Don't assume that "E" response to NEGOTIATE_SSL_CODE means pre-7.0 server.

These days, such a response is far more likely to signify a server-side
problem, such as fork failure.  Reporting "server does not support SSL"
(in sslmode=require) could be quite misleading.  But the results could
be even worse in sslmode=prefer: if the problem was transient and the
next connection attempt succeeds, we'll have silently fallen back to
protocol version 2.0, possibly disabling features the user needs.

Hence, it seems best to just eliminate the assumption that backing off
to non-SSL/2.0 protocol is the way to recover from an "E" response, and
instead treat the server error the same as we would in non-SSL cases.

I tested this change against a pre-7.0 server, and found that there
was a second logic bug in the "prefer" path: the test to decide whether
to make a fallback connection attempt assumed that we must have opened
conn->ssl, which in fact does not happen given an "E" response.  After
fixing that, the code does indeed connect successfully to pre-7.0,
as long as you didn't set sslmode=require.  (If you did, you get
"Unsupported frontend protocol", which isn't completely off base
given the server certainly doesn't support SSL.)

Since there seems no reason to believe that pre-7.0 servers exist anymore
in the wild, back-patch to all supported branches.

Author: tglsfdc

src/interfaces/libpq/fe-connect.c

@@ -1337,16 +1337,19 @@ PQconnectPoll(PGconn *conn)
 						/* should not happen really */
 						return PGRES_POLLING_READING;
 					}
-					/* mark byte consumed */
-					conn->inStart = conn->inCursor;
 					if (SSLok == 'S')
 					{
+						/* mark byte consumed */
+						conn->inStart = conn->inCursor;
 						/* Do one-time setup; this creates conn->ssl */
 						if (pqsecure_initialize(conn) == -1)
 							goto error_return;
 					}
 					else if (SSLok == 'N')
 					{
+						/* mark byte consumed */
+						conn->inStart = conn->inCursor;
+						/* OK to do without SSL? */
 						if (conn->sslmode[0] == 'r')	/* "require" */
 						{
 							/* Require SSL, but server does not want it */
@@ -1361,27 +1364,17 @@ PQconnectPoll(PGconn *conn)
 					}
 					else if (SSLok == 'E')
 					{
-						/* Received error - probably protocol mismatch */
-						if (conn->Pfdebug)
-							fprintf(conn->Pfdebug, "received error from server, attempting fallback to pre-7.0\n");
-						if (conn->sslmode[0] == 'r')	/* "require" */
-						{
-							/* Require SSL, but server is too old */
-							printfPQExpBuffer(&conn->errorMessage,
-											  libpq_gettext("server does not support SSL, but SSL was required\n"));
-							goto error_return;
-						}
-						/* Otherwise, try again without SSL */
-						conn->allow_ssl_try = false;
-						/* Assume it ain't gonna handle protocol 3, either */
-						conn->pversion = PG_PROTOCOL(2, 0);
-						/* Must drop the old connection */
-						closesocket(conn->sock);
-						conn->sock = -1;
-						conn->status = CONNECTION_NEEDED;
-						/* Discard any unread/unsent data */
-						conn->inStart = conn->inCursor = conn->inEnd = 0;
-						conn->outCount = 0;
+						/*
+						 * Server failure of some sort, such as failure to
+						 * fork a backend process.  We need to process and
+						 * report the error message, which might be formatted
+						 * according to either protocol 2 or protocol 3.
+						 * Rather than duplicate the code for that, we flip
+						 * into AWAITING_RESPONSE state and let the code there
+						 * deal with it.  Note we have *not* consumed the "E"
+						 * byte here.
+						 */
+						conn->status = CONNECTION_AWAITING_RESPONSE;
 						goto keep_going;
 					}
 					else
@@ -1614,8 +1607,7 @@ PQconnectPoll(PGconn *conn)
 					 * then do a non-SSL retry
 					 */
 					if (conn->sslmode[0] == 'p' /* "prefer" */
-						&& conn->ssl
-						&& conn->allow_ssl_try	/* redundant? */
+						&& conn->allow_ssl_try
 						&& !conn->wait_ssl_try) /* redundant? */
 					{
 						/* only retry once */