I added some comments to describe how query cancellation is
>> handled as I explained in [1].
>> 
>> Also, I found the previous patch didn't work on Windows so fixed it.
>> On non-Windows system, a thread waiting a response of long query can
>> be interrupted by SIGINT, but on Windows, threads do not return from
>> waiting until queries they are running are cancelled. This is because,
>> when the signal is received, the system just creates a new thread to
>> execute the callback function specified by setup_cancel_handler, and
>> other thread continue to run[3]. Therefore, the queries have to be
>> cancelled in the callback function.
>> 
>> [1] https://www.postgresql.org/message-id/a58388ac-5411-4760-ea46-71324d8324cb%40mines-paristech.fr
>> [2] https://www.postgresql.org/message-id/20230906004524.2fd6ee049f8a6c6f2690b99c%40sraoss.co.jp
>> [3] https://learn.microsoft.com/en-us/windows/console/handlerroutine
> 
> I found that --disable-thread-safety option was removed in 68a4b58eca0329.
> So, I removed codes involving ENABLE_THREAD_SAFETY from the patch.
> 
> Also, I wrote a commit log draft.

> Previously, Ctrl+C during benchmark killed pgbench immediately,
> but queries running at that time were not cancelled.

Better to mention explicitely that queries keep on running on the
backend. What about this?

Previously, Ctrl+C during benchmark killed pgbench immediately, but
queries were not canceled and they keep on running on the backend
until they tried to send the result to pgbench.

> The commit
> fixes this so that cancel requests are sent for all connections
> before pgbench exits.

"sent for" -> "sent to"

> Attached is the updated version, v4.

+/* send cancel requests to all connections */
+static void
+cancel_all()
+{
+    for (int i = 0; i < nclients; i++)
+    {
+        char errbuf[1];
+        if (client_states[i].cancel != NULL)
+            (void) PQcancel(client_states[i].cancel, errbuf, sizeof(errbuf));
+    }
+}
+

Why in case of errors from PQCancel the error message is neglected? I
think it's better to print out the error message in case of error.

+     * On non-Windows, any callback function is not set. When SIGINT is
+     * received, CancelRequested is just set, and only thread #0 is interrupted
+     * and returns from waiting input from the backend. After that, the thread
+     * sends cancel requests to all benchmark queries.

The second line is a little bit long according to the coding
standard. Fix like this?

     * On non-Windows, any callback function is not set. When SIGINT is
     * received, CancelRequested is just set, and only thread #0 is
     * interrupted and returns from waiting input from the backend. After
     * that, the thread sends cancel requests to all benchmark queries.

Best reagards,
--
Tatsuo Ishii
SRA OSS LLC
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp