postgres_fdw: Inherit the local transaction's access/deferrable modes.

Previously, postgres_fdw always 1) opened a remote transaction in READ
WRITE mode even when the local transaction was READ ONLY, causing a READ
ONLY transaction using it that references a foreign table mapped to a
remote view executing a volatile function to write in the remote side,
and 2) opened the remote transaction in NOT DEFERRABLE mode even when
the local transaction was DEFERRABLE, causing a SERIALIZABLE READ ONLY
DEFERRABLE transaction using it to abort due to a serialization failure
in the remote side.

To avoid these, modify postgres_fdw to open a remote transaction in the
same access/deferrable modes as the local transaction.  This commit also
modifies it to open a remote subtransaction in the same access mode as
the local subtransaction.

Although these issues exist since the introduction of postgres_fdw,
there have been no reports from the field.  So it seems fine to just fix
them in master only.

Author: Etsuro Fujita <etsuro.fujita@gmail.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAPmGK16n_hcUUWuOdmeUS%2Bw4Q6dZvTEDHb%3DOP%3D5JBzo-M3QmpQ%40mail.gmail.com

1 files changed, 28 insertions, 0 deletions

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index b885513f765..2e67e998adb 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -1045,6 +1045,34 @@ TransactionStartedDuringRecovery(void)
 }
 
 /*
+ *	GetTopReadOnlyTransactionNestLevel
+ *
+ * Note: this will return zero when not inside any transaction or when neither
+ * a top-level transaction nor subtransactions are read-only, one when the
+ * top-level transaction is read-only, two when one level of subtransaction is
+ * read-only, etc.
+ *
+ * Note: subtransactions of the topmost read-only transaction are also
+ * read-only, because they inherit read-only mode from the transaction, and
+ * thus can't change to read-write mode.  See check_transaction_read_only().
+ */
+int
+GetTopReadOnlyTransactionNestLevel(void)
+{
+	TransactionState s = CurrentTransactionState;
+
+	if (!XactReadOnly)
+		return 0;
+	while (s->nestingLevel > 1)
+	{
+		if (!s->prevXactReadOnly)
+			return s->nestingLevel;
+		s = s->parent;
+	}
+	return s->nestingLevel;
+}
+
+/*
  *	EnterParallelMode
  */
 void