Fix SPI error cleanup and memory leak

Since the SPI stack has been moved from TopTransactionContext to
TopMemoryContext, setting _SPI_stack to NULL in AtEOXact_SPI() leaks
memory.  In fact, we don't need to do that anymore: We just leave the
allocated stack around for the next SPI use.

Also, refactor the SPI cleanup so that it is run both at transaction end
and when returning to the main loop on an exception.  The latter is
necessary when a procedure calls a COMMIT or ROLLBACK command that
itself causes an error.

src/backend/executor/spi.c

@@ -260,35 +260,37 @@ SPI_rollback(void)
 	_SPI_current->internal_xact = false;
 }
 
+/*
+ * Clean up SPI state.  Called on transaction end (of non-SPI-internal
+ * transactions) and when returning to the main loop on error.
+ */
+void
+SPICleanup(void)
+{
+	_SPI_current = NULL;
+	_SPI_connected = -1;
+	SPI_processed = 0;
+	SPI_lastoid = InvalidOid;
+	SPI_tuptable = NULL;
+}
+
 /*
  * Clean up SPI state at transaction commit or abort.
  */
 void
 AtEOXact_SPI(bool isCommit)
 {
-	/*
-	 * Do nothing if the transaction end was initiated by SPI.
-	 */
+	/* Do nothing if the transaction end was initiated by SPI. */
 	if (_SPI_current && _SPI_current->internal_xact)
 		return;
 
-	/*
-	 * Note that memory contexts belonging to SPI stack entries will be freed
-	 * automatically, so we can ignore them here.  We just need to restore our
-	 * static variables to initial state.
-	 */
 	if (isCommit && _SPI_connected != -1)
 		ereport(WARNING,
 				(errcode(ERRCODE_WARNING),
 				 errmsg("transaction left non-empty SPI stack"),
 				 errhint("Check for missing \"SPI_finish\" calls.")));
 
-	_SPI_current = _SPI_stack = NULL;
-	_SPI_stack_depth = 0;
-	_SPI_connected = -1;
-	SPI_processed = 0;
-	SPI_lastoid = InvalidOid;
-	SPI_tuptable = NULL;
+	SPICleanup();
 }
 
 /*

src/backend/tcop/postgres.c

@@ -42,6 +42,7 @@
 #include "catalog/pg_type.h"
 #include "commands/async.h"
 #include "commands/prepare.h"
+#include "executor/spi.h"
 #include "jit/jit.h"
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"
@@ -3941,6 +3942,7 @@ PostgresMain(int argc, char *argv[],
 			WalSndErrorCleanup();
 
 		PortalErrorCleanup();
+		SPICleanup();
 
 		/*
 		 * We can't release replication slots inside AbortTransaction() as we

src/include/executor/spi.h

@@ -163,6 +163,7 @@ extern void SPI_start_transaction(void);
 extern void SPI_commit(void);
 extern void SPI_rollback(void);
 
+extern void SPICleanup(void);
 extern void AtEOXact_SPI(bool isCommit);
 extern void AtEOSubXact_SPI(bool isCommit, SubTransactionId mySubid);