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Blame SOURCES/0576-journald-never-block-when-sending-messages-on-NOTIFY.patch

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From c67c643418b1df5b5705b3a72eba1e6755830dc5 Mon Sep 17 00:00:00 2001
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From: Jan Synacek <jsynacek@redhat.com>
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Date: Tue, 21 Nov 2017 12:46:28 +0100
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Subject: [PATCH] journald: never block when sending messages on NOTIFY_SOCKET
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 socket
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Otherwise we might run into deadlocks, when journald blocks on the
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notify socket on PID 1, and PID 1 blocks on IPC to dbus-daemon and
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dbus-daemon blocks on logging to journald. Break this cycle by making
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sure that journald never ever blocks on PID 1.
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Note that this change disables support for event loop watchdog support,
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as these messages are sent in blocking style by sd-event. That should
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not be a big loss though, as people reported frequent problems with the
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watchdog hitting journald on excessively slow IO.
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Fixes: #1505.
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(cherry-picked from commit e22aa3d3284709234f086ebebc13a905a295b7a7)
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Resolves: #1511565
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---
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 src/journal/journald-server.c     | 130 +++++++++++++++++++++++++++++++++++++-
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 src/journal/journald-server.h     |  13 ++--
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 src/journal/journald-stream.c     |  68 ++++++++++++++++++--
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 src/journal/journald-stream.h     |   3 +
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 src/journal/journald.c            |   8 ---
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 units/systemd-journald.service.in |   1 -
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 6 files changed, 201 insertions(+), 22 deletions(-)
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diff --git a/src/journal/journald-server.c b/src/journal/journald-server.c
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index daeecd519..a810829b2 100644
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--- a/src/journal/journald-server.c
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+++ b/src/journal/journald-server.c
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@@ -67,6 +67,8 @@
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 #define RECHECK_AVAILABLE_SPACE_USEC (30*USEC_PER_SEC)
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+#define NOTIFY_SNDBUF_SIZE (8*1024*1024)
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+
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 /* Pick a good default that is likely to fit into AF_UNIX and AF_INET SOCK_DGRAM datagrams, and even leaves some room
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 + * for a bit of additional metadata. */
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 #define DEFAULT_LINE_MAX (48*1024)
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@@ -1556,6 +1558,126 @@ static int server_open_hostname(Server *s) {
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         return 0;
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 }
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+static int dispatch_notify_event(sd_event_source *es, int fd, uint32_t revents, void *userdata) {
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+        Server *s = userdata;
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+        int r;
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+
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+        assert(s);
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+        assert(s->notify_event_source == es);
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+        assert(s->notify_fd == fd);
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+
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+        if (revents != EPOLLOUT) {
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+                log_error("Invalid events on notify file descriptor.");
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+                return -EINVAL;
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+        }
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+
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+        /* The $NOTIFY_SOCKET is writable again, now send exactly one
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+         * message on it. Either it's the initial READY=1 event or an
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+         * stdout stream event. If there's nothing to write anymore,
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+         * turn our event source off. The next time there's something
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+         * to send it will be turned on again. */
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+
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+        if (!s->sent_notify_ready) {
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+                static const char p[] =
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+                        "READY=1\n"
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+                        "STATUS=Processing requests...";
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+                ssize_t l;
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+
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+                l = send(s->notify_fd, p, strlen(p), MSG_DONTWAIT);
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+                if (l < 0) {
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+                        if (errno == EAGAIN)
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+                                return 0;
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+
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+                        return log_error_errno(errno, "Failed to send READY=1 notification message: %m");
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+                }
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+
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+                s->sent_notify_ready = true;
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+                log_debug("Sent READY=1 notification.");
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+
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+        } else if (s->stdout_streams_notify_queue)
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+                /* Dispatch one stream notification event */
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+                stdout_stream_send_notify(s->stdout_streams_notify_queue);
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+
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+        /* Leave us enabled if there's still more to to do. */
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+        if (s->stdout_streams_notify_queue)
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+                return 0;
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+
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+        /* There was nothing to do anymore, let's turn ourselves off. */
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+        r = sd_event_source_set_enabled(es, SD_EVENT_OFF);
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+        if (r < 0)
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+                return log_error_errno(r, "Failed to turn off notify event source: %m");
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+
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+        return 0;
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+}
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+
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+static int server_connect_notify(Server *s) {
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+        union sockaddr_union sa = {
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+                .un.sun_family = AF_UNIX,
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+        };
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+        const char *e;
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+        int r;
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+
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+        assert(s);
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+        assert(s->notify_fd < 0);
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+        assert(!s->notify_event_source);
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+
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+        /*
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+          So here's the problem: we'd like to send notification
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+          messages to PID 1, but we cannot do that via sd_notify(),
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+          since that's synchronous, and we might end up blocking on
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+          it. Specifically: given that PID 1 might block on
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+          dbus-daemon during IPC, and dbus-daemon is logging to us,
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+          and might hence block on us, we might end up in a deadlock
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+          if we block on sending PID 1 notification messages -- by
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+          generating a full blocking circle. To avoid this, let's
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+          create a non-blocking socket, and connect it to the
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+          notification socket, and then wait for POLLOUT before we
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+          send anything. This should efficiently avoid any deadlocks,
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+          as we'll never block on PID 1, hence PID 1 can safely block
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+          on dbus-daemon which can safely block on us again.
