From 34aad21cbf03fd0d49e7322efdd18c9bf692f53d Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Fri, 19 Oct 2018 22:22:57 -0400 Subject: [PATCH] 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 --- doc/src/sgml/libpq.sgml | 3 ++- src/bin/psql/common.c | 4 +++- src/interfaces/ecpg/ecpglib/execute.c | 5 +++-- src/interfaces/libpq/fe-exec.c | 3 +++ src/test/examples/testlibpq2.c | 1 + 5 files changed, 12 insertions(+), 4 deletions(-) diff --git a/doc/src/sgml/libpq.sgml b/doc/src/sgml/libpq.sgml index aecd903dbe9..69ae6226998 100644 --- a/doc/src/sgml/libpq.sgml +++ b/doc/src/sgml/libpq.sgml @@ -5000,7 +5000,7 @@ typedef struct pgNotify PQnotifies does not actually read data from the server; it just returns messages previously absorbed by another - libpq function. In prior releases of + libpq function. In ancient releases of libpq, the only way to ensure timely receipt of NOTIFY messages was to constantly submit commands, even empty ones, and then check PQnotifies 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 edccc806314..5c028208d63 100644 --- 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 5b09233810d..5b32e7cbcb2 100644 --- 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 ca20dd130c3..7afdb1b226a 100644 --- 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 62ecd68b55e..6cdf8c8631b 100644 --- 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); } }