gdb_server: Fix confusing warning when entering noack mode

Comment in gdb_server code mistakenly attributed constant warnings
about unexpected acknowledgement to the ACK packet sent by GDB when
establishing connection. However that is not the case, OpenOCD
gdb_server is handling this packet correctly without an warnings. This
warning instead was caused by the ACK packet which GDB sends right after
going into noack mode. Because OpenOCD gdb_server is already in noack
mode at this state, it emits warning about it. This is a documented GDB
behaviour
(https://sourceware.org/gdb/onlinedocs/gdb/Packet-Acknowledgment.html)
so there is no reason to scare users with warnings in this case.

This patch basically introduces two-level noack mode: after receiving
QStartNoAckMode gdb_server will set noack_mode to 1, then it will
receive this last ACK packet, but instead of printing a warning,
noack_mode will be increased to 2. Should there be any other ACK packets
after that, they will be properly reported by warning. All other code
that relies on noack_mode checks it for "!= 0", so there will be no
difference if it is 1 or 2.

Change-Id: I0e9c57fd93293bfe010390db2f3f161528b11d86
Signed-off-by: Anton Kolesov <anton.kolesov@synopsys.com>
Reviewed-on: http://openocd.zylin.com/1895
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

1 files changed, 15 insertions, 5 deletions

diff --git a/src/server/gdb_server.c b/src/server/gdb_server.c
index ea821140..7bf7319c 100644
--- a/src/server/gdb_server.c
+++ b/src/server/gdb_server.c
@@ -617,11 +617,21 @@ static int gdb_get_packet_inner(struct connection *connection,
 				case '$':
 					break;
 				case '+':
-					/* gdb sends a dummy ack '+' at every remote connect - see
-					 * remote_start_remote (remote.c)
-					 * in case anyone tries to debug why they receive this
-					 * warning every time */
-					LOG_WARNING("acknowledgment received, but no packet pending");
+					/* According to the GDB documentation
+					 * (https://sourceware.org/gdb/onlinedocs/gdb/Packet-Acknowledgment.html):
+					 * "gdb sends a final `+` acknowledgment of the stub's `OK`
+					 * response, which can be safely ignored by the stub."
+					 * However OpenOCD server already is in noack mode at this
+					 * point and instead of ignoring this it was emitting a
+					 * warning. This code makes server ignore the first ACK
+					 * that will be received after going into noack mode,
+					 * warning only about subsequent ACK's. */
+					if (gdb_con->noack_mode > 1) {
+						LOG_WARNING("acknowledgment received, but no packet pending");
+					} else {
+						LOG_DEBUG("Received first acknowledgment after entering noack mode. Ignoring it.");
+						gdb_con->noack_mode = 2;
+					}
 					break;
 				case '-':
 					LOG_WARNING("negative acknowledgment, but no packet pending");