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/envelope/envelope.zig

@@ -72,6 +72,41 @@ pub const operation_subscribe: u32 = 2; // capability = the subscriber's endpoin
 pub const operation_unsubscribe: u32 = 3;
 pub const first_protocol_operation: u32 = 16;
 
+/// The optional body of a reserved `subscribe`: **which** of a provider's events
+/// the subscriber wants, as a bit mask whose meaning the protocol defines (the
+/// input service's device classes are the model). A reserved verb carries no
+/// typed request, so this rides the packet's tail — and zero, which is also what
+/// a subscribe that sent no body at all reads as, means *every* event.
+///
+/// The mask lives here rather than in each protocol because the subscriber
+/// machinery is the service harness's (library/kernel/service.zig): the harness
+/// records the number, the protocol decides what its bits mean, and neither has
+/// to know the other.
+pub const Subscription = extern struct { interest: u32 = 0 };
+
+/// Frame a `subscribe` request. The subscriber's own endpoint travels as the
+/// call's *capability*, never in the packet — that is what makes the reverse
+/// path unforgeable.
+pub fn encodeSubscribe(interest: u32, buffer: []u8) ?[]u8 {
+    const header = Header{ .operation = operation_subscribe };
+    const body = Subscription{ .interest = interest };
+    return frame(std.mem.asBytes(&header), std.mem.asBytes(&body), &.{}, buffer);
+}
+
+/// Frame a bare `unsubscribe`: it names no event and no endpoint, because it
+/// means "every subscription this task holds here" (one task, one voice).
+pub fn encodeUnsubscribe(buffer: []u8) ?[]u8 {
+    const header = Header{ .operation = operation_unsubscribe };
+    return frame(std.mem.asBytes(&header), &.{}, &.{}, buffer);
+}
+
+/// The interest mask out of a `subscribe` packet's tail, on the provider's side.
+/// A caller that sent no mask reads as the every-event mask.
+pub fn decodeSubscribe(tail: []const u8) Subscription {
+    if (tail.len < @sizeOf(Subscription)) return .{};
+    return std.mem.bytesToValue(Subscription, tail[0..@sizeOf(Subscription)]);
+}
+
 /// The `describe` reply's fixed part, followed inline by `name_len` bytes of the
 /// protocol's name. This is the version handshake: the version is asked for
 /// once, at connect time, rather than re-carried by every packet out of a
@@ -876,6 +911,21 @@ test "a truncated packet answers -EPROTO" {
 // operation with `.request = extern struct { bytes: [241]u8 }`, or an event with
 // `.payload = extern struct { bytes: [49]u8 }`, fails to compile with the
 // protocol, the verb, and the two numbers named in the message.
+test "a subscribe carries its interest mask in the reserved verb's tail" {
+    var buffer: [packet_maximum]u8 = undefined;
+    const packet = encodeSubscribe(0b101, &buffer).?;
+    try testing.expectEqual(operation_subscribe, headerOf(packet).?.operation);
+    try testing.expectEqual(@as(u32, 0b101), decodeSubscribe(packet[prefix_size..]).interest);
+    // No body at all — and a body too short to be one — read as "every event",
+    // which is what a subscriber that named nothing wants.
+    try testing.expectEqual(@as(u32, 0), decodeSubscribe(&.{}).interest);
+    try testing.expectEqual(@as(u32, 0), decodeSubscribe(&.{ 1, 2 }).interest);
+
+    const bare = encodeUnsubscribe(&buffer).?;
+    try testing.expectEqual(operation_unsubscribe, headerOf(bare).?.operation);
+    try testing.expectEqual(prefix_size, bare.len);
+}
+
 test "the floor counts the header once, and the boundary is exact" {
     try testing.expect(fitsPacket(extern struct { bytes: [240]u8 }));
     try testing.expect(!fitsPacket(extern struct { bytes: [241]u8 }));