Re: [HACKERS] listening addresses

Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
>> A small problem with it was reported to me a couple of days ago -
>> user had firewalled off all IP6 traffic. The stats collector happily
>> bound and connected to the socket, but all the packets fell in the
>> bit bucket. They found it quite hard to diagnose the problem.

> Revised patch attached. I think this is about as much trouble as this
> problem is worth ;-)

I thought the messages were a bit sloppy, which made the patch much less
useful than it should be: we are testing for a very specific failure
mode and we can give a very specific message. Patch as-applied is
attached.

I don't have any real convenient way to set up a situation where this
failure can actually occur. Anyone want to verify that the patch
acts as intended?

regards, tom lane

*** src/backend/postmaster/pgstat.c.orig Mon Mar 15 15:01:57 2004
--- src/backend/postmaster/pgstat.c Mon Mar 22 18:55:29 2004
***************
*** 191,196 ****
--- 191,202 ----
*addr,
hints;
int ret;
+ fd_set rset;
+ struct timeval tv;
+ char test_byte;
+ int sel_res;
+
+ #define TESTBYTEVAL ((char) 199)

/*
* Force start of collector daemon if something to collect
***************
*** 303,308 ****
--- 309,393 ----
ereport(LOG,
(errcode_for_socket_access(),
errmsg("could not connect socket for statistics collector: %m")));
+ closesocket(pgStatSock);
+ pgStatSock = -1;
+ continue;
+ }
+
+ /*
+ * Try to send and receive a one-byte test message on the socket.
+ * This is to catch situations where the socket can be created but
+ * will not actually pass data (for instance, because kernel packet
+ * filtering rules prevent it).
+ */
+ test_byte = TESTBYTEVAL;
+ if (send(pgStatSock, &test_byte, 1, 0) != 1)
+ {
+ ereport(LOG,
+ (errcode_for_socket_access(),
+ errmsg("could not send test message on socket for statistics collector: %m")));
+ closesocket(pgStatSock);
+ pgStatSock = -1;
+ continue;
+ }
+
+ /*
+ * There could possibly be a little delay before the message can be
+ * received. We arbitrarily allow up to half a second before deciding
+ * it's broken.
+ */
+ for (;;) /* need a loop to handle EINTR */
+ {
+ FD_ZERO(&rset);
+ FD_SET(pgStatSock, &rset);
+ tv.tv_sec = 0;
+ tv.tv_usec = 500000;
+ sel_res = select(pgStatSock+1, &rset, NULL, NULL, &tv);
+ if (sel_res >= 0 || errno != EINTR)
+ break;
+ }
+ if (sel_res < 0)
+ {
+ ereport(LOG,
+ (errcode_for_socket_access(),
+ errmsg("select() failed in statistics collector: %m")));
+ closesocket(pgStatSock);
+ pgStatSock = -1;
+ continue;
+ }
+ if (sel_res == 0 || !FD_ISSET(pgStatSock, &rset))
+ {
+ /*
+ * This is the case we actually think is likely, so take pains to
+ * give a specific message for it.
+ *
+ * errno will not be set meaningfully here, so don't use it.
+ */
+ ereport(LOG,
+ (ERRCODE_CONNECTION_FAILURE,
+ errmsg("test message did not get through on socket for statistics collector")));
+ closesocket(pgStatSock);
+ pgStatSock = -1;
+ continue;
+ }
+
+ test_byte++; /* just make sure variable is changed */
+
+ if (recv(pgStatSock, &test_byte, 1, 0) != 1)
+ {
+ ereport(LOG,
+ (errcode_for_socket_access(),
+ errmsg("could not receive test message on socket for statistics collector: %m")));
+ closesocket(pgStatSock);
+ pgStatSock = -1;
+ continue;
+ }
+
+ if (test_byte != TESTBYTEVAL) /* strictly paranoia ... */
+ {
+ ereport(LOG,
+ (ERRCODE_INTERNAL_ERROR,
+ errmsg("incorrect test message transmission on socket for statistics collector")));
closesocket(pgStatSock);
pgStatSock = -1;
continue;