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Handling EPERM in terminal_give_to_job()

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This commit is contained in:
Mahmoud Al-Qudsi
2017-07-27 16:23:36 -05:00
committed by Kurtis Rader
parent 8b8a21dcad
commit bdcd451030
1 changed files with 15 additions and 3 deletions
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18
src/proc.cpp
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@@ -789,6 +789,9 @@ bool terminal_give_to_job(job_t *j, int cont) {
return true;
}
//Previously, terminal_give_to_job was being called for each process in a job, hence all the comments
//and warnings below. It is now only called for the first process in a job.t d
//it may not be safe to call tcsetpgrp if we've already done so, as at that point we are no longer
//the controlling process group for the terminal and no longer have permission to set the process
//group that is in control, causing tcsetpgrp to return EPERM, even though that's not the documented
@@ -798,18 +801,27 @@ bool terminal_give_to_job(job_t *j, int cont) {
//for SIGTTOU are installed. Read: http://curiousthing.org/sigttin-sigttou-deep-dive-linux
//In all cases, our goal here was just to hand over control of the terminal to this process group,
//which is a no-op if it's already been done.
if (tcgetpgrp(STDIN_FILENO) == j->pgid) {
auto previous_owner = tcgetpgrp(STDIN_FILENO);
if (previous_owner == j->pgid) {
debug(2, L"Process group %d already has control of terminal\n", j->pgid);
return true;
}
debug(4, L"Attempting bring process group to foreground via tcsetpgrp for job->pgid %d\n", j->pgid);
debug(4, L"caller session id: %d, pgid %d has session id: %d\n", getsid(0), j->pgid, getsid(j->pgid));
//the tcsetpgrp(2) man page says that EPERM is thrown if "pgrp has a supported value, but is not the
//process group ID of a process in the same session as the calling process."
//Since we _guarantee_ that this isn't the case (the child calls setpgid before it calls SIGSTOP, and
//the child was created in the same session as us), it seems that EPERM is being thrown because of an
//caching issue - the call to tcsetpgrp isn't seeing the newly-created process group just yet. On this
//developer's test machine (WSL running Linux 4.4.0), EPERM does indeed disappear on retry. The important
//thing is that we can guarantee the process isn't going to exit while we wait (which would cause us to
//possibly block indefinitely).
signal_block(true);
int result = -1;
errno = EINTR;
while (result == -1 && errno == EINTR) {
while (result == -1 && (errno == EINTR || errno == EPERM)) {
result = tcsetpgrp(STDIN_FILENO, j->pgid);
}
if (result == -1) {