selftests/net/forwarding: add slowwait functions

Add slowwait functions to wait for some operations that may need a long time
to finish. The busywait executes the cmd too fast, which is kind of wasting
cpu in this scenario. At the same time, if shell debugging is enabled with
`set -x`. the busywait will output too much logs.

Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20240205130048.282087-2-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/forwarding/lib.sh

@@ -37,6 +37,32 @@ fi
 
 source "$net_forwarding_dir/../lib.sh"
 
+# timeout in seconds
+slowwait()
+{
+	local timeout=$1; shift
+
+	local start_time="$(date -u +%s)"
+	while true
+	do
+		local out
+		out=$("$@")
+		local ret=$?
+		if ((!ret)); then
+			echo -n "$out"
+			return 0
+		fi
+
+		local current_time="$(date -u +%s)"
+		if ((current_time - start_time > timeout)); then
+			echo -n "$out"
+			return 1
+		fi
+
+		sleep 0.1
+	done
+}
+
 ##############################################################################
 # Sanity checks
 
@@ -478,6 +504,15 @@ busywait_for_counter()
 	busywait "$timeout" until_counter_is ">= $((base + delta))" "$@"
 }
 
+slowwait_for_counter()
+{
+	local timeout=$1; shift
+	local delta=$1; shift
+
+	local base=$("$@")
+	slowwait "$timeout" until_counter_is ">= $((base + delta))" "$@"
+}
+
 setup_wait_dev()
 {
 	local dev=$1; shift