libpq: Add min/max_protocol_version connection options

All supported version of the PostgreSQL server send the
NegotiateProtocolVersion message when an unsupported minor protocol
version is requested by a client. But many other applications that
implement the PostgreSQL protocol (connection poolers, or other
databases) do not, and the same is true for PostgreSQL server versions
older than 9.3. Connecting to such other applications thus fails if a
client requests a protocol version different than 3.0.

This patch adds a max_protocol_version connection option to libpq that
specifies the protocol version that libpq should request from the
server. Currently only 3.0 is supported, but that will change in a
future commit that bumps the protocol version. Even after that version
bump the default will likely stay 3.0 for the time being. Once more of
the ecosystem supports the NegotiateProtocolVersion message we might
want to change the default to the latest minor version.

This also adds the similar min_protocol_version connection option, to
allow the client to specify that connecting should fail if a lower
protocol version is attempted by the server. This can be used to
ensure that certain protocol features are used, which can be
particularly useful if those features impact security.

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Reviewed-by: Robert Haas <robertmhaas@gmail.com> (earlier versions)
Discussion: https://www.postgresql.org/message-id/CAGECzQTfc_O%2BHXqAo5_-xG4r3EFVsTefUeQzSvhEyyLDba-O9w@mail.gmail.com
Discussion: https://www.postgresql.org/message-id/CAGECzQRbAGqJnnJJxTdKewTsNOovUt4bsx3NFfofz3m2j-t7tA@mail.gmail.com

src/interfaces/libpq/fe-connect.c

@@ -325,6 +325,16 @@ static const internalPQconninfoOption PQconninfoOptions[] = {
 		"Require-Auth", "", 14, /* sizeof("scram-sha-256") == 14 */
 	offsetof(struct pg_conn, require_auth)},
 
+	{"min_protocol_version", "PGMINPROTOCOLVERSION",
+		NULL, NULL,
+		"Min-Protocol-Version", "", 6,	/* sizeof("latest") = 6 */
+	offsetof(struct pg_conn, min_protocol_version)},
+
+	{"max_protocol_version", "PGMAXPROTOCOLVERSION",
+		NULL, NULL,
+		"Max-Protocol-Version", "", 6,	/* sizeof("latest") = 6 */
+	offsetof(struct pg_conn, max_protocol_version)},
+
 	{"ssl_min_protocol_version", "PGSSLMINPROTOCOLVERSION", "TLSv1.2", NULL,
 		"SSL-Minimum-Protocol-Version", "", 8,	/* sizeof("TLSv1.x") == 8 */
 	offsetof(struct pg_conn, ssl_min_protocol_version)},
@@ -483,6 +493,7 @@ static void pgpassfileWarning(PGconn *conn);
 static void default_threadlock(int acquire);
 static bool sslVerifyProtocolVersion(const char *version);
 static bool sslVerifyProtocolRange(const char *min, const char *max);
+static bool pqParseProtocolVersion(const char *value, ProtocolVersion *result, PGconn *conn, const char *context);
 
 
 /* global variable because fe-auth.c needs to access it */
@@ -2081,6 +2092,48 @@ pqConnectOptions2(PGconn *conn)
 		}
 	}
 
+	if (conn->min_protocol_version)
+	{
+		if (!pqParseProtocolVersion(conn->min_protocol_version, &conn->min_pversion, conn, "min_protocol_version"))
+		{
+			conn->status = CONNECTION_BAD;
+			return false;
+		}
+	}
+	else
+	{
+		conn->min_pversion = PG_PROTOCOL_EARLIEST;
+	}
+
+	if (conn->max_protocol_version)
+	{
+		if (!pqParseProtocolVersion(conn->max_protocol_version, &conn->max_pversion, conn, "max_protocol_version"))
+		{
+			conn->status = CONNECTION_BAD;
+			return false;
+		}
+	}
+	else
+	{
+		/*
+		 * To not break connecting to older servers/poolers that do not yet
+		 * support NegotiateProtocolVersion, default to the 3.0 protocol at
+		 * least for a while longer. Except when min_protocol_version is set
+		 * to something larger, then we might as well default to the latest.
+		 */
+		if (conn->min_pversion > PG_PROTOCOL(3, 0))
+			conn->max_pversion = PG_PROTOCOL_LATEST;
+		else
+			conn->max_pversion = PG_PROTOCOL(3, 0);
+	}
+
+	if (conn->min_pversion > conn->max_pversion)
+	{
+		conn->status = CONNECTION_BAD;
+		libpq_append_conn_error(conn, "min_protocol_version is greater than max_protocol_version");
+		return false;
+	}
+
 	/*
 	 * Resolve special "auto" client_encoding from the locale
 	 */
@@ -3084,7 +3137,7 @@ keep_going:						/* We will come back to here until there is
 		 * must persist across individual connection attempts, but we must
 		 * reset them when we start to consider a new server.
 		 */
-		conn->pversion = PG_PROTOCOL(3, 0);
+		conn->pversion = conn->max_pversion;
 		conn->send_appname = true;
 		conn->failed_enc_methods = 0;
 		conn->current_enc_method = 0;
@@ -4102,6 +4155,7 @@ keep_going:						/* We will come back to here until there is
 
