libpq: Process buffered SSL read bytes to support records >8kB on async API

The async API of libpq doesn't support SSL record sizes >8kB so far.
This size isn't exceeded by vanilla PostgreSQL, but by other products using
the postgres wire protocol 3.
PQconsumeInput() reads all data readable from the socket, so that the read
condition is cleared.
But it doesn't process all the data that is pending on the SSL layer.
Also a subsequent call to PQisBusy() doesn't process it, so that the client
is triggered to wait for more readable data on the socket.
But this never arrives, so that the connection blocks infinitely.

To fix this issue call pqReadData() repeatedly until there is no buffered
SSL data left to be read.

The synchronous libpq API isn't affected, since it supports arbitrary SSL
record sizes already.

Author: larskanis

src/interfaces/libpq/fe-exec.c

@@ -2006,6 +2006,19 @@ PQconsumeInput(PGconn *conn)
 	if (pqReadData(conn) < 0)
 		return 0;
 
+	#ifdef USE_SSL
+		/*
+		 * Ensure all buffered read bytes in the SSL library are processed,
+		 * which might be not the case, if the SSL record size exceeds 8k.
+		 * Otherwise parseInput can't process the data.
+		 */
+		while (conn->ssl_in_use && pgtls_read_pending(conn))
+		{
+			if (pqReadData(conn) < 0)
+				return 0;
+		}
+	#endif
+
 	/* Parsing of the data waits till later. */
 	return 1;
 }