Le 01/07/2021 à 18:47, Tom Lane a écrit :
Nicolas CHAHWEKILIAN <leptitstagia...@gmail.com> writes:
As far as I am concerned, I am totally awaiting for this kind of feature
exposed here, for one single reason at this time : the extension
pg_statement_rollback will be much more valuable with the ability of
processing "rollback to savepoint" without the need for explicit
instruction from client side (and this patch is giving this option).
What exactly do these hooks do that isn't done as well or better
by the RegisterXactCallback and RegisterSubXactCallback mechanisms?
Perhaps we need to define some additional event types for those?
Or pass more data to the callback functions?
I quite dislike inventing a hook that's defined as "run during
start_xact_command", because there is basically nothing that's
not ad-hoc about that function: it's internal to postgres.c
and both its responsibilities and its call sites have changed
over time. I think anyone hooking into that will be displeased
by the stability of their results.
Sorry Tom, it seems that I have totally misinterpreted your comments,
google translate was not a great help for my understanding but Julien
was. Thanks Julien.
I'm joining a new patch v4 that removes the need of any hook and adds a
new events XACT_EVENT_COMMAND_START and SUBXACT_EVENT_COMMAND_START that
can be cautch in the xact callbacks when a new command is to be executed.
--
Gilles Darold
http://www.darold.net/
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 441445927e..3b5f6bfc2d 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -6118,3 +6118,32 @@ MarkSubTransactionAssigned(void)
CurrentTransactionState->assigned = true;
}
+
+/*
+ * CallXactStartCommand
+ *
+ * Wrapper over CallXactCallbacks or CallSubXactCallbacks called in postgres.c
+ * at end of start_xact_command(). It allows to user-defined code to be executed
+ * for the start of any command through a xact registered callback. This function
+ * do nothing if a transaction is not started.
+ *
+ * The related events XactEvent/SubXactEvent are XACT_EVENT_COMMAND_START and
+ * SUBXACT_EVENT_COMMAND_START.
+ */
+void
+CallXactStartCommand(void)
+{
+ TransactionState s = CurrentTransactionState;
+
+ if (s->blockState == TBLOCK_DEFAULT || s->blockState == TBLOCK_STARTED)
+ return;
+
+ /*
+ * Call start-of-xact callbacks with start command event
+ */
+ if (s->parent && s->subTransactionId)
+ CallSubXactCallbacks(SUBXACT_EVENT_COMMAND_START, s->subTransactionId,
+ s->parent->subTransactionId);
+ else
+ CallXactCallbacks(XACT_EVENT_COMMAND_START);
+}
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 8cea10c901..a0f4a17c51 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -2708,6 +2708,16 @@ start_xact_command(void)
!get_timeout_active(CLIENT_CONNECTION_CHECK_TIMEOUT))
enable_timeout_after(CLIENT_CONNECTION_CHECK_TIMEOUT,
client_connection_check_interval);
+
+ /*
+ * Instruct registered callbacks on xact that a new command is to be
+ * executed. It allows to execute user-defined code before any new
+ * statement is executed. This is the responsability of the user code
+ * to take care of the state of the transaction, it can be in an ABORT
+ * state. The related xact events are XACT_EVENT_COMMAND_START and
+ * SUBXACT_EVENT_COMMAND_START.
+ */
+ CallXactStartCommand();
}
static void
diff --git a/src/include/access/xact.h b/src/include/access/xact.h
index 134f6862da..190fc7151b 100644
--- a/src/include/access/xact.h
+++ b/src/include/access/xact.h
@@ -119,7 +119,8 @@ typedef enum
XACT_EVENT_PREPARE,
XACT_EVENT_PRE_COMMIT,
XACT_EVENT_PARALLEL_PRE_COMMIT,
- XACT_EVENT_PRE_PREPARE
+ XACT_EVENT_PRE_PREPARE,
+ XACT_EVENT_COMMAND_START
} XactEvent;
typedef void (*XactCallback) (XactEvent event, void *arg);
@@ -129,7 +130,8 @@ typedef enum
SUBXACT_EVENT_START_SUB,
SUBXACT_EVENT_COMMIT_SUB,
SUBXACT_EVENT_ABORT_SUB,
- SUBXACT_EVENT_PRE_COMMIT_SUB
+ SUBXACT_EVENT_PRE_COMMIT_SUB,
+ SUBXACT_EVENT_COMMAND_START
} SubXactEvent;
typedef void (*SubXactCallback) (SubXactEvent event, SubTransactionId mySubid,
@@ -467,4 +469,6 @@ extern void EnterParallelMode(void);
extern void ExitParallelMode(void);
extern bool IsInParallelMode(void);
+extern void CallXactStartCommand(void);
+
#endif /* XACT_H */
