From 723027fe61a6f34acb7d7c0d518b4dbff03af9cd Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <peter@eisentraut.org>
Date: Thu, 20 Oct 2022 11:18:50 +0200
Subject: [PATCH v2] libpq: Handle NegotiateProtocolVersion message

Before, receiving a NegotiateProtocolVersion message would result in a
confusing error message like

    expected authentication request from server, but received v

This adds proper handling of this protocol message and produces an
on-topic error message from it.

Discussion: https://www.postgresql.org/message-id/flat/f9c7862f-b864-8ef7-a861-c4638c83e209%40enterprisedb.com
---
 src/interfaces/libpq/fe-connect.c   | 18 +++++++++--
 src/interfaces/libpq/fe-protocol3.c | 50 +++++++++++++++++++++++++++++
 src/interfaces/libpq/libpq-int.h    |  1 +
 3 files changed, 66 insertions(+), 3 deletions(-)

diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index 746e9b4f1efc..1c0d8243a6ca 100644
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -3246,10 +3246,11 @@ PQconnectPoll(PGconn *conn)
 
 				/*
 				 * Validate message type: we expect only an authentication
-				 * request or an error here.  Anything else probably means
-				 * it's not Postgres on the other end at all.
+				 * request, NegotiateProtocolVersion, or an error here.
+				 * Anything else probably means it's not Postgres on the other
+				 * end at all.
 				 */
-				if (!(beresp == 'R' || beresp == 'E'))
+				if (!(beresp == 'R' || beresp == 'v' || beresp == 'E'))
 				{
 					appendPQExpBuffer(&conn->errorMessage,
 									  libpq_gettext("expected authentication request from server, but received %c\n"),
@@ -3405,6 +3406,17 @@ PQconnectPoll(PGconn *conn)
 
 					goto error_return;
 				}
+				else if (beresp == 'v')
+				{
+					if (pqGetNegotiateProtocolVersion3(conn))
+					{
+						/* We'll come back when there is more data */
+						return PGRES_POLLING_READING;
+					}
+					/* OK, we read the message; mark data consumed */
+					conn->inStart = conn->inCursor;
+					goto error_return;
+				}
 
 				/* It is an authentication request. */
 				conn->auth_req_received = true;
diff --git a/src/interfaces/libpq/fe-protocol3.c b/src/interfaces/libpq/fe-protocol3.c
index f001137b7692..7bc81740ab35 100644
--- a/src/interfaces/libpq/fe-protocol3.c
+++ b/src/interfaces/libpq/fe-protocol3.c
@@ -1397,6 +1397,56 @@ reportErrorPosition(PQExpBuffer msg, const char *query, int loc, int encoding)
 }
 
 
+/*
+ * Attempt to read a NegotiateProtocolVersion message.
+ * Entry: 'v' message type and length have already been consumed.
+ * Exit: returns 0 if successfully consumed message.
+ *		 returns EOF if not enough data.
+ */
+int
+pqGetNegotiateProtocolVersion3(PGconn *conn)
+{
+	int			tmp;
+	ProtocolVersion their_version;
+	int			num;
+	PQExpBufferData buf;
+
+	initPQExpBuffer(&buf);
+	if (pqGetInt(&tmp, 4, conn) != 0)
+		return EOF;
+	their_version = tmp;
+	if (pqGetInt(&num, 4, conn) != 0)
+		return EOF;
+	for (int i = 0; i < num; i++)
+	{
+		if (pqGets(&conn->workBuffer, conn))
+			return EOF;
+		if (buf.len > 0)
+			appendPQExpBufferChar(&buf, ' ');
+		appendPQExpBufferStr(&buf, conn->workBuffer.data);
+	}
+
+	if (their_version < conn->pversion)
+		appendPQExpBuffer(&conn->errorMessage,
+						  libpq_gettext("protocol version not supported by server: client uses %u.%u, server supports up to %u.%u\n"),
+						  PG_PROTOCOL_MAJOR(conn->pversion), PG_PROTOCOL_MINOR(conn->pversion),
+						  PG_PROTOCOL_MAJOR(their_version), PG_PROTOCOL_MINOR(their_version));
+	if (num > 0)
+		appendPQExpBuffer(&conn->errorMessage,
+						  libpq_ngettext("protocol extension not supported by server: %s\n",
+										 "protocol extensions not supported by server: %s\n", num),
+						  buf.data);
+
+	/* neither -- server shouldn't have sent it */
+	if (!(their_version < conn->pversion) && !(num > 0))
+		appendPQExpBuffer(&conn->errorMessage,
+						  libpq_gettext("invalid %s message"), "NegotiateProtocolVersion");
+
+	termPQExpBuffer(&buf);
+	return 0;
+}
+
+
 /*
  * Attempt to read a ParameterStatus message.
  * This is possible in several places, so we break it out as a subroutine.
diff --git a/src/interfaces/libpq/libpq-int.h b/src/interfaces/libpq/libpq-int.h
index c75ed63a2c62..c59afac7a086 100644
--- a/src/interfaces/libpq/libpq-int.h
+++ b/src/interfaces/libpq/libpq-int.h
@@ -685,6 +685,7 @@ extern void pqParseInput3(PGconn *conn);
 extern int	pqGetErrorNotice3(PGconn *conn, bool isError);
 extern void pqBuildErrorMessage3(PQExpBuffer msg, const PGresult *res,
 								 PGVerbosity verbosity, PGContextVisibility show_context);
+extern int	pqGetNegotiateProtocolVersion3(PGconn *conn);
 extern int	pqGetCopyData3(PGconn *conn, char **buffer, int async);
 extern int	pqGetline3(PGconn *conn, char *s, int maxlen);
 extern int	pqGetlineAsync3(PGconn *conn, char *buffer, int bufsize);

base-commit: 39b8c293fcde1d845da4d7127a25d41df53faab5
-- 
2.37.3