Before deciding we can use a socket for statistics collection, test to

ensure that it actually passes data.  This catches cases such as a kernel
packet filter rule that makes the socket useless.

Andrew Dunstan

diff --git a/src/backend/postmaster/pgstat.c b/src/backend/postmaster/pgstat.c
index 15cff4d..74ba693 100644 (file)
--- a/src/backend/postmaster/pgstat.c
+++ b/src/backend/postmaster/pgstat.c
@@ -13,7 +13,7 @@
  *
  *     Copyright (c) 2001-2003, PostgreSQL Global Development Group
  *
- *     $PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.61 2004/03/15 16:21:37 momjian Exp $
+ *     $PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.62 2004/03/22 23:55:29 tgl Exp $
  * ----------
  */
 #include "postgres.h"
@@ -191,6 +191,12 @@ pgstat_init(void)
                           *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
@@ -308,6 +314,85 @@ pgstat_init(void)
                        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;
+               }
+
                /* If we get here, we have a working socket */
                break;
        }