Publish exit events to subscribers; the VFS releases dead clients' handles (M17.3)

process_subscribe adds an endpoint to a bounded, ref-counted subscriber
table; every death posts the same badge encoding a supervisor's exit
notification uses, equally late, so subscribers observe a fully-released
child. A dying subscriber's own subscriptions are removed first — it never
hears about itself. The VFS is the first subscriber: open handles now
record their owner and are swept when the owner dies, because a service
must never depend on clients cleaning up after themselves
(docs/process-lifecycle.md). Proven by the vfs-client-death scenario.

system/services/vfs/vfs.zig

@@ -23,6 +23,10 @@ const Node = struct {
 const OpenFile = struct {
     used: bool = false,
     node: usize = 0,
+    // The client (task id — an IPC badge is one) that opened this handle. What
+    // release-on-death sweeps by: a service must never depend on its clients
+    // cleaning up after themselves (docs/process-lifecycle.md).
+    owner: u32 = 0,
 };
 
 var nodes = [_]Node{.{}} ** 8;
@@ -65,8 +69,29 @@ fn fail(out: []u8) usize {
     return writeReply(out, .{ .status = -1 }, &.{});
 }
 
-/// Handle one request; write the reply into `out`, return its length.
-fn handle(message: []const u8, out: []u8) usize {
+/// Format one whole log line and emit it in a single `debug_write`, so lines from
+/// concurrent processes can never land in the middle of it.
+fn writeLine(comptime fmt: []const u8, arguments: anytype) void {
+    var line: [96]u8 = undefined;
+    _ = runtime.system.write(std.fmt.bufPrint(&line, fmt, arguments) catch return);
+}
+
+/// Release every open handle `client` held — called on that client's published
+/// exit event. The nodes (the files) stay: ramfs contents outlive their writers,
+/// only the dead client's handles go.
+fn releaseClientHandles(client: u32) void {
+    var released: u32 = 0;
+    for (&opens) |*o| {
+        if (o.used and o.owner == client) {
+            o.used = false;
+            released += 1;
+        }
+    }
+    if (released != 0) writeLine("vfs: released {d} handle(s) for dead client {d}\n", .{ released, client });
+}
+
+/// Handle one request from `sender`; write the reply into `out`, return its length.
+fn handle(message: []const u8, out: []u8, sender: u32) usize {
     if (message.len < protocol.request_size) return fail(out);
     const request = std.mem.bytesToValue(protocol.Request, message[0..protocol.request_size]);
     const payload = message[protocol.request_size..];
@@ -77,7 +102,7 @@ fn handle(message: []const u8, out: []u8) usize {
             const ni = findNode(name) orelse createNode(name) orelse return fail(out);
             for (&opens, 0..) |*o, i| {
                 if (!o.used) {
-                    o.* = .{ .used = true, .node = ni };
+                    o.* = .{ .used = true, .node = ni, .owner = sender };
                     return writeReply(out, .{ .status = 0, .node = i }, &.{});
                 }
             }
@@ -122,6 +147,12 @@ pub fn main() void {
         _ = runtime.system.write("vfs: register failed\n");
         return;
     }
+    // First subscriber of the published exit events (docs/process-lifecycle.md):
+    // when a client dies holding open handles, the notification below is how the
+    // VFS learns to release them.
+    if (!runtime.process.subscribeExits(endpoint)) {
+        _ = runtime.system.write("vfs: exit subscription failed\n");
+    }
     _ = runtime.system.write("vfs: ready\n");
 
     var reply_buffer: [protocol.message_maximum]u8 = undefined;
@@ -129,8 +160,17 @@ pub fn main() void {
     var receive: [protocol.message_maximum]u8 = undefined;
     while (true) {
         const got = runtime.ipc.replyWait(endpoint, reply_buffer[0..reply_len], &receive, null);
-        // Ignore notifications (none expected here); handle a request.
-        reply_len = handle(receive[0..got.len], &reply_buffer);
+        if (got.isChildExit()) {
+            // A published exit event: a process died; drop whatever it held.
+            releaseClientHandles(got.childProcessId());
+            reply_len = 0;
+            continue;
+        }
+        if (got.isNotification()) {
+            reply_len = 0; // not a request; nothing owed
+            continue;
+        }
+        reply_len = handle(receive[0..got.len], &reply_buffer, got.senderTaskId());
     }
 }