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+
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+          Don't think that this issue is real? It is, see:
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+          https://github.com/systemd/systemd/issues/1505
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+        */
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+
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+        e = getenv("NOTIFY_SOCKET");
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+        if (!e)
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+                return 0;
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+
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+        if ((e[0] != '@' && e[0] != '/') || e[1] == 0) {
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+                log_error("NOTIFY_SOCKET set to an invalid value: %s", e);
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+                return -EINVAL;
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+        }
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+
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+        if (strlen(e) > sizeof(sa.un.sun_path)) {
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+                log_error("NOTIFY_SOCKET path too long: %s", e);
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+                return -EINVAL;
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+        }
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+
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+        s->notify_fd = socket(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
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+        if (s->notify_fd < 0)
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+                return log_error_errno(errno, "Failed to create notify socket: %m");
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+
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+        (void) fd_inc_sndbuf(s->notify_fd, NOTIFY_SNDBUF_SIZE);
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+
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+        strncpy(sa.un.sun_path, e, sizeof(sa.un.sun_path));
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+        if (sa.un.sun_path[0] == '@')
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+                sa.un.sun_path[0] = 0;
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+
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+        r = connect(s->notify_fd, &sa.sa, offsetof(struct sockaddr_un, sun_path) + strlen(e));
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+        if (r < 0)
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+                return log_error_errno(errno, "Failed to connect to notify socket: %m");
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+
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+        r = sd_event_add_io(s->event, &s->notify_event_source, s->notify_fd, EPOLLOUT, dispatch_notify_event, s);
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+        if (r < 0)
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+                return log_error_errno(r, "Failed to watch notification socket: %m");
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+
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+        /* This should fire pretty soon, which we'll use to send the
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+         * READY=1 event. */
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+
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+        return 0;
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+}
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+
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 int server_init(Server *s) {
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         _cleanup_fdset_free_ FDSet *fds = NULL;
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         int n, r, fd;
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@@ -1563,7 +1685,7 @@ int server_init(Server *s) {
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         assert(s);
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         zero(*s);
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-        s->syslog_fd = s->native_fd = s->stdout_fd = s->dev_kmsg_fd = s->audit_fd = s->hostname_fd = -1;
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+        s->syslog_fd = s->native_fd = s->stdout_fd = s->dev_kmsg_fd = s->audit_fd = s->hostname_fd = s->notify_fd = -1;
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         s->compress = true;
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         s->seal = true;
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@@ -1611,8 +1733,6 @@ int server_init(Server *s) {
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         if (r < 0)
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                 return log_error_errno(r, "Failed to create event loop: %m");
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-        sd_event_set_watchdog(s->event, true);
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-
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         n = sd_listen_fds(true);
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         if (n < 0)
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                 return log_error_errno(n, "Failed to read listening file descriptors from environment: %m");
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@@ -1718,6 +1838,8 @@ int server_init(Server *s) {
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         server_cache_boot_id(s);
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         server_cache_machine_id(s);
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+        (void) server_connect_notify(s);
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+
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         r = system_journal_open(s, false);
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         if (r < 0)
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                 return r;
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@@ -1770,6 +1892,7 @@ void server_done(Server *s) {
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         sd_event_source_unref(s->sigterm_event_source);
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         sd_event_source_unref(s->sigint_event_source);
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         sd_event_source_unref(s->hostname_event_source);
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+        sd_event_source_unref(s->notify_event_source);
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         sd_event_unref(s->event);
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         safe_close(s->syslog_fd);
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@@ -1778,6 +1901,7 @@ void server_done(Server *s) {
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         safe_close(s->dev_kmsg_fd);
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         safe_close(s->audit_fd);
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         safe_close(s->hostname_fd);
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+        safe_close(s->notify_fd);
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         if (s->rate_limit)
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                 journal_rate_limit_free(s->rate_limit);
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diff --git a/src/journal/journald-server.h b/src/journal/journald-server.h
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index b29410778..e59ff35e2 100644
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--- a/src/journal/journald-server.h
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+++ b/src/journal/journald-server.h
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@@ -27,12 +27,15 @@
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 #include <sys/types.h>
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 #include <sys/socket.h>
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+typedef struct Server Server;
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+
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 #include "sd-event.h"
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 #include "journal-file.h"
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 #include "hashmap.h"
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 #include "util.h"
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 #include "audit.h"
