gfhome: derive the service hostname from the effective serial

The gf_hostname override is gone (the forgectrl UI no longer offers
it): the hostname is a pure derivation of the serial - base 23 over
the factory consonant alphabet - so a gf_serial override re-derives
MACHINE.HOSTNAME and the fuse derivation stands otherwise. BRINGUP
records the panel rework (units, fuse identity, always-on position)
as offline-verified; board deploy and pin bumps held during the
firmware-upgrade bench testing.

meta-forgefirm/recipes-forgefirm/gfhome/files/gfhome.conf.sample

@@ -1,8 +1,9 @@
 # gfhome - Glowforge web-service homing configuration.
 # Copied to /data/etc/gfhome.conf on first run; edit the copy.
 # Machine identity (serial/password) is read from the OCOTP fuses;
-# gf_serial / gf_password / gf_hostname in /data/forgefirm.conf (the
-# forgectrl UI, GF Cloud tab) override it.
+# gf_serial / gf_password in /data/forgefirm.conf (the forgectrl UI,
+# GF Cloud tab) override it. The service hostname always derives from
+# whichever serial is in effect.
 
 [SERVICE]
 server_url: https://app.glowforge.com

meta-forgefirm/recipes-forgefirm/gfhome/files/gfhome.py

@@ -14,8 +14,9 @@ homing_mode = gfcloud is set in /data/forgefirm.conf, releasing
 /dev/glowforge for the duration of the run. It can also be run by hand
 (with the controller stopped or its homing session active). The same
 shared config supplies optional identity overrides (gf_serial /
-gf_password / gf_hostname; the fuse identity is the fallback), managed
-from the forgectrl UI.
+gf_password; the fuse identity is the fallback), managed from the
+forgectrl UI. The service hostname is always derived from whichever
+serial is in effect - it is never set independently.
 
 The service ends the sequence silently - there is no completion
 message - so the run is considered homed once a hunt and at least one
@@ -75,11 +76,25 @@ def load_config(path: str) -> bool:
     return True
 
 
+def _hostname_for(serial):
+    """The factory serial -> hostname encoding (base 23 over the
+    consonant alphabet, up to six characters, split XXX-YYY) - the
+    same derivation gfhardware applies to the fuse serial."""
+    enc = ''
+    serial = int(serial)
+    while serial > 0 and len(enc) < 6:
+        enc = 'BCDFGHJKMQRTVWXY2346789'[serial % 23] + enc
+        serial //= 23
+    return '{}-{}'.format(enc[:3], enc[3:])
+
+
 def apply_identity_overrides():
     """Identity overrides from the shared machine config (set in the
-    forgectrl UI): non-empty gf_serial / gf_password / gf_hostname beat
-    the OCOTP fuse identity - Machine.__init__ sets its fuse values
-    with keep_value, so whatever is in the config store first wins."""
+    forgectrl UI): non-empty gf_serial / gf_password beat the OCOTP
+    fuse identity - Machine.__init__ sets its fuse values with
+    keep_value, so whatever is in the config store first wins. The
+    hostname is never overridden independently: it derives from the
+    serial, so a serial override re-derives it."""
     keys = {}
     try:
         with open(MACHINE_CONF) as f:
@@ -92,11 +107,18 @@ def apply_identity_overrides():
     except OSError:
         return
     for key, cfg in (('gf_serial', 'MACHINE.SERIAL'),
-                     ('gf_password', 'MACHINE.PASSWORD'),
-                     ('gf_hostname', 'MACHINE.HOSTNAME')):
+                     ('gf_password', 'MACHINE.PASSWORD')):
         if keys.get(key):
             set_cfg(cfg, keys[key])
             logger.info('identity override: %s from %s', cfg, MACHINE_CONF)
+    if keys.get('gf_serial'):
+        try:
+            set_cfg('MACHINE.HOSTNAME', _hostname_for(keys['gf_serial']))
+            logger.info('identity override: MACHINE.HOSTNAME derived '
+                        'from gf_serial')
+        except ValueError:
+            logger.warning('gf_serial is not numeric; hostname left '
+                           'at the fuse derivation')
 
 
 def make_machine():