net: simplify the TcpConn.read_line/0 method, accumulate partially read lines, use a string builder, instead of concatenation

vlib/net/tcp_read_line.v

@@ -1,9 +1,12 @@
 module net
 
+import strings
+
 const (
-	crlf     = '\r\n'
-	msg_peek = 0x02
-	max_read = 400
+	crlf              = '\r\n'
+	msg_peek          = 0x02
+	max_read          = 400
+	max_read_line_len = 1048576
 )
 
 // get_blocking returns whether the connection is in a blocking state,
@@ -39,52 +42,59 @@ pub fn (mut con TcpConn) set_blocking(state bool) ? {
 }
 
 // read_line is a *simple*, *non customizable*, blocking line reader.
-// It will *always* return a line, ending with CRLF, or just '', on EOF.
+// It will return a line, ending with LF, or just '', on EOF.
 // NB: if you want more control over the buffer, please use a buffered IO
 // reader instead: `io.new_buffered_reader({reader: io.make_reader(con)})`
 pub fn (mut con TcpConn) read_line() string {
-	mut buf := [net.max_read]byte{} // where C.recv will store the network data
-	mut res := '' // The final result, including the ending \n.
+	return con.read_line_max(net.max_read_line_len)
+}
+
+// read_line_max is a *simple*, *non customizable*, blocking line reader.
+// It will return a line, ending with LF, '' on EOF.
+// It stops reading, when the result line length exceeds max_line_len.
+[manualfree]
+pub fn (mut con TcpConn) read_line_max(max_line_len int) string {
 	if !con.is_blocking {
 		con.set_blocking(true) or {}
 	}
+	mut buf := [net.max_read]byte{} // where C.recv will store the network data
+	mut res := strings.new_builder(net.max_read) // The final result, including the ending \n.
+	defer {
+		unsafe { res.free() }
+	}
+	bstart := unsafe { &buf[0] }
 	for {
-		mut line := '' // The current line. Can be a partial without \n in it.
-		n := C.recv(con.sock.handle, &buf[0], net.max_read - 1, net.msg_peek | msg_nosignal)
-		if n == -1 {
-			return res
-		}
-		if n == 0 {
-			return res
+		n := C.recv(con.sock.handle, bstart, net.max_read - 1, net.msg_peek | msg_nosignal)
+		if n <= 0 {
+			return res.str()
 		}
 		buf[n] = `\0`
 		mut eol_idx := -1
+		mut lend := n
 		for i in 0 .. n {
-			if int(buf[i]) == `\n` {
+			if buf[i] == `\n` {
 				eol_idx = i
-				// Ensure that tos_clone(buf) later,
-				// will return *only* the first line (including \n),
-				// and ignore the rest
-				buf[i + 1] = `\0`
+				lend = i + 1
+				buf[lend] = `\0`
 				break
 			}
 		}
-		line = unsafe { tos_clone(&buf[0]) }
 		if eol_idx > 0 {
 			// At this point, we are sure that recv returned valid data,
 			// that contains *at least* one line.
 			// Ensure that the block till the first \n (including it)
 			// is removed from the socket's receive queue, so that it does
 			// not get read again.
-			C.recv(con.sock.handle, &buf[0], eol_idx + 1, msg_nosignal)
-			res += line
+			C.recv(con.sock.handle, bstart, lend, msg_nosignal)
+			unsafe { res.write_ptr(bstart, lend) }
+			break
+		}
+		// recv returned a buffer without \n in it, just store it for now:
+		C.recv(con.sock.handle, bstart, n, msg_nosignal)
+		unsafe { res.write_ptr(bstart, lend) }
+		if res.len > max_line_len {
 			break
 		}
-		// recv returned a buffer without \n in it .
-		C.recv(con.sock.handle, &buf[0], n, msg_nosignal)
-		res += line
-		res += net.crlf
-		break
 	}
-	return res
+	return res.str()
 }