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 #include "journald-rate-limit.h"
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+#include "journald-stream.h"
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 #include "list.h"
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 typedef enum Storage {
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@@ -52,15 +55,14 @@ typedef enum SplitMode {
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         _SPLIT_INVALID = -1
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 } SplitMode;
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-typedef struct StdoutStream StdoutStream;
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-
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-typedef struct Server {
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+struct Server {
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         int syslog_fd;
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         int native_fd;
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         int stdout_fd;
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         int dev_kmsg_fd;
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         int audit_fd;
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         int hostname_fd;
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+        int notify_fd;
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         sd_event *event;
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@@ -75,6 +77,7 @@ typedef struct Server {
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         sd_event_source *sigterm_event_source;
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         sd_event_source *sigint_event_source;
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         sd_event_source *hostname_event_source;
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+        sd_event_source *notify_event_source;
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         JournalFile *runtime_journal;
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         JournalFile *system_journal;
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@@ -114,6 +117,7 @@ typedef struct Server {
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         usec_t oldest_file_usec;
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         LIST_HEAD(StdoutStream, stdout_streams);
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+        LIST_HEAD(StdoutStream, stdout_streams_notify_queue);
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         unsigned n_stdout_streams;
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         char *tty_path;
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@@ -135,6 +139,7 @@ typedef struct Server {
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         struct udev *udev;
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+        bool sent_notify_ready;
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         bool sync_scheduled;
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         char machine_id_field[sizeof("_MACHINE_ID=") + 32];
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@@ -145,7 +150,7 @@ typedef struct Server {
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         char *cgroup_root;
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         size_t line_max;
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-} Server;
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+};
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 #define N_IOVEC_META_FIELDS 20
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 #define N_IOVEC_KERNEL_FIELDS 64
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diff --git a/src/journal/journald-stream.c b/src/journal/journald-stream.c
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index 4d6b7ad18..9118d1a31 100644
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--- a/src/journal/journald-stream.c
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+++ b/src/journal/journald-stream.c
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@@ -80,6 +80,7 @@ struct StdoutStream {
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         bool forward_to_console:1;
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         bool fdstore:1;
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+        bool in_notify_queue:1;
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         char *buffer;
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         size_t length;
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@@ -90,6 +91,7 @@ struct StdoutStream {
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         char *state_file;
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         LIST_FIELDS(StdoutStream, stdout_stream);
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+        LIST_FIELDS(StdoutStream, stdout_stream_notify_queue);
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         char id_field[sizeof("_STREAM_ID=")-1 + SD_ID128_STRING_MAX];
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 };
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@@ -102,6 +104,9 @@ void stdout_stream_free(StdoutStream *s) {
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                 assert(s->server->n_stdout_streams > 0);
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                 s->server->n_stdout_streams --;
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                 LIST_REMOVE(stdout_stream, s->server->stdout_streams, s);
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+
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+                if (s->in_notify_queue)
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+                        LIST_REMOVE(stdout_stream_notify_queue, s->server->stdout_streams_notify_queue, s);
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         }
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         if (s->event_source) {
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@@ -126,7 +131,7 @@ static void stdout_stream_destroy(StdoutStream *s) {
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                 return;
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         if (s->state_file)
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-                unlink(s->state_file);
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+                (void) unlink(s->state_file);
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         stdout_stream_free(s);
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 }
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@@ -210,11 +215,15 @@ static int stdout_stream_save(StdoutStream *s) {
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         free(temp_path);
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         temp_path = NULL;
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-        /* Store the connection fd in PID 1, so that we get it passed
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-         * in again on next start */
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-        if (!s->fdstore) {
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-                sd_pid_notify_with_fds(0, false, "FDSTORE=1", &s->fd, 1);
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-                s->fdstore = true;
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+        if (!s->fdstore && !s->in_notify_queue) {
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+                LIST_PREPEND(stdout_stream_notify_queue, s->server->stdout_streams_notify_queue, s);
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+                s->in_notify_queue = true;
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+
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+                if (s->server->notify_event_source) {
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+                        r = sd_event_source_set_enabled(s->server->notify_event_source, SD_EVENT_ON);
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+                        if (r < 0)
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+                                log_warning_errno(r, "Failed to enable notify event source: %m");
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+                }
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         }
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 finish:
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@@ -801,3 +810,50 @@ int server_open_stdout_socket(Server *s, FDSet *fds) {
