Re: Patch avoid call strlen repeatedly in loop.

On 11/8/19 9:41 AM, Ranier VF wrote:
> --- \dll\postgresql-12.0\a\backend\libpq\auth.c Mon Sep 30 17:06:55 2019
> +++ auth.c Fri Nov 08 14:27:17 2019
> @@ -1815,6 +1815,7 @@
> char ident_user[IDENT_USERNAME_MAX + 1];
> pgsocket sock_fd = PGINVALID_SOCKET; /* for talking to Ident server */
> int rc; /* Return code from a locally called function */
> + int ident_query_len;
> bool ident_return;
> char remote_addr_s[NI_MAXHOST];
> char remote_port[NI_MAXSERV];
> @@ -1913,7 +1914,7 @@
> }
>
> /* The query we send to the Ident server */
> - snprintf(ident_query, sizeof(ident_query), "%s,%s\r\n",
> + ident_query_len = snprintf(ident_query, sizeof(ident_query), "%s,%s\r\n",
> remote_port, local_port);
>
> /* loop in case send is interrupted */
> @@ -1921,7 +1922,7 @@
> {
> CHECK_FOR_INTERRUPTS();
>
> - rc = send(sock_fd, ident_query, strlen(ident_query), 0);
> + rc = send(sock_fd, ident_query, ident_query_len, 0);

Hello Ranier,

In general, writing a string with snprintf and then calling strlen on
that same string is not guaranteed to give the same lengths. You can
easily construct a case where they differ:

char foo[3] = {0};
int foolen;
foolen = snprintf(foo, sizeof(foo), "%s", "xxxxxxxx");
printf("strlen(foo) = %u, foolen = %u, foo = '%s'\n", strlen(foo),
foolen, foo);

Using standard snprintf (and not pg_snprintf), I get:

strlen(foo) = 2, foolen = 8, foo = 'xx'

Perhaps an analysis of the surrounding code would prove that in all
cases this particular snprintf will return the same result that
strlen(ident_query) would return, but I don't care to do the analysis.
I think the way it is coded is easier to read, and probably more robust
against future changes, even if your proposed change happens to be safe
today.

As for calling strlen(ident_query) just once, caching that result, and
then looping, I don't immediately see a problem, but I also don't expect
that loop to run more than one iteration except under unusual instances.
Do you find that send() gets interrupted a lot? Is
strlen(ident_query) taking long enough to be significant compared to how
long send() takes?

A bit more information about the performance problem you are
encountering might make it easier to understand the motivation for this
patch.

--
Mark Dilger