Fix SPI_cursor_open() and SPI_is_cursor_plan() to push the SPI stack before

doing anything interesting, such as calling RevalidateCachedPlan().  The
necessity of this is demonstrated by an example from Willem Buitendyk:
during a replan, the planner might try to evaluate SPI-using functions,
and so we'd better be in a clean SPI context.

A small downside of this fix is that these two functions will now fail
outright if called when not inside a SPI-using procedure (ie, a
SPI_connect/SPI_finish pair).  The documentation never promised or suggested
that that would work, though; and they are normally used in concert with
other functions, mainly SPI_prepare, that always have failed in such a case.
So the odds of breaking something seem pretty low.

In passing, make SPI_is_cursor_plan's error handling convention clearer,
and fix documentation's erroneous claim that SPI_cursor_open would
return NULL on error.

Before 8.3 these functions could not invoke replanning, so there is probably
no need for back-patching.

doc/src/sgml/spi.sgml

@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/spi.sgml,v 1.59 2007/09/14 04:18:27 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/spi.sgml,v 1.60 2008/02/12 04:09:44 tgl Exp $ -->
 
 <chapter id="spi">
  <title>Server Programming Interface</title>
@@ -1077,9 +1077,12 @@ bool SPI_is_cursor_plan(SPIPlanPtr <parameter>plan</parameter>)
   <title>Return Value</title>
   <para>
     <symbol>true</symbol> or <symbol>false</symbol> to indicate if the
-    <parameter>plan</parameter> can produce a cursor or not.
+    <parameter>plan</parameter> can produce a cursor or not, with
-    If the <parameter>plan</parameter> is <symbol>NULL</symbol> or invalid,
+    <varname>SPI_result</varname> set to zero.
-    <varname>SPI_result</varname> is set to <symbol>SPI_ERROR_ARGUMENT</symbol>
+    If it is not possible to determine the answer (for example,
+    if the <parameter>plan</parameter> is <symbol>NULL</symbol> or invalid,
+    or if called when not connected to SPI), then
+    <varname>SPI_result</varname> is set to a suitable error code
     and <symbol>false</symbol> is returned.
   </para>
  </refsect1>
@@ -1442,8 +1445,8 @@ Portal SPI_cursor_open(const char * <parameter>name</parameter>, SPIPlanPtr <par
   <title>Return Value</title>
 
   <para>
-   pointer to portal containing the cursor, or <symbol>NULL</symbol>
+   Pointer to portal containing the cursor.  Note there is no error
-   on error
+   return convention; any error will be reported via <function>elog</>.
   </para>
  </refsect1>
 </refentry>

src/backend/executor/spi.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/spi.c,v 1.187 2008/01/01 19:45:49 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/spi.c,v 1.188 2008/02/12 04:09:44 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -886,6 +886,10 @@ SPI_cursor_open(const char *name, SPIPlanPtr plan,
 	Assert(list_length(plan->plancache_list) == 1);
 	plansource = (CachedPlanSource *) linitial(plan->plancache_list);
 
+	/* Push the SPI stack */
+	if (_SPI_begin_call(false) < 0)
+		elog(ERROR, "SPI_cursor_open called while not connected");
+
 	/* Reset SPI result (note we deliberately don't touch lastoid) */
 	SPI_processed = 0;
 	SPI_tuptable = NULL;
@@ -1041,6 +1045,9 @@ SPI_cursor_open(const char *name, SPIPlanPtr plan,
 
 	Assert(portal->strategy != PORTAL_MULTI_QUERY);
 
+	/* Pop the SPI stack */
+	_SPI_end_call(false);
+
 	/* Return the created portal */
 	return portal;
 }
@@ -1180,9 +1187,17 @@ SPI_is_cursor_plan(SPIPlanPtr plan)
 	}
 
 	if (list_length(plan->plancache_list) != 1)
+	{
+		SPI_result = 0;
 		return false;			/* not exactly 1 pre-rewrite command */
+	}
 	plansource = (CachedPlanSource *) linitial(plan->plancache_list);
 
+	/* Need _SPI_begin_call in case replanning invokes SPI-using functions */
+	SPI_result = _SPI_begin_call(false);
+	if (SPI_result < 0)
+		return false;
+
 	if (plan->saved)
 	{
 		/* Make sure the plan is up to date */
@@ -1190,6 +1205,9 @@ SPI_is_cursor_plan(SPIPlanPtr plan)
 		ReleaseCachedPlan(cplan, true);
 	}
 
+	_SPI_end_call(false);
+	SPI_result = 0;
+
 	/* Does it return tuples? */
 	if (plansource->resultDesc)
 		return true;