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 aecd903dbe9b85bd06420ab1dfdf6f98f2a19baf..69ae62269989b8a3caba4eb6c3445cc36fd0afe1 100644 (file)
--- a/doc/src/sgml/libpq.sgml
+++ b/doc/src/sgml/libpq.sgml
@@ -5000,7 +5000,7 @@ 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</> messages was to constantly submit commands, even
    empty ones, and then check <function>PQnotifies</function> after each
@@ -8313,6 +8313,7 @@ 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 edccc806314be3a2fc815faec7a327a4ec6b2eaf..5c028208d63789b2d5b2f2166c3daac02615f727 100644 (file)
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -693,7 +693,8 @@ 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])
@@ -704,6 +705,7 @@ 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 5b09233810d6ebffeac01079fe7336db6baf064e..5b32e7cbcb2187e6e3874b9c1ffa1230005ef893 100644 (file)
--- a/src/interfaces/ecpg/ecpglib/execute.c
+++ b/src/interfaces/ecpg/ecpglib/execute.c
@@ -1730,12 +1730,13 @@ 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 ca20dd130c30487cf107eec07e0713e8edf53e21..7afdb1b226a4e060d90e3dd5ed589a7e89c02c5d 100644 (file)
--- a/src/interfaces/libpq/fe-exec.c
+++ b/src/interfaces/libpq/fe-exec.c
@@ -2239,6 +2239,9 @@ 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);
                }
        }