shared: add PTY helper

This Pty API wraps the ugliness that is POSIX PTY. It takes care of:
  - edge-triggered HUP handling (avoid heavy CPU-usage on vhangup)
  - HUP vs. input-queue draining (handle HUP _after_ draining the whole
    input queue)
  - SIGCHLD vs. HUP (HUP is no reliable way to catch PTY deaths, always
    use SIGCHLD. Otherwise, vhangup() and friends will break.)
  - Output queue buffering (async EPOLLOUT handling)
  - synchronous setup (via Barrier API)

At the same time, the PTY API does not execve(). It simply fork()s and
leaves everything else to the caller. Usually, they execve() but we
support other setups, too.

This will be needed by multiple UI binaries (systemd-console, systemd-er,
...) so it's placed in src/shared/. It's not strictly related to
libsystemd-terminal, so it's not included there.

src/test/test-pty.c

@@ -0,0 +1,143 @@
+/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
+
+/***
+  This file is part of systemd.
+
+  Copyright 2014 David Herrmann <dh.herrmann@gmail.com>
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+***/
+
+#include <errno.h>
+#include <fcntl.h>
+#include <locale.h>
+#include <string.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include "def.h"
+#include "pty.h"
+#include "util.h"
+
+static const char sndmsg[] = "message\n";
+static const char rcvmsg[] = "message\r\n";
+static char rcvbuf[128];
+static size_t rcvsiz = 0;
+static sd_event *event;
+
+static void run_child(Pty *pty) {
+        int r, l;
+        char buf[512];
+
+        r = read(0, buf, sizeof(buf));
+        assert_se(r == strlen(sndmsg));
+        assert_se(!strncmp(buf, sndmsg, r));
+
+        l = write(1, buf, r);
+        assert_se(l == r);
+}
+
+static int pty_fn(Pty *pty, void *userdata, unsigned int ev, const void *ptr, size_t size) {
+        switch (ev) {
+        case PTY_DATA:
+                assert_se(rcvsiz < strlen(rcvmsg) * 2);
+                assert_se(rcvsiz + size < sizeof(rcvbuf));
+
+                memcpy(&rcvbuf[rcvsiz], ptr, size);
+                rcvsiz += size;
+
+                if (rcvsiz >= strlen(rcvmsg) * 2) {
+                        assert_se(rcvsiz == strlen(rcvmsg) * 2);
+                        assert_se(!memcmp(rcvbuf, rcvmsg, strlen(rcvmsg)));
+                        assert_se(!memcmp(&rcvbuf[strlen(rcvmsg)], rcvmsg, strlen(rcvmsg)));
+                }
+
+                break;
+        case PTY_HUP:
+                /* This is guaranteed to appear _after_ the input queues are
+                 * drained! */
+                assert_se(rcvsiz == strlen(rcvmsg) * 2);
+                break;
+        case PTY_CHILD:
+                /* this may appear at any time */
+                break;
+        default:
+                assert_se(0);
+                break;
+        }
+
+        /* if we got HUP _and_ CHILD, exit */
+        if (pty_get_fd(pty) < 0 && pty_get_child(pty) < 0)
+                sd_event_exit(event, 0);
+
+        return 0;
+}
+
+static void run_parent(Pty *pty) {
+        int r;
+
+        /* write message to pty, ECHO mode guarantees that we get it back
+         * twice: once via ECHO, once from the run_child() fn */
+        assert_se(pty_write(pty, sndmsg, strlen(sndmsg)) >= 0);
+
+        r = sd_event_loop(event);
+        assert_se(r >= 0);
+}
+
+static void test_pty(void) {
+        pid_t pid;
+        Pty *pty;
+
+        rcvsiz = 0;
+        memset(rcvbuf, 0, sizeof(rcvbuf));
+
+        assert_se(sd_event_default(&event) >= 0);
+
+        pid = pty_fork(&pty, event, pty_fn, NULL, 80, 25);
+        assert_se(pid >= 0);
+
+        if (pid == 0) {
+                /* child */
+                run_child(pty);
+                exit(0);
+        }
+
+        /* parent */
+        run_parent(pty);
+
+        /* Make sure the PTY recycled the child; yeah, this is racy if the
+         * PID was already reused; but that seems fine for a test. */
+        assert_se(waitpid(pid, NULL, WNOHANG) < 0 && errno == ECHILD);
+
+        pty_unref(pty);
+        sd_event_unref(event);
+}
+
+int main(int argc, char *argv[]) {
+        unsigned int i;
+
+        log_parse_environment();
+        log_open();
+
+        assert_se(sigprocmask_many(SIG_BLOCK, SIGCHLD, -1) >= 0);
+
+        /* Oh, there're ugly races in the TTY layer regarding HUP vs IN. Turns
+         * out they appear only 10% of the time. I fixed all of them and
+         * don't see them, anymore. But lets be safe and run this 1000 times
+         * so we catch any new ones, in case they appear again. */
+        for (i = 0; i < 1000; ++i)
+                test_pty();
+
+        return 0;
+}