test-zsh-subreaper

README.md

zsh with subreaper

• compile the test program with gcc -o test-zsh main.c
• run it as test-zsh no-subreaper|subreaper|subreaper-reap

There are 3 modes:

• no-subreaper: do not invoke the program with PR_SET_CHILD_SUBREAPER, so the child process of zsh will be reaped by something else ( either zsh? or PID 1)
• subreaper: set PR_SET_CHILD_SUBREAPER, but doesnt reap child processes when receiving SIGCHLD
• subreaper-reap: set PR_SET_CHILD_SUBREAPER and reap child processes when receiving SIGCHLD

In case of sourcing docker-zsh-test, no-subreaper and subreaper-reap will behave correctly i.e. properly reap defunct childs.

docker-zsh-test.sh

#!/bin/zsh
# docker-zsh-test.sh
if [[ "$(echo "the black cat was chased by the brown dog" | sed -e 's/cat/fox/g' | grep fox)" = "" ]]; then
  echo "cat"
else
  echo "fox"
fi

main.c

// formated with indent -linux main.c

#include <errno.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/prctl.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

#define pr_perror(fmt, ...) fprintf(stderr, "error: " fmt ": %m\n", ##__VA_ARGS__)

void child_handler(int sig)
{
	while (waitpid(-1, NULL, WNOHANG) > 0) {
	}
}

void main(int argc, char *argv[])
{
	if (argc < 2) {
		pr_perror("%s no-subreaper|subreaper|subreaper-reap", argv[0]);
		exit(1);
	}

	if (strcmp(argv[1], "subreaper") == 0) {
		if (prctl(PR_SET_CHILD_SUBREAPER, 1, 0, 0, 0) == -1) {
			pr_perror("Failed to set child subreaper");
			exit(1);
		}
	} else if (strcmp(argv[1], "subreaper-reap") == 0) {
		if (prctl(PR_SET_CHILD_SUBREAPER, 1, 0, 0, 0) == -1) {
			pr_perror("Failed to set child subreaper");
			exit(1);
		}
		// setup SIGCHLD handler to reap zombie processes
		struct sigaction sa;
		sigemptyset(&sa.sa_mask);
		sa.sa_handler = &child_handler;
		sa.sa_flags = SA_RESTART | SA_NOCLDSTOP;
		if (sigaction(SIGCHLD, &sa, NULL) == -1) {
			pr_perror("Failed to set SIGCHLD handler");
			exit(1);
		}
	} else if (strcmp(argv[1], "no-subreaper") == 0) {
		// do nothing here
	} else {
		pr_perror("%s no-subreaper|subreaper|subreaper-reap", argv[0]);
		exit(1);
	}

	// We must fork to actually enter the PID namespace.
	int child = fork();
	if (child == -1) {
		pr_perror("Unable to fork a process");
		exit(1);
	}
	if (child == 0) {
		char *args[2];
		args[0] = "zsh";
		args[1] = NULL;
		execvp("zsh", args);
		pr_perror("failed to execvp");
		exit(1);
	} else {
		// Parent, wait for the child.
		int status = 0;
		if (waitpid(child, &status, 0) == -1) {
			pr_perror("Failed to waitpid with error");
			exit(1);
		}
		// Forward the child's exit code or re-send its death signal.
		if (WIFEXITED(status)) {
			exit(WEXITSTATUS(status));
		} else if (WIFSIGNALED(status)) {
			kill(getpid(), WTERMSIG(status));
		}

		exit(1);
	}

	return;
}