Hi Takashi,
This v7 LGTM. OK to push.
Thanks,
..mark
On 6/23/2026 3:51 PM, Takashi Yano wrote:
Without this patch, when a non-cygwin program (A) is about to exit, and
another non-cygwin program (B) is started, input transferring between
cyg-pipe and nat-pipe may not work as expected. When the non-cygwin
program (A) exits, input transferring from nat-pipe to cyg-pipe will be
performed. However, the the non-cygwin program (B) will performs input
transferring from cyg-pipe to nat-pipe at the same time. The mechanism
of the problem is as follows.
1) The the non-cygwin program (A) checks current input pipe state,
then it is nat-pipe since the this program is a non-cygwin program.
The program (A) also checks if any handover target exists, but
it is not found since the program (B) is not started yet. So,
the program (A) decided to transfer input form nat-pipe to cyg-
pipe.
2) Before the non-cygwin (A) program performs input transferring,
if the non-cygwin program (B) is started and checks the input
pipe state, it is nat-pipe state, so the non-cygwin program (B)
does not perform input transferring.
3) However, just after that, the non-cygwin program (A) performs
input transferring from nat-pipe to cyg-pipe, so typeahead input
will be stored in cyg-pipe.
4) The non-cygwin program (B) cannot read the typeahead input
because it is now in the cyg-pipe.
The following code demonstrates the issue.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
int n = 1;
if (argc > 1)
n = atoi(argv[1]);
if (fork()) {
execlp("cmd.exe", "cmd", NULL);
perror("execlp(\"cmd\"): ");
}
for (int i=0; i<n; i++) {
if (fork() == 0) {
execlp("./winsleep.exe", "winsleep", "0", NULL);
perror("execlp(\"winsleep\"): ");
}
}
return 0;
}
Transferring input itself is guarded by input_mutex, but the pre-
check is not. With this patch, the guard is enhanced so that the
state check and transferring input are done in atomic way.
Fixes: f20641789427 ("Cygwin: pty: Reduce unecessary input transfer.")
Signed-off-by: Takashi Yano <[email protected]>
Reviewed-by: Mark Geisert <[email protected]>
---
winsup/cygwin/fhandler/pty.cc | 99 +++++++++++++++----------
winsup/cygwin/local_includes/fhandler.h | 2 +
2 files changed, 60 insertions(+), 41 deletions(-)
[...]
