vtysh: optimize printout routine

When printing responses from a client, vtysh searches through every
response it receives twice. Once is to look for the terminating
sequence and the other is to translate \n to \0, which is used with the
line processing callback capability. However:

* There's no need to search all of the output for the terminator, we can
  just check the last 4 bytes.
* In most scenarios we won't have a callback. Therefore we don't need to
  process the output and can just dump it.

Together these optimizations have a significant impact on the total
runtime of talkative CLI commands, such as `sh ip bgp json` which runs
roughly 1000% faster when dumping 1,000,000 routes.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>

vtysh/vtysh.c

@@ -138,16 +138,25 @@ static int vtysh_client_run(struct vtysh_client *vclient, const char *line,
 
 		bufvalid += nread;
 
-		end = memmem(buf, bufvalid - buf, terminator,
-			     sizeof(terminator));
-		if (end + sizeof(terminator) + 1 > bufvalid)
+		if (bufvalid - buf >= 4)
+			end = memmem(bufvalid - 4, 4, terminator,
+				     sizeof(terminator));
+
+		if (end && end + sizeof(terminator) + 1 > bufvalid)
 			/* found \0\0\0 but return code hasn't been read yet */
 			end = NULL;
 		if (end)
 			ret = end[sizeof(terminator)];
 
-		while (bufvalid > buf && (end > buf || !end)) {
-			size_t textlen = (end ? end : bufvalid) - buf;
+		/*
+		 * calculate # bytes we have, up to & not including the
+		 * terminator if present
+		 */
+		size_t textlen = (end ? end : bufvalid) - buf;
+
+		/* feed line processing callback if present */
+		while (callback && bufvalid > buf && (end > buf || !end)) {
+			textlen = (end ? end : bufvalid) - buf;
 			char *eol = memchr(buf, '\n', textlen);
 			if (eol)
 				/* line break */
@@ -165,8 +174,7 @@ static int vtysh_client_run(struct vtysh_client *vclient, const char *line,
 				/* continue reading */
 				break;
 
-			/* eol is at a line end now, either \n => \0 or \0\0\0
-			 */
+			/* eol is at line end now, either \n => \0 or \0\0\0 */
 			assert(eol && eol <= bufvalid);
 
 			if (fp) {
@@ -186,6 +194,14 @@ static int vtysh_client_run(struct vtysh_client *vclient, const char *line,
 				end -= eol - buf;
 		}
 
+		/* else if no callback, dump raw */
+		if (!callback) {
+			if (fp)
+				fwrite(buf, 1, textlen, fp);
+			memmove(buf, buf + textlen, bufvalid - buf - textlen);
+			bufvalid -= textlen;
+		}
+
 		if (bufvalid == buf + bufsz) {
 			char *new;
 			bufsz *= 2;