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         return 0;
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 }
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+
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+void stdout_stream_send_notify(StdoutStream *s) {
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+        struct iovec iovec = {
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+                .iov_base = (char*) "FDSTORE=1",
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+                .iov_len = strlen("FDSTORE=1"),
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+        };
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+        struct msghdr msghdr = {
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+                .msg_iov = &iovec,
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+                .msg_iovlen = 1,
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+        };
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+        struct cmsghdr *cmsg;
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+        ssize_t l;
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+
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+        assert(s);
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+        assert(!s->fdstore);
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+        assert(s->in_notify_queue);
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+        assert(s->server);
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+        assert(s->server->notify_fd >= 0);
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+
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+        /* Store the connection fd in PID 1, so that we get it passed
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+         * in again on next start */
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+
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+        msghdr.msg_controllen = CMSG_SPACE(sizeof(int));
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+        msghdr.msg_control = alloca0(msghdr.msg_controllen);
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+
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+        cmsg = CMSG_FIRSTHDR(&msghdr);
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+        cmsg->cmsg_level = SOL_SOCKET;
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+        cmsg->cmsg_type = SCM_RIGHTS;
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+        cmsg->cmsg_len = CMSG_LEN(sizeof(int));
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+
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+        memcpy(CMSG_DATA(cmsg), &s->fd, sizeof(int));
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+
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+        l = sendmsg(s->server->notify_fd, &msghdr, MSG_DONTWAIT|MSG_NOSIGNAL);
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+        if (l < 0) {
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+                if (errno == EAGAIN)
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+                        return;
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+
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+                log_error_errno(errno, "Failed to send stream file descriptor to service manager: %m");
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+        } else {
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+                log_debug("Successfully sent stream file descriptor to service manager.");
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+                s->fdstore = 1;
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+        }
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+
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+        LIST_REMOVE(stdout_stream_notify_queue, s->server->stdout_streams_notify_queue, s);
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+        s->in_notify_queue = false;
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+
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+}
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diff --git a/src/journal/journald-stream.h b/src/journal/journald-stream.h
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index 94bf955d7..fd8c94fb6 100644
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--- a/src/journal/journald-stream.h
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+++ b/src/journal/journald-stream.h
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@@ -21,9 +21,12 @@
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   along with systemd; If not, see <http://www.gnu.org/licenses/>.
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 ***/
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+typedef struct StdoutStream StdoutStream;
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+
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 #include "fdset.h"
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 #include "journald-server.h"
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 int server_open_stdout_socket(Server *s, FDSet *fds);
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 void stdout_stream_free(StdoutStream *s);
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+void stdout_stream_send_notify(StdoutStream *s);
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diff --git a/src/journal/journald.c b/src/journal/journald.c
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index 15bbcbe3d..b7ba2b6ec 100644
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--- a/src/journal/journald.c
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+++ b/src/journal/journald.c
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@@ -64,10 +64,6 @@ int main(int argc, char *argv[]) {
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         log_debug("systemd-journald running as pid "PID_FMT, getpid());
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         server_driver_message(&server, SD_MESSAGE_JOURNAL_START, "Journal started");
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-        sd_notify(false,
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-                  "READY=1\n"
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-                  "STATUS=Processing requests...");
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-
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         for (;;) {
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                 usec_t t = USEC_INFINITY, n;
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@@ -120,10 +116,6 @@ int main(int argc, char *argv[]) {
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         server_driver_message(&server, SD_MESSAGE_JOURNAL_STOP, "Journal stopped");
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 finish:
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-        sd_notify(false,
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-                  "STOPPING=1\n"
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-                  "STATUS=Shutting down...");
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-
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         server_done(&server);
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         return r < 0 ? EXIT_FAILURE : EXIT_SUCCESS;
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diff --git a/units/systemd-journald.service.in b/units/systemd-journald.service.in
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index 0d1ea61fe..c94c0bfba 100644
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--- a/units/systemd-journald.service.in
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+++ b/units/systemd-journald.service.in
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@@ -22,7 +22,6 @@ RestartSec=0
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 StandardOutput=null
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 FileDescriptorStoreMax=4224
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 CapabilityBoundingSet=CAP_SYS_ADMIN CAP_DAC_OVERRIDE CAP_SYS_PTRACE CAP_SYSLOG CAP_AUDIT_CONTROL CAP_AUDIT_READ CAP_CHOWN CAP_DAC_READ_SEARCH CAP_FOWNER CAP_SETUID CAP_SETGID CAP_MAC_OVERRIDE
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-WatchdogSec=3min
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 # Increase the default a bit in order to allow many simultaneous
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 # services being run since we keep one fd open per service. Also, when