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termio/exec: fix 100% CPU usage after wait-after-command process exits (#4171)

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read(2) returning 0 means that the other end of the pipe/pty has been
closed (EOF), so there cannot be any more output to read on the pipe,
and
the io reader thread can just exit.

If exec.wait_after_command=false, the read thread's quit pipe is
immediately written to after the child process dies, so all is well.
However, if wait_after_command=true (which is the case when using
Ghostty
to run a .command/.sh file on macOS), the read thread keeps spinning,
causing persistent 100% CPU usage per exited process.

Fix it by exiting the reader thread on EOF.
This commit is contained in:
Mitchell Hashimoto
2025-01-08 13:10:20 -08:00
committed by GitHub
parent 542c655348 3e24e96af5
commit 6f720a0b11
1 changed files with 16 additions and 9 deletions
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25
src/termio/Exec.zig
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@ -179,8 +179,11 @@ pub fn threadExit(self: *Exec, td: *termio.Termio.ThreadData) void {
// Quit our read thread after exiting the subprocess so that
// we don't get stuck waiting for data to stop flowing if it is
// a particularly noisy process.
_ = posix.write(exec.read_thread_pipe, "x") catch |err|
log.warn("error writing to read thread quit pipe err={}", .{err});
if (exec.read_thread_pipe) |pipe| {
posix.close(pipe);
// Tell deinit that we've already closed the pipe
exec.read_thread_pipe = null;
}
if (comptime builtin.os.tag == .windows) {
// Interrupt the blocking read so the thread can see the quit message
@ -639,7 +642,7 @@ pub const ThreadData = struct {
/// Reader thread state
read_thread: std.Thread,
read_thread_pipe: posix.fd_t,
read_thread_pipe: ?posix.fd_t,
read_thread_fd: posix.fd_t,
/// The timer to detect termios state changes.
@ -652,7 +655,8 @@ pub const ThreadData = struct {
termios_mode: ptypkg.Mode = .{},
pub fn deinit(self: *ThreadData, alloc: Allocator) void {
posix.close(self.read_thread_pipe);
// If the pipe isn't closed, close it.
if (self.read_thread_pipe) |pipe| posix.close(pipe);
// Clear our write pools. We know we aren't ever going to do
// any more IO since we stop our data stream below so we can just
@ -1433,9 +1437,12 @@ pub const ReadThread = struct {
};
// This happens on macOS instead of WouldBlock when the
// child process dies. To be safe, we just break the loop
// and let our poll happen.
if (n == 0) break;
// child process dies. It's equivalent to NotOpenForReading
// so we can just exit.
if (n == 0) {
log.info("io reader exiting", .{});
return;
}
// log.info("DATA: {d}", .{n});
@call(.always_inline, termio.Termio.processOutput, .{ io, buf[0..n] });
@ -1447,8 +1454,8 @@ pub const ReadThread = struct {
return;
};
// If our quit fd is set, we're done.
if (pollfds[1].revents & posix.POLL.IN != 0) {
// If our quit fd is closed, we're done.
if (pollfds[1].revents & posix.POLL.HUP != 0) {
log.info("read thread got quit signal", .{});
return;
}