1 files changed, 39 insertions, 4 deletions

diff --git a/src/pardes.zig b/src/pardes.zig
index e92fb3f7..7863e272 100644
--- a/src/pardes.zig
+++ b/src/pardes.zig
@@ -1361,6 +1361,36 @@ test "Exec in a terminal whose tty is taken runs as a command pane instead of ty
     try std.testing.expect(spawned);
 }
 
+test "a spawn is for the pane it was made for, not whatever took the slot since" {
+    const gpa = std.testing.allocator;
+    const p = try Pardes.init(gpa, .{ .tty_only = true });
+    defer p.deinit();
+    while (p.nextEffect()) |_| {}
+    const Count = struct {
+        var spawns: usize = 0;
+        var command: [32]u8 = undefined;
+        var core: *Pardes = undefined;
+        const vtable: Host.VTable = .{ .spawn = spawn };
+        fn spawn(_: ?*anyopaque, pane: u8, _: []const u8) void {
+            spawns += 1;
+            const line = core.panes[pane].?.command.?;
+            @memcpy(command[0..line.len], line);
+        }
+    };
+    Count.core = p;
+    p.host = .{ .vtable = &Count.vtable };
+    // A command pane closed and its slot taken, in one step, before the
+    // effects run: only the second's spawn is performed.
+    const slot = p.freeSlot().?;
+    _ = try p.newCommand(slot, "", "first");
+    try p.removePane(slot, null);
+    try std.testing.expectEqual(slot, p.freeSlot().?);
+    _ = try p.newCommand(slot, "", "second");
+    while (p.nextEffect()) |effect| p.perform(effect);
+    try std.testing.expectEqual(@as(usize, 1), Count.spawns);
+    try std.testing.expectEqualStrings("second", Count.command[0.."second".len]);
+}
+
 test "Tty+fish, one word a tag can hold, opens a terminal on that shell" {
     const p = try Pardes.init(std.testing.allocator, .{ .tty_only = true });
     defer p.deinit();
@@ -3136,7 +3166,10 @@ pub const WatchMode = enum { reconcile, baseline_disk };
 pub const effect_path_cap = 256;
 
 pub const Effect = union(enum) {
-    spawn: struct { pane: u8, cwd: Buf(effect_path_cap) },
+    /// A child for the pane in slot `pane` -- the one whose serial is
+    /// `serial`: a pane closed and its slot taken before the effect is
+    /// performed is not the one it was for (perform).
+    spawn: struct { pane: u8, serial: u32, cwd: Buf(effect_path_cap) },
     write: struct { pane: u8, bytes: Buf(64) },
     resize_pty: struct { pane: u8, cols: u16, rows: u16 },
     signal_pty: struct { pane: u8, sig: PtySignal },
@@ -4243,7 +4276,7 @@ pub const Pardes = struct {
         const pane = try panes.Terminal.create(p.gpa, p.screen_w, p.screen_h);
         panes.Terminal.armShellSpawn(pane);
         p.installPane(id, pane);
-        p.emit(.{ .spawn = .{ .pane = @intCast(id), .cwd = .from(cwd) } });
+        p.emit(.{ .spawn = .{ .pane = @intCast(id), .serial = pane.serial, .cwd = .from(cwd) } });
         return pane;
     }
 
@@ -4262,7 +4295,7 @@ pub const Pardes = struct {
         // directory's commands reuse it; the child's own cd does not move it.
         pane.setOwnedCwd(cwd) catch {};
         p.installPane(id, pane);
-        p.emit(.{ .spawn = .{ .pane = @intCast(id), .cwd = .from(cwd) } });
+        p.emit(.{ .spawn = .{ .pane = @intCast(id), .serial = pane.serial, .cwd = .from(cwd) } });
         return pane;
     }
 
@@ -4608,7 +4641,9 @@ pub const Pardes = struct {
     pub fn perform(p: *Pardes, e: Effect) void {
         const v = p.host.vtable;
         switch (e) {
-            .spawn => |s| if (v.spawn) |f| f(p.host.ctx, s.pane, s.cwd.slice()) else {
+            // The host reads the pane (its command, its shell) as it forks,
+            // so the slot must still hold the pane the spawn was for.
+            .spawn => |s| if (if (p.panes[s.pane]) |pane| pane.serial != s.serial else true) {} else if (v.spawn) |f| f(p.host.ctx, s.pane, s.cwd.slice()) else {
                 p.fallback.spawned[s.pane] = true;
             },
             // A pane with no child is silent: nothing invents output on its