Treat "busy" as retryable and pace jobs behind the printer

The VC-500W accepts one job at a time; anything sent while the tape is
moving comes back server-error-busy. Add IPPBusy, wait_until_idle and
print_job_paced so a batch goes through instead of failing from the
second label on.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Vunz4VLxAYDBnU5tLciLDC

app/services/ipp_client.py

@@ -24,6 +24,7 @@ Created by: Jess Rogerson (yelling commands at Claude.AI)
 import logging
 import random
 import struct
+import time
 
 import requests
 
@@ -71,6 +72,24 @@ class IPPError(Exception):
     """Anything that stopped us talking to the printer."""
 
 
+class IPPBusy(IPPError):
+    """
+    The printer refused the job because it is still working.
+
+    Worth separating from a real failure: a ZINK tape printer runs at
+    about 7.5mm/s, so a 15mm label takes a couple of seconds plus feed and
+    cut. Fire a batch at it with no pacing and everything after the first
+    job comes back busy. The cure is to wait and try again, not to give up.
+    """
+
+
+# Statuses that mean "not now" rather than "no".
+RETRYABLE_STATUSES = {
+    0x0503,  # server-error-service-unavailable
+    0x0507,  # server-error-busy
+}
+
+
 def _encode_attribute(tag, name, value):
     """One IPP attribute: tag, name, value. Empty name = extra set member."""
     if isinstance(value, bool):
@@ -304,6 +323,8 @@ def print_job(host, document, mime_type, job_name='LEGO part labels',
 
     if not result['ok']:
         detail = result['status_name']
+        if result['status'] in RETRYABLE_STATUSES:
+            raise IPPBusy(f'Printer is busy ({detail}).')
         if result['status'] == 0x040A:
             detail += (f' - the printer will not accept {mime_type}. '
                        'Run Test connection to see what it does accept.')
@@ -322,3 +343,58 @@ JOB_STATES = {
     3: 'pending', 4: 'pending-held', 5: 'processing', 6: 'processing-stopped',
     7: 'canceled', 8: 'aborted', 9: 'completed',
 }
+
+
+# --------------------------------------------------------------- pacing
+
+IDLE_STATES = ('idle',)
+BUSY_STATES = ('processing', 'stopped')
+
+
+def wait_until_idle(host, port=DEFAULT_PORT, path='/ipp/print',
+                    timeout=120.0, poll=1.0):
+    """
+    Block until the printer reports idle again, or the timeout runs out.
+
+    Returns True if it went idle. A probe failure returns False rather than
+    raising: the caller should still try the job and let that report the
+    real problem. A state we don't recognise also returns straight away -
+    waiting on a reading we can't interpret would just stall the batch for
+    the whole timeout on every single label.
+    """
+    deadline = time.monotonic() + timeout
+    while time.monotonic() < deadline:
+        try:
+            info = get_printer_attributes(host, port, path)
+        except IPPError:
+            return False
+        state = info.get('state')
+        if state in IDLE_STATES:
+            return True
+        if state not in BUSY_STATES:
+            return False
+        time.sleep(poll)
+    return False
+
+
+def print_job_paced(host, document, mime_type, job_name='LEGO part labels',
+                    port=DEFAULT_PORT, path='/ipp/print', user='lim',
+                    attempts=6, settle=1.5, wait_timeout=120.0):
+    """
+    Send one job, waiting for the printer and retrying while it is busy.
+
+    This is what a batch should use. print_job on its own fires and hopes,
+    which is fine for one label and reliably fails from the second one on.
+    """
+    last = None
+    for attempt in range(attempts):
+        wait_until_idle(host, port, path, timeout=wait_timeout)
+        try:
+            return print_job(host, document, mime_type, job_name=job_name,
+                             port=port, path=path, user=user)
+        except IPPBusy as exc:
+            last = exc
+            # Back off a little further each time round.
+            time.sleep(settle * (attempt + 1))
+    raise IPPError(f'Printer stayed busy after {attempts} attempts'
+                   + (f' ({last})' if last else '') + '.')