fat/harness: filesystems coexist without the shared vfs name; two-volume proof (S3)

A second usb-storage device (a generated data volume, serial da7a0001,
an empty FAT with no /system) plugged in beside the boot volume: the
volume manager adopts both devices and spawns a confined fat per volume,
each mounted at its own content id-path.

The test surfaced a real coexistence bug. Every filesystem bound the
single "vfs" contract name under /protocol; the second volume's fat lost
the race, service.run refused-and-exited on the held name, and that
volume never mounted. Clients don't reach filesystems by that name —
fs_resolve routes a path to its backing endpoint through the kernel
mount table by prefix — and nothing consumes "vfs", so the fix is to
bind no shared name: the harness's service_name now defaults to null.
This is the "this fades" the harness comment anticipated for the
volume-manager era; a filesystem's endpoint still serves as its mount
backend without a name.

fat logs "is a data volume" for the non-system branch so the test can
positively assert content-based detection. make-fat-image gains
--serial/--label (default unchanged) so a second image gets a distinct
id-path; the data image is generated per run, never committed. The case
fails against the pre-fix harness (the data volume's fat exits on the
refused bind) — toggle-demonstrated.

Full suite 129/129 (128 + two-volumes); the single-volume path is
unaffected by dropping the vestigial name bind.

tools/make-fat-image.py

@@ -47,8 +47,14 @@ def fat32_geometry(total_sectors):
 
 
 class Fat32Image:
-    def __init__(self, total_sectors):
+    def __init__(self, total_sectors, volume_id=0x12345678, label="DANOS"):
         self.total_sectors = total_sectors
+        # The FAT volume serial (its content identity — the /volumes/fat-<id>
+        # mount path danos derives from it) and the display label. A second image
+        # needs a distinct serial so its id-path does not collide with the boot
+        # volume's.
+        self.volume_id = volume_id & 0xFFFFFFFF
+        self.label = label
         self.fat_size, self.cluster_count = fat32_geometry(total_sectors)
         if self.cluster_count < 65525:
             sys.exit(f"error: image too small for FAT32 ({self.cluster_count} clusters "
@@ -146,8 +152,8 @@ class Fat32Image:
             0x80,                    # drive number
             0,                       # reserved
             0x29,                    # extended boot signature
-            0x12345678,              # volume id
-            b"DANOS      ",          # volume label
+            self.volume_id,          # volume id
+            self.label.encode("ascii", "replace")[:11].ljust(11, b" "),  # volume label
             b"FAT32   ",             # filesystem type
         )
         sector[510] = 0x55
@@ -276,9 +282,9 @@ def build_tree(pairs):
     return root
 
 
-def build(out_path, size_mib, pairs):
+def build(out_path, size_mib, pairs, volume_id=0x12345678, label="DANOS"):
     total_sectors = size_mib * 1024 * 1024 // SECTOR
-    image = Fat32Image(total_sectors)
+    image = Fat32Image(total_sectors, volume_id, label)
     tree = build_tree(pairs)
     write_directory(image, 2, tree, 0, True)
     with open(out_path, "wb") as handle:
@@ -350,14 +356,32 @@ def main(argv):
     if len(argv) == 3 and argv[1] == "--verify":
         verify(argv[2])
         return 0
+    # Optional flags ahead of the positionals: --serial <hex> sets the FAT volume
+    # id (the /volumes/fat-<id> content identity), --label <name> its display
+    # label. A second FAT image passes a distinct --serial so its id-path cannot
+    # collide with the boot volume's.
+    argv = list(argv)
+    volume_id = 0x12345678
+    label = "DANOS"
+    i = 1
+    while i < len(argv):
+        if argv[i] == "--serial" and i + 1 < len(argv):
+            volume_id = int(argv[i + 1], 16)
+            del argv[i:i + 2]
+        elif argv[i] == "--label" and i + 1 < len(argv):
+            label = argv[i + 1]
+            del argv[i:i + 2]
+        else:
+            i += 1
     if len(argv) < 3 or (len(argv) - 3) % 2 != 0:
-        sys.exit("usage: make-fat-image.py <out.img> <size-MiB> [<dest> <host>]...\n"
+        sys.exit("usage: make-fat-image.py [--serial <hex>] [--label <name>] "
+                 "<out.img> <size-MiB> [<dest> <host>]...\n"
                  "       make-fat-image.py --verify <out.img>")
     out_path = argv[1]
     size_mib = int(argv[2])
     rest = argv[3:]
     pairs = [(rest[i], rest[i + 1]) for i in range(0, len(rest), 2)]
-    build(out_path, size_mib, pairs)
+    build(out_path, size_mib, pairs, volume_id, label)
     return 0