termio/exec: if pty fd HUP, end read thread (#5351)

Fixes #4884

When our command exits, it will close the pty slave fd. This will
trigger a HUP on our poll. Previously, we only checked for IN. When a fd
is closed, IN triggers forever which would leave to an infinite loop and
100% CPU.

Now, detect the HUP and exit the read thread.

src/termio/Exec.zig

@@ -179,8 +179,17 @@ 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|
+    _ = posix.write(exec.read_thread_pipe, "x") catch |err| switch (err) {
-        log.warn("error writing to read thread quit pipe err={}", .{err});
+        // BrokenPipe means that our read thread is closed already,
+        // which is completely fine since that is what we were trying
+        // to achieve.
+        error.BrokenPipe => {},
+
+        else => log.warn(
+            "error writing to read thread quit pipe err={}",
+            .{err},
+        ),
+    };
 
     if (comptime builtin.os.tag == .windows) {
         // Interrupt the blocking read so the thread can see the quit message
@@ -1467,6 +1476,13 @@ pub const ReadThread = struct {
                 log.info("read thread got quit signal", .{});
                 return;
             }
+
+            // If our pty fd is closed, then we're also done with our
+            // read thread.
+            if (pollfds[0].revents & posix.POLL.HUP != 0) {
+                log.info("pty fd closed, read thread exiting", .{});
+                return;
+            }
         }
     }