httpd: make the /cmd answer usable when a command fails or floods

Three ways the answer could mislead the caller, all in the paths that
matter most when something is wrong.

The capture sat in write_char(), so anything printed through
print_string_no_syslog() escaped it. That is exactly the replies of the
syslog commands, which deliberately avoid the syslog path: "syslog on"
answered with the bare address and no sentence around it, "syslog off"
answered with nothing at all. Moving the capture one level down, into
write_char_no_syslog(), catches both paths and costs nothing, since
write_char() reaches it anyway.

On a parse error send_bad_request() rewrote the buffer from the start,
throwing away the very text that explains the error and replacing it
with the 400 HTML page. Keeping the captured text and restating only the
status keeps the parser's own message; "400 NO" is as long as "200 OK",
so the header does not have to be rebuilt.

An overlong listing was cut at the buffer end with nothing to show for
it, so a clipped answer read as a short one. It now ends in a marker,
and the reserved room is accounted for in the guard.

While here: a command that configures something prints nothing, and an
empty body reads the same as "nothing ran", so a successful silent
command now answers "OK".

No internal RAM and no xdata; 174 bytes of BANK1, which has 7356 left.

httpd/httpd.c

@@ -16,12 +16,17 @@
 
 #define CMARK_S 6
 
+/* Answer to POST /cmd. Kept as a macro so that its length is known at compile
+ * time: an unchanged slen afterwards means the command printed nothing. */
+#define CMD_RESPONSE_HEADER "HTTP/1.1 200 OK\r\nConnection: close\r\n" \
+			    "Content-Type: text/plain\r\n\r\n"
+
 #pragma codeseg BANK1
 #pragma constseg BANK1
 
 extern volatile __xdata uint8_t sfr_data[4];
 extern volatile __xdata uint32_t ticks;
-extern __xdata uint8_t cmd_capture;	/* owned by rtlplayground.c, see write_char() */
+extern __xdata uint8_t cmd_capture;	/* owned by rtlplayground.c, see write_char_no_syslog() */
 extern __code uint8_t * __code hex;
 extern __code struct f_data f_data[];
 extern __code char * __code mime_strings[];
@@ -626,14 +631,26 @@ static void handle_firmware_fragment(__xdata uint8_t *p)
 
 static void run_cmd_body(__xdata uint8_t *body)
 {
-	slen = strtox(outbuf, "HTTP/1.1 200 OK\r\nConnection: close\r\n" \
-			      "Content-Type: text/plain\r\n\r\n");
+	slen = strtox(outbuf, CMD_RESPONSE_HEADER);
 	cmd_capture = 1;
 	execute_commands(body);
+	if (cmd_capture == 2)
+		slen += strtox(outbuf + slen, CMD_TRUNCATED);
 	cmd_capture = 0;
+	/* Commands that configure something print nothing at all. Saying
+	 * so beats an empty body, which reads the same as "nothing ran".
+	 * Only on success: a silent failure must not answer with "OK". */
+	if (err_status == ERR_OK && slen == sizeof(CMD_RESPONSE_HEADER) - 1)
+		slen += strtox(outbuf + slen, "OK\n");
+	/* On a parse error keep what the parser printed, because that text
+	 * is the explanation, and only restate the status. "400 NO" is as
+	 * long as "200 OK", so the header does not have to be rebuilt. */
 	if (err_status != ERR_OK) {
-		send_bad_request();
-		return;
+		outbuf[9] = '4';
+		outbuf[10] = '0';
+		outbuf[11] = '0';
+		outbuf[13] = 'N';
+		outbuf[14] = 'O';
 	}
 }