Hi hackers,
I found a problem when executing the plpython function:
After the plpython function returns an error, in the same session, if we
continue to execute
plpython function, the server panic will be caused.
*Reproduce*
preparation
SET max_parallel_workers_per_gather=4;
SET parallel_setup_cost=1;
SET min_parallel_table_scan_size ='4kB';
CREATE TABLE t(i int);
INSERT INTO t SELECT generate_series(1, 10000);
CREATE EXTENSION plpython3u;
CREATE OR REPLACE FUNCTION test_func() RETURNS SETOF int AS
$$
plpy.execute("select pg_backend_pid()")
for i in range(0, 5):
yield (i)
$$ LANGUAGE plpython3u parallel safe;
execute the function twice in the same session
postgres=# SELECT test_func() from t where i>10 and i<100;
ERROR: error fetching next item from iterator
DETAIL: Exception: cannot start subtransactions during a parallel
operation
CONTEXT: Traceback (most recent call last):
PL/Python function "test_func"
postgres=# SELECT test_func();
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
The connection to the server was lost. Attempting reset: Failed.
*Analysis*
- There is an SPI call in test_func(): plpy.execute().
- Then the server will start a subtransaction by
PLy_spi_subtransaction_begin(); BUT! The Python processor does not know
whether an error happened during PLy_spi_subtransaction_begin().
- If there is an error that occurs in PLy_spi_subtransaction_begin(),
the SPI call will be terminated but the python error indicator won't be set
and the PyObject won't be free.
- Then the next plpython UDF in the same session will fail due to the
wrong Python environment.
*Solution*
Use try-catch to catch the error that occurs in
PLy_spi_subtransaction_begin(), and set the python error indicator.
With Regards
Hao Zhang
From 891ff26ba9b52f3eba6f0d112657c3b288524de2 Mon Sep 17 00:00:00 2001
From: Hao Zhang <zhrt1446384...@gmail.com>
Date: Thu, 30 Nov 2023 14:51:07 +0800
Subject: [PATCH v1] Fix bug: plpython function causes server panic.
---
src/pl/plpython/plpy_cursorobject.c | 12 ++++++---
src/pl/plpython/plpy_spi.c | 41 +++++++++++++++++++++++------
src/pl/plpython/plpy_spi.h | 2 +-
3 files changed, 42 insertions(+), 13 deletions(-)
diff --git a/src/pl/plpython/plpy_cursorobject.c b/src/pl/plpython/plpy_cursorobject.c
index 57e8f8ec21..8bfe05815b 100644
--- a/src/pl/plpython/plpy_cursorobject.c
+++ b/src/pl/plpython/plpy_cursorobject.c
@@ -98,7 +98,8 @@ PLy_cursor_query(const char *query)
oldcontext = CurrentMemoryContext;
oldowner = CurrentResourceOwner;
- PLy_spi_subtransaction_begin(oldcontext, oldowner);
+ if (!PLy_spi_subtransaction_begin(oldcontext, oldowner))
+ return NULL;
PG_TRY();
{
@@ -196,7 +197,8 @@ PLy_cursor_plan(PyObject *ob, PyObject *args)
oldcontext = CurrentMemoryContext;
oldowner = CurrentResourceOwner;
- PLy_spi_subtransaction_begin(oldcontext, oldowner);
+ if (!PLy_spi_subtransaction_begin(oldcontext, oldowner))
+ return NULL;
PG_TRY();
{
@@ -333,7 +335,8 @@ PLy_cursor_iternext(PyObject *self)
oldcontext = CurrentMemoryContext;
oldowner = CurrentResourceOwner;
- PLy_spi_subtransaction_begin(oldcontext, oldowner);
+ if (!PLy_spi_subtransaction_begin(oldcontext, oldowner))
+ return NULL;
PG_TRY();
{
@@ -403,7 +406,8 @@ PLy_cursor_fetch(PyObject *self, PyObject *args)
