gatchat: add g_at_chat_retry()

The current API doesn't support canceling an in-progress command;
instead g_at_chat_cancel() simply removes the callback.

In cases where the modem doesn't respond at all to a command, a chat is
simply stalled without any way to write new commands to the modem.

Support that case by adding a g_at_chat_retry() function to the API. The
function does nothing if the command is not yet in-progress, or if the
command is finished. Otherwise, it resets the bytes-written counter to
re-write the command string.

gatchat/gatchat.c

@@ -1047,6 +1047,29 @@ static guint at_chat_send_common(struct at_chat *chat, guint gid,
 	return c->id;
 }
 
+static gboolean at_chat_retry(struct at_chat *chat, guint id)
+{
+	struct at_command *cmd = g_queue_peek_head(chat->command_queue);
+
+	if (!cmd)
+		return FALSE;
+
+	/* do nothing if command is not yet started, or already finished */
+	if (cmd->id != id)
+		return FALSE;
+
+	/* do nothing if command is not fully written */
+	if (chat->cmd_bytes_written != strlen(cmd->cmd))
+		return FALSE;
+
+	/* reset number of written bytes to re-write command */
+	chat->cmd_bytes_written = 0;
+
+	chat_wakeup_writer(chat);
+
+	return TRUE;
+}
+
 static struct at_notify *at_notify_create(struct at_chat *chat,
 						const char *prefix,
 						gboolean pdu)
@@ -1543,6 +1566,14 @@ guint g_at_chat_send_and_expect_short_prompt(GAtChat *chat, const char *cmd,
 					NULL, func, user_data, notify);
 }
 
+gboolean g_at_chat_retry(GAtChat *chat, guint id)
+{
+	if (chat == NULL || id == 0)
+		return FALSE;
+
+	return at_chat_retry(chat->parent, id);
+}
+
 gboolean g_at_chat_cancel(GAtChat *chat, guint id)
 {
 	/* We use id 0 for wakeup commands */

gatchat/gatchat.h

@@ -147,6 +147,13 @@ guint g_at_chat_send_and_expect_short_prompt(GAtChat *chat, const char *cmd,
 				const char **valid_resp, GAtResultFunc func,
 				gpointer user_data, GDestroyNotify notify);
 
+/*!
+ * Retry an already created command. This does nothing if the command is
+ * still waiting in the queue. If the command has been written to the channel,
+ * but no response is received yet, the retry writes the command again.
+ */
+gboolean g_at_chat_retry(GAtChat *chat, guint id);
+
 gboolean g_at_chat_cancel(GAtChat *chat, guint id);
 gboolean g_at_chat_cancel_all(GAtChat *chat);