pty/run's busy names the program holding the terminal

busy said only that something ran. Where the host can tell (Linux, the
tty's foreground group's /proc comm, a new fg_name host call), it says
busy: <program> is running; elsewhere, busy as before. The GUI's host
gains the one small callback (render agent's file: gui.zig, fgName).

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

7 files changed, 52 insertions, 2 deletions

diff --git a/src/detached/server.zig b/src/detached/server.zig
index af299051..bb66b841 100644
--- a/src/detached/server.zig
+++ b/src/detached/server.zig
@@ -373,6 +373,7 @@ pub const Session = struct {
         .pty_signal = ptySignal,
         .close_pty = closePaneShell,
         .tty_taken = ttyTaken,
+        .fg_name = fgName,
         .kill_job = killJob,
         .write_file = writeFile,
         .write_dump = writeDump,
@@ -544,6 +545,14 @@ pub const Session = struct {
         return host_io.ttyTaken(pt.pid, pt.fd);
     }
 
+    fn fgName(ctx: ?*anyopaque, pane: u8, buf: []u8) ?[]const u8 {
+        const s = of(ctx);
+        if (pane >= s.ptys.len) return null;
+        const pt = s.ptys[pane];
+        if (pt.fd < 0) return null;
+        return host_io.foregroundName(pt.pid, pt.fd, buf);
+    }
+
     fn killJob(ctx: ?*anyopaque, pane: u8) bool {
         const s = of(ctx);
         if (pane >= s.ptys.len) return false;
diff --git a/src/gui/gui.zig b/src/gui/gui.zig
index 491373a8..de5da1e7 100644
--- a/src/gui/gui.zig
+++ b/src/gui/gui.zig
@@ -4045,6 +4045,7 @@ const Shell = struct {
         .pty_signal = ptySignal,
         .close_pty = closePaneShell,
         .tty_taken = ttyTaken,
+        .fg_name = fgName,
         .kill_job = killJob,
         .write_file = writeFile,
         .write_dump = writeDump,
@@ -4507,6 +4508,13 @@ fn ttyTaken(ctx: ?*anyopaque, pane: u8) bool {
     return host_io.ttyTaken(pt.pid, pt.fd);
 }
 
+fn fgName(ctx: ?*anyopaque, pane: u8, buf: []u8) ?[]const u8 {
+    const s = shellOf(ctx);
+    const pt = s.ptys[pane] orelse return null;
+    if (pt.fd < 0) return null;
+    return host_io.foregroundName(pt.pid, pt.fd, buf);
+}
+
 fn killJob(ctx: ?*anyopaque, pane: u8) bool {
     const s = shellOf(ctx);
     const pt = s.ptys[pane] orelse return false;
diff --git a/src/host_io.zig b/src/host_io.zig
index 10a85d8f..1db0dac9 100644
--- a/src/host_io.zig
+++ b/src/host_io.zig
@@ -25,6 +25,9 @@ pub const Host = struct {
         pty_signal: ?*const fn (ctx: ?*anyopaque, pane: u8, sig: pardes.PtySignal) void = null,
         close_pty: ?*const fn (ctx: ?*anyopaque, pane: u8) void = null,
         tty_taken: ?*const fn (ctx: ?*anyopaque, pane: u8) bool = null,
+        /// The name of the program holding the tty, when it is not the
+        /// shell (host_io.foregroundName), in `buf`.
+        fg_name: ?*const fn (ctx: ?*anyopaque, pane: u8, buf: []u8) ?[]const u8 = null,
         /// Answered at once, not queued as an effect: Kill asks while the
         /// job it saw running is still the one in the tty (host_io.killJob).
         kill_job: ?*const fn (ctx: ?*anyopaque, pane: u8) bool = null,
@@ -1751,6 +1754,18 @@ pub fn killJob(shell_pid: libc.pid_t, master_fd: c_int) bool {
     return libc.kill(-fg, libc.SIG.TERM) == 0;
 }
 
