Answer a held read by its cloud9 ticket, refuse a second, and say a pane shut down

The held read is now kept by the Ticket cloud9's Conn.hold() gives its
park and answered through Conn.answerWith(), which makes the answer only
while that very park still waits, instead of walking the engine's slots
by tag; pardes no longer reaches into the engine for it. A second read on
an open whose read is held fails with file in use rather than sitting
parked where nothing answers it, and a read that waits with no open
record to hold it is logged and asserted on. A read on an event or
pty/data open whose pane closed answers acme's "window shut down"
(editors/acme/xfid.c:1005). The pane keeps its run's and its lock's open
handles, checked through openOf on use, not record indices. Docs: lock
from a shell needs a held fd, and a command that clears the screen may
read as cut.

Needs cloud9 zvuqvnzy (cca47d63), which adds Conn.hold, waiting and
answerWith; build.zig.zon still pins 82d8152c until that is pushed and
re-pinned.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 82 insertions, 41 deletions

diff --git a/src/9p_io.zig b/src/9p_io.zig
index 7c366aa8..7b0976bc 100644
--- a/src/9p_io.zig
+++ b/src/9p_io.zig
@@ -228,10 +228,29 @@ pub const Listener = struct {
         const reply = core.serveFs(req);
         if (req.op == .release and quiet) l.collectOs();
         // A read with nothing yet stays parked in the engine, and the core
-        // keeps it to answer when what it waits on has something.
-        if (reply.status == .again and req.op == .read) if (pardes.ctlfs.openOf(core, req)) |o| {
-            o.held = .{ .asker = conn, .req = req };
-        };
+        // keeps its ticket to answer it when what it waits on has something.
+        if (reply.status == .again and req.op == .read) {
+            // Only an open's record can hold a read; one that waits with
+            // none would sit parked until some unrelated write parks.
+            const o = pardes.ctlfs.openOf(core, req) orelse {
+                log.err("a read of node {x} waits with no open record to hold it", .{req.node});
+                std.debug.assert(false);
+                return conn.reply(&reply, "");
+            };
+            // One read waits on an open at a time, as acme's window keeps
+            // one `eventx`; a second is refused rather than left parked
+            // where nothing would ever answer it. The first asked again by
+            // a retry is the same request, and holds its place.
+            if (o.held) |held| if (held.req.tag != req.tag) {
+                const other: *Runner.Conn = @ptrCast(@alignCast(held.asker));
+                if (other.waiting(held.ticket))
+                    return conn.reply(&pardes.ctlfs.failText(req.tag, pardes.ctlfs.E.BUSY, pardes.ctlfs.e_in_use), "");
+            };
+            o.held = null;
+            const ticket = conn.hold(&reply) orelse return;
+            o.held = .{ .asker = conn, .req = req, .ticket = ticket };
+            return;
+        }
         if (!pardes.ctlfs.changesPane(req)) return conn.reply(&reply, core.fsPayload(reply));
         // The editor draws the change and performs what it asked for; when
         // it asked for something -- a save, a shell, a watch -- the answer
@@ -253,10 +272,11 @@ pub const Listener = struct {
 
     /// With the turn, as it is given up: answers each read the core holds
     /// whose file now has something, on the connection that asked, with no
-    /// retry of anything else parked there. Only a read its engine still
-    /// holds parked is answered: Tflush drops one without a word to the
-    /// backend, and a `retry` takes one out to ask again, and a record spent
-    /// on either would be lost to the reader that comes next.
+    /// retry of anything else parked there. Only while its ticket still
+    /// waits, asked in the same hold of the engine as the answer: Tflush
+    /// and clunk drop a park without a word to the backend, a `retry` takes
+    /// one out to ask again (and that asking answers it), and a record spent
+    /// on any of them would be lost to the reader that comes next.
     fn answerHeld(ctx: ?*anyopaque) void {
         if (comptime !supported) return;
         const l = of(ctx);
@@ -267,27 +287,22 @@ pub const Listener = struct {
         for (&core.fs.opens) |*o| {
             const held = o.held orelse continue;
             const conn: *Runner.Conn = @ptrCast(@alignCast(held.asker));
-            conn.lock();
-            const parked: ?bool = for (conn.engine.slots) |sl| {
-                if (sl.used and sl.req.tag == held.req.tag) break sl.parked;
-            } else null;
-            // Gone (flushed, answered, hung up) it is forgotten; out being
-            // asked again, the asking answers it.
-            if (parked != true) {
-                if (parked == null) o.held = null;
-                conn.unlock();
-                continue;
-            }
-            const reply = core.serveFs(held.req);
-            if (reply.status != .again) {
-                o.held = null;
-                conn.engine.reply(&reply, core.fsPayload(reply));
-            }
-            conn.unlock();
-            conn.flush();
+            if (!conn.answerWith(held.ticket, Held{ .core = core, .req = held.req }, Held.make)) o.held = null;
         }
     }
 
