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Using SIGSTOP/SIGCONT instead of mmap & sem_t to synchronize jobs

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This code should be more portable, and certainly cleaner. We are
currently always sending SIGCONT to the last process (if it was part of
a job chain) regardless of whether it called SIGSTOP on itself or not,
which should be fine.

Need to explore whether or not the other forks in src/exec.cpp need to
be SIGSTOP'd on run or only the one that we included in this patch.
This commit is contained in:
Mahmoud Al-Qudsi 2017-07-25 20:25:22 -05:00 committed by Kurtis Rader
parent 47d8a7e882
commit cafd856831
1 changed files with 15 additions and 23 deletions
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38
src/exec.cpp
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@ -23,8 +23,6 @@
#include <string>
#include <type_traits>
#include <vector>
#include <sys/mman.h>
#include <semaphore.h>
#include "builtin.h"
#include "common.h"
@ -495,7 +493,7 @@ void exec_job(parser_t &parser, job_t *j) {
//
// We are careful to set these to -1 when closed, so if we exit the loop abruptly, we can still
// close them.
sem_t *chained_wait_prev = nullptr;
int last_pid = -1;
int pipe_current_read = -1, pipe_current_write = -1, pipe_next_read = -1;
for (std::unique_ptr<process_t> &unique_p : j->processes) {
if (exec_error) {
@ -517,10 +515,6 @@ void exec_job(parser_t &parser, job_t *j) {
//these semaphores will be used to make sure the first process lives long enough for the
//next process in the chain to open its handles, process group, etc.
//this child will block on this one, the next child will have to call sem_post against it.
sem_t *chained_wait_next = !pipes_to_next_command ? nullptr : (sem_t*)mmap(NULL, sizeof(sem_t*), PROT_READ|PROT_WRITE,MAP_SHARED|MAP_ANONYMOUS,-1, 0);
if (chained_wait_next != nullptr) {
sem_init(chained_wait_next, 1, 0);
}
// The pipes the current process write to and read from. Unfortunately these can't be just
// allocated on the stack, since j->io wants shared_ptr.
@ -1071,17 +1065,17 @@ void exec_job(parser_t &parser, job_t *j) {
{
pid = execute_fork(false);
if (pid == 0) {
// usleep is a hack that fixes any tcsetpgrp errors caused by race conditions
// usleep(20 * 1000);
// it should no longer be needed with the chained_wait_next code below.
if (chained_wait_next != nullptr) {
debug(3, L"Waiting for next command in chain to start.\n");
sem_wait(chained_wait_next);
sem_destroy(chained_wait_next);
munmap(chained_wait_next, sizeof(sem_t));
}
// This is the child process.
p->pid = getpid();
// start child processes that are part of a job in a stopped state
// to ensure that they are still running when the next command in the
// chain is started.
if (pipes_to_next_command) {
debug(3, L"Blocking process %d waiting for next command in chain.\n", p->pid);
kill(p->pid, SIGSTOP);
}
// the process will be resumed by the shell when the next command in the
// chain is started
setup_child_process(j, p, process_net_io_chain);
safe_launch_process(p, actual_cmd, argv, envv);
// safe_launch_process _never_ returns...
@ -1123,14 +1117,12 @@ void exec_job(parser_t &parser, job_t *j) {
}
//now that next command in the chain has been started, unblock the previous command
if (chained_wait_prev != nullptr) {
debug(3, L"Unblocking previous command in chain.\n");
sem_post(chained_wait_prev);
munmap(chained_wait_prev, sizeof(sem_t));
chained_wait_prev = nullptr;
if (last_pid != -1) {
debug(3, L"Unblocking process %d.\n", last_pid);
kill(last_pid, SIGCONT);
}
if (chained_wait_next != nullptr) {
chained_wait_prev = chained_wait_next;
if (pid != 0) {
last_pid = pid;
}
}