coredumpctl: fflush() stdout before invoking gdb

Fixes: #18936

src/basic/process-util.c

@@ -1230,6 +1230,11 @@ int safe_fork_full(
 
         original_pid = getpid_cached();
 
+        if (flags & FORK_FLUSH_STDIO) {
+                fflush(stdout);
+                fflush(stderr); /* This one shouldn't be necessary, stderr should be unbuffered anyway, but let's better be safe than sorry */
+        }
+
         if (flags & (FORK_RESET_SIGNALS|FORK_DEATHSIG)) {
                 /* We temporarily block all signals, so that the new child has them blocked initially. This way, we can
                  * be sure that SIGTERMs are not lost we might send to the child. */

src/basic/process-util.h

@@ -162,6 +162,7 @@ typedef enum ForkFlags {
         FORK_MOUNTNS_SLAVE      = 1 <<  9, /* Make child's mount namespace MS_SLAVE */
         FORK_RLIMIT_NOFILE_SAFE = 1 << 10, /* Set RLIMIT_NOFILE soft limit to 1K for select() compat */
         FORK_STDOUT_TO_STDERR   = 1 << 11, /* Make stdout a copy of stderr */
+        FORK_FLUSH_STDIO        = 1 << 12, /* fflush() stdout (and stderr) before forking */
 } ForkFlags;
 
 int safe_fork_full(const char *name, const int except_fds[], size_t n_except_fds, ForkFlags flags, pid_t *ret_pid);