Client-side fixes for delayed NOTIFY receipt.

PQnotifies() is defined to just process already-read data, not try to read
any more from the socket.  (This is a debatable decision, perhaps, but I'm
hesitant to change longstanding library behavior.)  The documentation has
long recommended calling PQconsumeInput() before PQnotifies() to ensure
that any already-arrived message would get absorbed and processed.
However, psql did not get that memo, which explains why it's not very
reliable about reporting notifications promptly.

Also, most (not quite all) callers called PQconsumeInput() just once before
a PQnotifies() loop.  Taking this recommendation seriously implies that we
should do PQconsumeInput() before each call.  This is more important now
that we have "payload" strings in notification messages than it was before;
that increases the probability of having more than one packet's worth
of notify messages.  Hence, adjust code as well as documentation examples
to do it like that.

Back-patch to 9.5 to match related server fixes.  In principle we could
probably go back further with these changes, but given lack of field
complaints I doubt it's worthwhile.

Discussion: https://postgr.es/m/CAOYf6ec-TmRYjKBXLLaGaB-jrd=mjG1Hzn1a1wufUAR39PQYhw@mail.gmail.com

diff --git a/doc/src/sgml/libpq.sgml b/doc/src/sgml/libpq.sgml
index 06d909e80493ee94f9410ffdd67d9052c70aace3..82a440531b5163f5c6fe5804db89ebb5f1292c4d 100644 (file)
--- a/doc/src/sgml/libpq.sgml
+++ b/doc/src/sgml/libpq.sgml
@@ -5284,7 +5284,7 @@ typedef struct pgNotify
   <para>
    <function>PQnotifies</function> does not actually read data from the
    server; it just returns messages previously absorbed by another
-   <application>libpq</application> function.  In prior releases of
+   <application>libpq</application> function.  In ancient releases of
    <application>libpq</application>, the only way to ensure timely receipt
    of <command>NOTIFY</command> messages was to constantly submit commands, even
    empty ones, and then check <function>PQnotifies</function> after each
@@ -8711,6 +8711,7 @@ main(int argc, char **argv)
                     notify->relname, notify->be_pid);
             PQfreemem(notify);
             nnotifies++;
+            PQconsumeInput(conn);
         }
     }
 

diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index b56995925bd8fbb640b9c0c3d96b7a9e23cca522..62c2928e6bbcdc83cfcfc4644cd786c54c92aeee 100644 (file)
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -836,7 +836,8 @@ PrintNotifications(void)
 {
        PGnotify   *notify;
 
-       while ((notify = PQnotifies(pset.db)))
+       PQconsumeInput(pset.db);
+       while ((notify = PQnotifies(pset.db)) != NULL)
        {
                /* for backward compatibility, only show payload if nonempty */
                if (notify->extra[0])
@@ -847,6 +848,7 @@ PrintNotifications(void)
                                        notify->relname, notify->be_pid);
                fflush(pset.queryFout);
                PQfreemem(notify);
+               PQconsumeInput(pset.db);
        }
 }
 

diff --git a/src/interfaces/ecpg/ecpglib/execute.c b/src/interfaces/ecpg/ecpglib/execute.c
index ff244498da4bf913b0d98a8ecf1fd78463687b51..42640ba9be1a614b1b8eac519d8b22a6862fc830 100644 (file)
--- a/src/interfaces/ecpg/ecpglib/execute.c
+++ b/src/interfaces/ecpg/ecpglib/execute.c
@@ -1722,12 +1722,13 @@ ecpg_process_output(struct statement *stmt, bool clear_result)
        }
 
        /* check for asynchronous returns */
-       notify = PQnotifies(stmt->connection->connection);
-       if (notify)
+       PQconsumeInput(stmt->connection->connection);
+       while ((notify = PQnotifies(stmt->connection->connection)) != NULL)
        {
                ecpg_log("ecpg_process_output on line %d: asynchronous notification of \"%s\" from backend PID %d received\n",
                                 stmt->lineno, notify->relname, notify->be_pid);
                PQfreemem(notify);
+               PQconsumeInput(stmt->connection->connection);
        }
 
        return status;

diff --git a/src/interfaces/libpq/fe-exec.c b/src/interfaces/libpq/fe-exec.c
index e8b28d9ccf9525162093b963feb28f7561cbb581..6aed8c87c7a283c4e823dc94237cad3714a2fba5 100644 (file)
--- a/src/interfaces/libpq/fe-exec.c
+++ b/src/interfaces/libpq/fe-exec.c
@@ -2265,6 +2265,9 @@ sendFailed:
  * no unhandled async notification from the backend
  *
  * the CALLER is responsible for FREE'ing the structure returned
+ *
+ * Note that this function does not read any new data from the socket;
+ * so usually, caller should call PQconsumeInput() first.
  */
 PGnotify *
 PQnotifies(PGconn *conn)

diff --git a/src/test/examples/testlibpq2.c b/src/test/examples/testlibpq2.c
index 62ecd68b55e39799d27789a4d980799fbcb604cb..6cdf8c8631ba89c46f7b9b271dd45c88d0762557 100644 (file)
--- a/src/test/examples/testlibpq2.c
+++ b/src/test/examples/testlibpq2.c
@@ -140,6 +140,7 @@ main(int argc, char **argv)
                                        notify->relname, notify->be_pid);
                        PQfreemem(notify);
                        nnotifies++;
+                       PQconsumeInput(conn);
                }
        }