library: the harness keeps the subscribers, and an id belongs to whoever opened it

Three services had each written the same thing and got it three different
ways: input polled the process list to notice a dead subscriber, and only
when someone else subscribed; the power service never noticed at all; the
device manager noticed drivers but not subscribers. The harness owns the
table now, driven by the events a protocol declares — it registers on the
reserved verb, frames each event once, posts to everyone interested without
waiting on any of them, and reclaims a slot when the kernel says its owner
died. Interest masks moved to the envelope, so a subscriber that wants only
mice asks the same way everywhere.

Two consequences the plan had not foreseen. The device manager now hears a
supervised child's death twice, once as its supervisor and once as a
subscriber, so restart backoff counted every crash twice and gave up after
half as many; it retires the id before counting. And the kernel's published
exit table had eight slots for what is now six subscriptions in a plain
boot, so it holds sixteen.

The other half is a hole the design named early and left standing: a
backend handed out a small integer and then honoured it from anyone. A
process that guessed a file's node id read another client's file; a display
layer had no owner at all, so any client could reconfigure or destroy any
layer; a USB device token was never checked against the client that opened
it. Each is now bound to the task that opened it, and a wrong owner gets
exactly what an unknown id gets — the refusal must not become the oracle
the identical answers elsewhere were designed to remove. Closing a file
changed with it: it used to succeed unconditionally, which would have told
a caller which ids existed.

Suite 111/111, with a new case in which one process holds a file and a
layer, hands both ids to a second process, and finds them untouched after
that process has tried everything with them.

library/protocol/input/input-protocol.zig

@@ -13,7 +13,8 @@
 //! - **subscribe** is the *reserved* verb, not one of this protocol's own: its shape — a
 //!   synchronous call whose attached capability is the subscriber's endpoint — is exactly
 //!   what `envelope.operation_subscribe` means everywhere. The interest mask travels as the
-//!   packet's tail (`Subscribe`), because a reserved verb carries no typed request.
+//!   packet's tail (`envelope.Subscription`), because a reserved verb carries no typed
+//!   request; what this protocol supplies is the *meaning* of its bits — the device classes.
 //! - **publish** is this protocol's one verb: a source sends one `InputEvent` and the
 //!   service answers at once, so publishing never blocks on a slow subscriber.
 //! - **delivery** is an event push: the service `ipc_send`s each event to every interested
@@ -294,12 +295,6 @@ pub const InputEvent = extern struct {
 
 // --- the contract -----------------------------------------------------------
 
-/// The body of a `subscribe` — the envelope's reserved verb 2, whose shape (a call whose
-/// capability is the subscriber's own endpoint) this protocol adopts wholesale. A reserved
-/// verb has no typed request, so the mask travels as the packet's tail and `encodeSubscribe`
-/// is how a client lays it down. Zero means every class.
-pub const Subscribe = extern struct { device_mask: u32 = 0 };
-
 pub const Protocol = envelope.Define(.{
     .name = "input",
     .version = 1,
@@ -334,23 +329,12 @@ pub fn eventOfDevice(device: u32) ?Event {
 }
 
 /// Frame a `subscribe` request: the reserved verb's header, then the interest mask. Null if
-/// the buffer is too small. Spelled here rather than at each caller so the one place that
-/// knows a reserved verb carries its body in the tail is the protocol module.
+/// the buffer is too small. The mask itself is the envelope's `Subscription` — the interest
+/// a reserved subscribe carries is universal, and the *meaning* of its bits (here: the
+/// device classes above) is what each protocol supplies. Kept as a named helper because
+/// `device_mask` is what an input caller calls it.
 pub fn encodeSubscribe(device_mask: u32, buffer: []u8) ?[]u8 {
-    const total = envelope.prefix_size + @sizeOf(Subscribe);
-    if (buffer.len < total) return null;
-    const header = envelope.Header{ .operation = envelope.operation_subscribe };
-    const body = Subscribe{ .device_mask = device_mask };
-    @memcpy(buffer[0..envelope.prefix_size], std.mem.asBytes(&header));
-    @memcpy(buffer[envelope.prefix_size..][0..@sizeOf(Subscribe)], std.mem.asBytes(&body));
-    return buffer[0..total];
-}
-
-/// The interest mask out of a `subscribe` packet's tail, on the provider's side. A caller
-/// that sent no mask at all means every class, which is what a zero mask means anyway.
-pub fn decodeSubscribe(tail: []const u8) Subscribe {
-    if (tail.len < @sizeOf(Subscribe)) return .{};
-    return std.mem.bytesToValue(Subscribe, tail[0..@sizeOf(Subscribe)]);
+    return envelope.encodeSubscribe(device_mask, buffer);
 }
 
 test "an event of every class fits the push floor, header included" {
@@ -388,7 +372,9 @@ test "a subscribe carries its mask in the tail of the reserved verb" {
     var buffer: [envelope.packet_maximum]u8 = undefined;
     const packet = encodeSubscribe(device_mouse, &buffer).?;
     try std.testing.expectEqual(envelope.operation_subscribe, envelope.headerOf(packet).?.operation);
-    try std.testing.expectEqual(device_mouse, decodeSubscribe(packet[envelope.prefix_size..]).device_mask);
+    // The provider side of this is the service harness's, which reads the same
+    // interest mask out of the tail for every protocol.
+    try std.testing.expectEqual(device_mouse, envelope.decodeSubscribe(packet[envelope.prefix_size..]).interest);
     // A caller that sent nothing at all reads as the every-class mask.
-    try std.testing.expectEqual(@as(u32, 0), decodeSubscribe(&.{}).device_mask);
+    try std.testing.expectEqual(@as(u32, 0), envelope.decodeSubscribe(&.{}).interest);
 }