+/// The command name of the tty's foreground job, when that is not the
+/// shell: what `pty/run` says is running. Linux only (/proc/<pgrp>/comm).
+pub fn foregroundName(shell_pid: libc.pid_t, master_fd: c_int, buf: []u8) ?[]const u8 {
+    if (comptime builtin.os.tag != .linux) return null;
+    const fg = tcgetpgrp(master_fd);
+    if (fg <= 0 or fg == shell_pid) return null;
+    var name: [64:0]u8 = undefined;
+    const path = std.fmt.bufPrintSentinel(&name, "/proc/{d}/comm", .{@as(u32, @intCast(fg))}, 0) catch return null;
+    const comm = std.mem.trimEnd(u8, readProc(path, buf) orelse return null, "\n");
+    return if (comm.len > 0) comm else null;
+}
+
 fn readProc(path: [*:0]const u8, buf: []u8) ?[]const u8 {
     const fd = libc.open(path, .{ .ACCMODE = .RDONLY });
     if (fd < 0) return null;
diff --git a/src/ninep/pty.zig b/src/ninep/pty.zig
index 393516ce..6b84af61 100644
--- a/src/ninep/pty.zig
+++ b/src/ninep/pty.zig
@@ -189,7 +189,12 @@ pub fn writeRun(p: *Pardes, req: Req, id: usize, pane: *Pane) Reply {
         // marks: a nested shell's prompt (ssh, a shell with its own
         // integration) looks like an empty prompt to ghostty but is not the
         // shell this run knows.
-        answer(p, slot, "busy", .{});
+        // Naming what runs, where the host can tell (Linux).
+        var name: [64]u8 = undefined;
+        if (p.hostForegroundName(id, &name)) |program|
+            answer(p, slot, "busy: {s} is running", .{program})
+        else
+            answer(p, slot, "busy", .{});
     } else {
         // Sent now if the shell is at its prompt, or at its first prompt
         // when it has not drawn one yet (noteMarks).
diff --git a/src/pardes.zig b/src/pardes.zig
index c103b9d7..d6859f86 100644
--- a/src/pardes.zig
+++ b/src/pardes.zig
@@ -5124,6 +5124,11 @@ pub const Pardes = struct {
         return ev;
     }
 
+    pub fn hostForegroundName(p: *const Pardes, id: usize, buf: []u8) ?[]const u8 {
+        const f = p.host.vtable.fg_name orelse return null;
+        return f(p.host.ctx, @intCast(id), buf);
+    }
+
     pub fn hostTtyTaken(p: *const Pardes, id: usize) bool {
         const f = p.host.vtable.tty_taken orelse return false;
         return f(p.host.ctx, @intCast(id));
diff --git a/src/tty/tty.zig b/src/tty/tty.zig
index 754b4b96..e7eddec4 100644
--- a/src/tty/tty.zig
+++ b/src/tty/tty.zig
@@ -946,6 +946,7 @@ const Shell = struct {
         .pty_signal = ptySignal,
         .close_pty = closePty,
         .tty_taken = ttyTaken,
+        .fg_name = fgName,
         .kill_job = killJob,
         .write_file = writeFile,
         .write_dump = writeDump,
@@ -1295,6 +1296,12 @@ const Shell = struct {
         return host_io.ttyTaken(pt.pid, pt.file.handle);
     }
 
+    fn fgName(ctx: ?*anyopaque, pane: u8, buf: []u8) ?[]const u8 {
+        const s = of(ctx);
+        const pt = s.ptys[pane] orelse return null;
+        return host_io.foregroundName(pt.pid, pt.file.handle, buf);
+    }
+
     /// The command panes' exits, told once their output is in (host_io).
     fn takeExits(s: *@This()) void {
         host_io.takeExits(s.core, &s.ptys, s, closeWatched);
diff --git a/test/fs.py b/test/fs.py
index bac7020c..b97c6967 100644
--- a/test/fs.py
+++ b/test/fs.py
@@ -428,7 +428,8 @@ def run_file(binary):
                 waiter = threading.Thread(target=lambda: slow.append(run(other, term, b'sleep 1\n')), daemon=True)
                 waiter.start()
                 time.sleep(.3)
-                assert run(client, term, b'true\n') == b'busy\n'
+                # The job the other client ran holds the tty, and says so.
+                assert run(client, term, b'true\n') == b'busy: sleep is running\n'
                 waiter.join(5)
                 assert slow == [b'exit 0\n'], slow
                 closing = []