Treat a silent printer as normal, and flush before closing

The QL-820NWB does not report status over TCP 9100 — brother_ql documents the
same limitation for its network backend. queryStatus therefore became
probeStatus, which reports reachable-with-unknown-state instead of failing.

Also fixes a real bug: the socket was destroyed straight after write, which can
discard data still queued in the kernel send buffer and truncate a ~190 KB job.
Now half-closes with end() and waits for flush, with a grace window for units
that never send FIN.

src/printing/ql-transport.js

@@ -42,85 +42,100 @@ function withSocket(host, port, connectTimeout, handler) {
 }
 
 /**
- * Ask the printer what state it's in without sending a print job.
- * Safe to call on a schedule for a "printer offline" banner in the admin console.
+ * Probe the printer.
  *
- * @returns {Promise<Object>} decoded status frame
+ * Never throws for the "connected but silent" case, because that is the normal
+ * result on this hardware: the QL-820NWB accepts jobs on port 9100 but does not
+ * report state back over TCP — status frames only come back over USB. brother_ql
+ * documents the same limitation for its network backend. A silent printer is
+ * therefore reachable-and-probably-fine, not broken, and callers must treat a
+ * null status as "unknown" rather than "bad".
+ *
+ * @returns {Promise<{reachable: boolean, status: Object|null, error: string|null}>}
  */
-async function queryStatus(host, options = {}) {
+async function probeStatus(host, options = {}) {
   const {
     port = DEFAULT_PORT,
     connectTimeout = DEFAULT_CONNECT_TIMEOUT,
-    replyTimeout = 5000,
+    replyTimeout = 2000,
   } = options;
 
-  return withSocket(host, port, connectTimeout, (socket, finish) => {
-    let received = Buffer.alloc(0);
+  try {
+    return await withSocket(host, port, connectTimeout, (socket, finish) => {
+      let received = Buffer.alloc(0);
 
-    const timer = setTimeout(() => {
-      finish(
-        new Error(
-          `Printer at ${host}:${port} accepted the connection but sent no status. ` +
-            'It may be mid-job, or another host may be holding the port.'
-        )
+      // No reply is the expected outcome over the network, so this timer is the
+      // normal path rather than an error path. Keep it short.
+      const timer = setTimeout(
+        () => finish(null, { reachable: true, status: null, error: null }),
+        replyTimeout
       );
-    }, replyTimeout);
 
-    socket.on('data', (chunk) => {
-      received = Buffer.concat([received, chunk]);
-      if (received.length >= FRAME_LENGTH) {
-        clearTimeout(timer);
-        const frames = decodeAll(received);
-        if (frames.length === 0) {
-          finish(new Error('Printer sent an unrecognisable status frame'));
-        } else {
-          finish(null, frames[frames.length - 1]);
+      socket.on('data', (chunk) => {
+        received = Buffer.concat([received, chunk]);
+        if (received.length >= FRAME_LENGTH) {
+          clearTimeout(timer);
+          const frames = decodeAll(received);
+          finish(null, {
+            reachable: true,
+            status: frames.length ? frames[frames.length - 1] : null,
+            error: null,
+          });
         }
-      }
-    });
+      });
 
-    socket.write(buildStatusRequest(), (err) => {
-      if (err) {
-        clearTimeout(timer);
-        finish(err);
-      }
+      socket.write(buildStatusRequest(), (err) => {
+        if (err) {
+          clearTimeout(timer);
+          finish(null, { reachable: false, status: null, error: err.message });
+        }
+      });
     });
-  });
+  } catch (err) {
+    return { reachable: false, status: null, error: err.message };
+  }
 }
 
 /**
- * Send a prepared job and wait for the printer to report completion.
+ * Send a prepared job.
  *
- * Resolves with every status frame received. The caller decides what counts
- * as success — a job can print fine and still emit a notification frame.
+ * Completion is signalled by the write flushing and the socket closing, NOT by
+ * a status frame — see probeStatus. If the printer does happen to answer, the
+ * frames come back as a bonus and a reported error is treated as authoritative.
  *
- * @param {string} host
- * @param {Buffer} job
- * @returns {Promise<{frames: Object[], completed: boolean, timedOut?: boolean}>}
+ * The write is followed by end(), not destroy(). destroy() can discard data
+ * still sitting in the kernel send buffer, which for a ~190 KB label job means
+ * a silently truncated print.
+ *
+ * @returns {Promise<{frames: Object[], completed: boolean, statusAvailable: boolean}>}
  */
 async function sendJob(host, job, options = {}) {
   const {
     port = DEFAULT_PORT,
     connectTimeout = DEFAULT_CONNECT_TIMEOUT,
     jobTimeout = DEFAULT_JOB_TIMEOUT,
-    waitForCompletion = true,
+    graceMs = 3000,
   } = options;
 
   return withSocket(host, port, connectTimeout, (socket, finish) => {
     let received = Buffer.alloc(0);
     const frames = [];
+    let sawCompleted = false;
+    let flushed = false;
+
+    const done = () =>
+      finish(null, {
+        frames,
+        completed: sawCompleted || flushed,
+        statusAvailable: frames.length > 0,
+      });
 
     const timer = setTimeout(() => {
-      if (frames.length > 0) {
-        // We heard something; report what we know rather than a bare timeout.
-        finish(null, { frames, completed: false, timedOut: true });
-      } else {
-        finish(
-          new Error(
-            `Printer at ${host}:${port} did not acknowledge the job within ${jobTimeout} ms`
-          )
-        );
-      }
+      finish(
+        new Error(
+          `Printer at ${host}:${port} did not accept the whole job within ${jobTimeout} ms`
+        )
+      );
     }, jobTimeout);
 
     socket.on('data', (chunk) => {
@@ -133,30 +148,38 @@ async function sendJob(host, job, options = {}) {
 
         if (frame.hasError) {
           clearTimeout(timer);
-          finish(null, { frames, completed: false });
-          return;
-        }
-        if (frame.statusType === 'printing_completed') {
-          clearTimeout(timer);
-          finish(null, { frames, completed: true });
+          finish(null, { frames, completed: false, statusAvailable: true });
           return;
         }
+        if (frame.statusType === 'printing_completed') sawCompleted = true;
       }
     });
 
+    // Peer closed and our data has gone out: the job is delivered.
+    socket.once('close', () => {
+      clearTimeout(timer);
+      done();
+    });
+
     socket.write(job, (err) => {
       if (err) {
         clearTimeout(timer);
         finish(err);
         return;
       }
-      if (!waitForCompletion) {
+      flushed = true;
+      // Half-close: flushes everything queued, then sends FIN.
+      socket.end();
+
+      // Some units never close their half of the connection. Once our data is
+      // out the job is delivered, so don't hold the queue open waiting on a FIN
+      // that may never arrive.
+      setTimeout(() => {
         clearTimeout(timer);
-        // Give the printer a beat to reject the job outright.
-        setTimeout(() => finish(null, { frames, completed: false }), 250);
-      }
+        done();
+      }, graceMs);
     });
   });
 }
 
-export { DEFAULT_PORT, queryStatus, sendJob };
+export { DEFAULT_PORT, probeStatus, sendJob };