+    /// A held read asked again, to make its answer while its park waits.
+    const Held = struct {
+        core: *pardes.Pardes,
+        req: pardes.ctlfs.Req,
+
+        fn make(h: Held) ?Runner.Conn.Answer {
+            const reply = h.core.serveFs(h.req);
+            if (reply.status == .again) return null;
+            return .{ .reply = reply, .bytes = h.core.fsPayload(reply) };
+        }
+    };
+
     /// The turn was given up quiet with a request parked: every connection
     /// retries what it parked.
     fn wakeParked(ctx: ?*anyopaque) void {
@@ -1407,41 +1422,65 @@ test "a held read is answered when the log has news, and a flushed one spends no
     _ = try s.ask(.{ .write = .{ .fid = 1, .offset = 0, .data = "follow\n" } }, &remote);
 
     const heldNow = struct {
-        fn check(core: *pardes.Pardes) !void {
+        /// Waits until the core holds `n` reads.
+        fn count(core: *pardes.Pardes, n: usize) !void {
             const deadline = Client.nowMs() + Client.budget_ms;
             while (Client.nowMs() < deadline) {
                 pardes.turn.wake();
-                const any = for (core.fs.opens) |o| {
-                    if (o.held != null) break true;
-                } else false;
+                var got: usize = 0;
+                for (core.fs.opens) |o| got += @intFromBool(o.held != null);
                 pardes.turn.rest();
-                if (any) return;
+                if (got == n) return;
                 Client.nap(1);
             }
             return error.Timeout;
         }
     };
+    const serial = p.panes[0].?.serial;
+    var event_path: [32]u8 = undefined;
+    var event_names: [3][]const u8 = .{ "pane", try std.fmt.bufPrint(&event_path, "{d}", .{serial}), "event" };
+    _ = try s.ask(.{ .walk = .{ .fid = 0, .newfid = 2, .names = &event_names } }, &remote);
+    _ = try s.ask(.{ .open = .{ .fid = 2, .mode = ninep.oread } }, &remote);
 
     // Flushed while held, a read is gone from the engine without the core
-    // hearing of it; the record logged next must reach the read after it.
+    // hearing of it. Its tag, asked again at once for a read of the pane's
+    // event, must not be answered with the log's next record, and that
+    // record must reach the log's next read.
     const flushed = try s.cl.submit(.{ .read = .{ .fid = 1, .offset = 0, .count = 4096 } });
     try s.flush();
-    try heldNow.check(p);
+    try heldNow.count(p, 1);
     _ = try s.cl.submit(.{ .flush = .{ .oldtag = flushed } });
     const interrupted = try s.settle();
     try testing.expectEqual(flushed, interrupted.tag);
     try testing.expect(interrupted.result == .fail);
     try testing.expect((try s.settle()).result == .flush);
+    const reused = try s.cl.submit(.{ .read = .{ .fid = 2, .offset = 0, .count = 4096 } });
+    try testing.expectEqual(flushed, reused);
+    try s.flush();
+    try heldNow.count(p, 2);
     pardes.turn.wake();
     p.setMessage(0, "after the flush");
     pardes.turn.rest();
+    try heldNow.count(p, 1);
+    pardes.turn.wake();
+    p.fs.origin = 'K';
+    _ = pardes.ctlfs.events.noteAction(p, 0, .body_exec, 0, 4, 0, "held");
+    pardes.turn.rest();
+    const clicked = try s.settle();
+    try testing.expectEqual(reused, clicked.tag);
+    try testing.expectEqualStrings("KX0 4 0 4 held\n", clicked.result.read);
     try testing.expect(std.mem.indexOf(u8, (try s.ask(.{ .read = .{ .fid = 1, .offset = 0, .count = 4096 } }, &remote)).read, "after the flush") != null);
 
     // Held, a read is answered by the record that arrives, on its own
-    // connection, with no retry of every parked request asked for.
+    // connection, with no retry of every parked request asked for; a
+    // second read on the same open meanwhile is refused, not orphaned.
     const waiting = try s.cl.submit(.{ .read = .{ .fid = 1, .offset = 0, .count = 4096 } });
     try s.flush();
-    try heldNow.check(p);
+    try heldNow.count(p, 1);
+    const second = try s.cl.submit(.{ .read = .{ .fid = 1, .offset = 0, .count = 4096 } });
+    const refused = try s.settle();
+    try testing.expectEqual(second, refused.tag);
+    try testing.expectEqualStrings(pardes.ctlfs.e_in_use, refused.result.fail);
     pardes.turn.wake();
     p.setMessage(0, "while held");
     const retry_all = pardes.turn.parked;
@@ -1450,13 +1489,15 @@ test "a held read is answered when the log has news, and a flushed one spends no
     const answered = try s.settle();
     try testing.expectEqual(waiting, answered.tag);
     try testing.expect(std.mem.indexOf(u8, answered.result.read, "while held") != null);
+
+    // Clunked, the log's held read is interrupted by the engine and the
+    // record it would have had is spent on nobody.
+    _ = try s.cl.submit(.{ .read = .{ .fid = 1, .offset = 0, .count = 4096 } });
+    try s.flush();
+    try heldNow.count(p, 1);
     s.drop(1);
-    pardes.turn.wake();
-    const left = for (p.fs.opens) |o| {
-        if (o.held != null) break true;
-    } else false;
-    pardes.turn.rest();
-    try testing.expect(!left);
+    s.drop(2);
+    try heldNow.count(p, 0);
 }
 
 extern "c" fn setenv(name: [*:0]const u8, value: [*:0]const u8, overwrite: c_int) c_int;