oldcontext = CurrentMemoryContext;
oldowner = CurrentResourceOwner;
- PLy_spi_subtransaction_begin(oldcontext, oldowner);
+ if (!PLy_spi_subtransaction_begin(oldcontext, oldowner))
+ return NULL;
PG_TRY();
{
diff --git a/src/pl/plpython/plpy_spi.c b/src/pl/plpython/plpy_spi.c
index ff87b27de0..58a274eda9 100644
--- a/src/pl/plpython/plpy_spi.c
+++ b/src/pl/plpython/plpy_spi.c
@@ -77,7 +77,8 @@ PLy_spi_prepare(PyObject *self, PyObject *args)
oldcontext = CurrentMemoryContext;
oldowner = CurrentResourceOwner;
- PLy_spi_subtransaction_begin(oldcontext, oldowner);
+ if (!PLy_spi_subtransaction_begin(oldcontext, oldowner))
+ return NULL;
PG_TRY();
{
@@ -217,7 +218,8 @@ PLy_spi_execute_plan(PyObject *ob, PyObject *list, long limit)
oldcontext = CurrentMemoryContext;
oldowner = CurrentResourceOwner;
- PLy_spi_subtransaction_begin(oldcontext, oldowner);
+ if (!PLy_spi_subtransaction_begin(oldcontext, oldowner))
+ return NULL;
PG_TRY();
{
@@ -313,7 +315,8 @@ PLy_spi_execute_query(char *query, long limit)
oldcontext = CurrentMemoryContext;
oldowner = CurrentResourceOwner;
- PLy_spi_subtransaction_begin(oldcontext, oldowner);
+ if (!PLy_spi_subtransaction_begin(oldcontext, oldowner))
+ return NULL;
PG_TRY();
{
@@ -556,7 +559,9 @@ PLy_rollback(PyObject *self, PyObject *args)
* MemoryContext oldcontext = CurrentMemoryContext;
* ResourceOwner oldowner = CurrentResourceOwner;
*
- * PLy_spi_subtransaction_begin(oldcontext, oldowner);
+ * if (!PLy_spi_subtransaction_begin(oldcontext, oldowner))
+ * return NULL;
+ *
* PG_TRY();
* {
* <call SPI functions>
@@ -573,12 +578,32 @@ PLy_rollback(PyObject *self, PyObject *args)
* These utilities take care of restoring connection to the SPI manager and
* setting a Python exception in case of an abort.
*/
-void
+bool
PLy_spi_subtransaction_begin(MemoryContext oldcontext, ResourceOwner oldowner)
{
- BeginInternalSubTransaction(NULL);
- /* Want to run inside function's memory context */
- MemoryContextSwitchTo(oldcontext);
+ /*
+ * Catch the error when begin a subtransaction.
+ * Set the python error indicator if any error occurs to free
+ * the traced pyobjects.
+ */
+ PG_TRY();
+ {
+ BeginInternalSubTransaction(NULL);
+ /* Want to run inside function's memory context */
+ MemoryContextSwitchTo(oldcontext);
+ }
+ PG_CATCH();
+ {
+ MemoryContextSwitchTo(oldcontext);
+ CurrentResourceOwner = oldowner;
+
+ ErrorData *edata = CopyErrorData();
+ PLy_spi_exception_set(PyExc_Exception, edata);
+ FreeErrorData(edata);
+ return false;
+ }
+ PG_END_TRY();
+ return true;
}
void
diff --git a/src/pl/plpython/plpy_spi.h b/src/pl/plpython/plpy_spi.h
index 98ccd21093..a43d803928 100644
--- a/src/pl/plpython/plpy_spi.h
+++ b/src/pl/plpython/plpy_spi.h
@@ -22,7 +22,7 @@ typedef struct PLyExceptionEntry
} PLyExceptionEntry;
/* handling of SPI operations inside subtransactions */
-extern void PLy_spi_subtransaction_begin(MemoryContext oldcontext, ResourceOwner oldowner);
+extern bool PLy_spi_subtransaction_begin(MemoryContext oldcontext, ResourceOwner oldowner);
extern void PLy_spi_subtransaction_commit(MemoryContext oldcontext, ResourceOwner oldowner);
extern void PLy_spi_subtransaction_abort(MemoryContext oldcontext, ResourceOwner oldowner);
--
2.43.0
