net_irc: More robustly prevent message truncation.

Be significantly more conservative in where we split
long messages to really ensure that messages are not
truncated when they are relayed to IRC. This fixes
truncation for long messages relayed to Libera chat.

nets/net_irc.c

@@ -1057,13 +1057,22 @@ static int transform_and_send(const char *channel, enum channel_user_modes modes
 	 * and this could be 40-50 bytes, potentially.
 	 * Being conservative on message length here reduces the chance that
 	 * the message will be within bounds within the BBS but exceed 512 when it hits lirc
-	 * to actually generate the final message. */
-	while (linelen > 470) {
-		char c;
+	 * to actually generate the final message.
+	 *
+	 * On top of that, libera seems to be truncating messages at 470 characters,
+	 * which is far under the 512 character limit. Clients as well attempt to split
+	 * messages under the 512 character limit, with Ambassador doing so around 462.
+	 * So, we actually want to aim somewhere around 100 fewer characters than the 512 limit,
+	 * to be conservative and ensure truncation doesn't occur when relaying to IRC.
+	 */
+
+#define MAX_PREFORMATTED_RELAY_MSGLEN 412
+	while (linelen > MAX_PREFORMATTED_RELAY_MSGLEN) {
+		char c1, c2, c3, c4;
 		/* Message is too long to be a single message towards IRC.
 		 * Split it up. */
-		char *end = line + 465; /* Leave room for the relay module to prefix a username later. */
-		const char *midpoint = line + 235;
+		char *end = line + MAX_PREFORMATTED_RELAY_MSGLEN - 10; /* Leave room for the relay module to prefix a username later. */
+		const char *midpoint = line + MAX_PREFORMATTED_RELAY_MSGLEN / 2;
 		/* Look for a graceful position to split the message (a space).
 		 * In the worst case, there are no spaces, and we should split the
 		 * message in the second half no matter what, so we send a maximum
@@ -1076,12 +1085,29 @@ static int transform_and_send(const char *channel, enum channel_user_modes modes
 #pragma GCC diagnostic pop
 		/* We are guaranteed here that *end is not the last character in the message. */
 		end++; /* Move back forward past the space. */
-		c = *end; /* Save character at this position. */
-		*end = '\0'; /* NUL terminate to split message. */
+		c1 = *end; /* Save character at this position. */
+		c2 = *(end + 1);
+		c3 = *(end + 2);
+		c4 = *(end + 3);
+		/* Use ... to indicate payload will be continued in another message.
+		 * It will be prefixed by a space, which is fine since it likely differentiates from user-added '...',
+		 * since user-created messages are likely to end "EOM..." not "EOM ..." */
+		*end = '.';
+		*(end + 1) = '.';
+		*(end + 2) = '.';
+		*(end + 3) = '\0'; /* NUL terminate to split message. */
 		_irc_relay_send(channel, modes, relayname, sender, hostsender, final, ircuser, 0, mod);
-		*end = c; /* Restore character */
+		*end = c1; /* Restore character */
+		*(end + 1) = c2;
+		*(end + 2) = c3;
+		*(end + 3) = c4;
+		end -= 3;
 		final = line = end; /* This is the beginning of the next chunk to send. */
 		linelen = strlen(line);
+		/* Since we subtracted 3 from the end, we can use the first 3 characters to put '...' for message continuation indication */
+		*end = '.';
+		*(end + 1) = '.';
+		*(end + 2) = '.';
 	}
 
 	/* Send entire message, if less than max message length, or the last chunk, if it was larger. */