coredump: Try to write journald coredump metadata to the journal

Currently, if journald coredumps, the coredump is written to
/var/lib/systemd/coredump but the coredump metadata is not written
to  the journal meaning we can't find out about the coredump's
existence via the journal. This means that coredumpctl can't be
used to work with journald coredumps, as well as any other tools
that rely on journald to know about coredumps.

To solve the issue, let's have systemd-coredump try to write
systemd-journald coredump metadata to the journal. We have to be
careful though, since if journald coredumps, there's no active
reader on the receive end of the journal socket, so we have to make
sure we don't deadlock trying to write to the socket. To avoid the
deadlock, we put the socket in nonblocking mode before trying to
write to it.

src/coredump/coredump.c

@@ -29,6 +29,7 @@
 #include "fs-util.h"
 #include "io-util.h"
 #include "journal-importer.h"
+#include "journal-send.h"
 #include "log.h"
 #include "macro.h"
 #include "main-func.h"
@@ -842,12 +843,10 @@ log:
 
         core_message = strjoina(core_message, stacktrace ? "\n\n" : NULL, stacktrace);
 
-        if (context->is_journald) {
-                /* We cannot log to the journal, so just print the message.
-                 * The target was set previously to something safe. */
+        if (context->is_journald)
+                /* We might not be able to log to the journal, so let's always print the message to another
+                 * log target. The target was set previously to something safe. */
                 log_dispatch(LOG_ERR, 0, core_message);
-                return 0;
-        }
 
         (void) iovw_put_string_field(iovw, "MESSAGE=", core_message);
 
@@ -903,8 +902,29 @@ log:
                                  coredump_size, arg_journal_size_max);
         }
 
+        /* If journald is coredumping, we have to be careful that we don't deadlock when trying to write the
+         * coredump to the journal, so we put the journal socket in nonblocking mode before trying to write
+         * the coredump to the socket. */
+
+        if (context->is_journald) {
+                r = journal_fd_nonblock(true);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to make journal socket non-blocking: %m");
+        }
+
         r = sd_journal_sendv(iovw->iovec, iovw->count);
-        if (r < 0)
+
+        if (context->is_journald) {
+                int k;
+
+                k = journal_fd_nonblock(false);
+                if (k < 0)
+                        return log_error_errno(k, "Failed to make journal socket blocking: %m");
+        }
+
+        if (r == -EAGAIN && context->is_journald)
+                log_warning_errno(r, "Failed to log journal coredump, ignoring: %m");
+        else if (r < 0)
                 return log_error_errno(r, "Failed to log coredump: %m");
 
         return 0;

src/libsystemd/sd-journal/journal-send.c

@@ -66,6 +66,16 @@ retry:
         return fd;
 }
 
+int journal_fd_nonblock(bool nonblock) {
+        int r;
+
+        r = journal_fd();
+        if (r < 0)
+                return r;
+
+        return fd_nonblock(r, nonblock);
+}
+
 #if VALGRIND
 void close_journal_fd(void) {
         /* Be nice to valgrind. This is not atomic. This must be used only in tests. */
@@ -318,7 +328,7 @@ _public_ int sd_journal_sendv(const struct iovec *iov, int n) {
         if (errno == ENOENT)
                 return 0;
 
-        if (!IN_SET(errno, EMSGSIZE, ENOBUFS))
+        if (!IN_SET(errno, EMSGSIZE, ENOBUFS, EAGAIN))
                 return -errno;
 
         /* Message doesn't fit... Let's dump the data in a memfd or

src/libsystemd/sd-journal/journal-send.h

@@ -1,6 +1,10 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include <stdbool.h>
+
+int journal_fd_nonblock(bool nonblock);
+
 #if VALGRIND
 void close_journal_fd(void);
 #else