 					/* OK, we read the message; mark data consumed */
 					pqParseDone(conn, conn->inCursor);
+
 					goto keep_going;
 				}
 
@@ -8157,6 +8211,38 @@ error:
 	return false;
 }
 
+/*
+ * Parse and try to interpret "value" as a ProtocolVersion value, and if
+ * successful, store it in *result.
+ */
+static bool
+pqParseProtocolVersion(const char *value, ProtocolVersion *result, PGconn *conn,
+					   const char *context)
+{
+	if (strcmp(value, "latest") == 0)
+	{
+		*result = PG_PROTOCOL_LATEST;
+		return true;
+	}
+	if (strcmp(value, "3.0") == 0)
+	{
+		*result = PG_PROTOCOL(3, 0);
+		return true;
+	}
+
+	/* 3.1 never existed, we went straight from 3.0 to 3.2 */
+
+	if (strcmp(value, "3.2") == 0)
+	{
+		*result = PG_PROTOCOL(3, 2);
+		return true;
+	}
+
+	libpq_append_conn_error(conn, "invalid %s value: \"%s\"",
+							context, value);
+	return false;
+}
+
 /*
  * To keep the API consistent, the locking stubs are always provided, even
  * if they are not required.

src/interfaces/libpq/fe-protocol3.c

@@ -1432,6 +1432,13 @@ pqGetNegotiateProtocolVersion3(PGconn *conn)
 		goto failure;
 	}
 
+	/* 3.1 never existed, we went straight from 3.0 to 3.2 */
+	if (their_version == PG_PROTOCOL(3, 1))
+	{
+		libpq_append_conn_error(conn, "received invalid protocol negotiation message: server requests downgrade to non-existent 3.1 protocol version");
+		goto failure;
+	}
+
 	if (num < 0)
 	{
 		libpq_append_conn_error(conn, "received invalid protocol negotiation message: server reported negative number of unsupported parameters");
@@ -1444,6 +1451,17 @@ pqGetNegotiateProtocolVersion3(PGconn *conn)
 		goto failure;
 	}
 
+	if (their_version < conn->min_pversion)
+	{
+		libpq_append_conn_error(conn, "server only supports protocol version %d.%d, but min_protocol_version was set to %d.%d",
+								PG_PROTOCOL_MAJOR(their_version),
+								PG_PROTOCOL_MINOR(their_version),
+								PG_PROTOCOL_MAJOR(conn->min_pversion),
+								PG_PROTOCOL_MINOR(conn->min_pversion));
+
+		goto failure;
+	}
+
 	/* the version is acceptable */
 	conn->pversion = their_version;
 

src/interfaces/libpq/libpq-int.h

@@ -417,6 +417,8 @@ struct pg_conn
 	char	   *gsslib;			/* What GSS library to use ("gssapi" or
 								 * "sspi") */
 	char	   *gssdelegation;	/* Try to delegate GSS credentials? (0 or 1) */
+	char	   *min_protocol_version;	/* minimum used protocol version */
+	char	   *max_protocol_version;	/* maximum used protocol version */
 	char	   *ssl_min_protocol_version;	/* minimum TLS protocol version */
 	char	   *ssl_max_protocol_version;	/* maximum TLS protocol version */
 	char	   *target_session_attrs;	/* desired session properties */
@@ -539,6 +541,8 @@ struct pg_conn
 	void	   *scram_client_key_binary;	/* binary SCRAM client key */
 	size_t		scram_server_key_len;
 	void	   *scram_server_key_binary;	/* binary SCRAM server key */
+	ProtocolVersion min_pversion;	/* protocol version to request */
+	ProtocolVersion max_pversion;	/* protocol version to request */
 
 	/* Miscellaneous stuff */
 	int			be_pid;			/* PID of backend --- needed for cancels */