fatal-signal: Log backtrace when no monitor daemon.

Currently the backtrace logging is only available when monitor
daemon is running.  This patch enables backtrace logging when
no monitor daemon exists.  At signal handling context, it detects
whether monitor daemon exists.  If not, write directly the backtrace
to the vlog fd.  Note that using VLOG_* macro doesn't work due to
it's buffer I/O, so this patch directly issue write() syscall to
the file descriptor.

For some system we stop using monitor daemon and use systemd to
monitor ovs-vswitchd, thus need this patch. Example of
ovs-vswitchd.log (note that there is no timestamp printed):
  2020-03-23T14:42:12.949Z|00049|memory|INFO|175332 kB peak resident
  2020-03-23T14:42:12.949Z|00050|memory|INFO|handlers:2 ports:3 reva
  SIGSEGV detected, backtrace:
  0x0000000000486969 <fatal_signal_handler+0x49>
  0x00007f7f5e57f4b0 <killpg+0x40>
  0x000000000047daa8 <pmd_thread_main+0x238>
  0x0000000000504edd <ovsthread_wrapper+0x7d>
  0x00007f7f5f0476ba <start_thread+0xca>
  0x00007f7f5e65141d <clone+0x6d>
  0x0000000000000000 <+0x0>

Acked-by: Ben Pfaff <blp@ovn.org>
Signed-off-by: William Tu <u9012063@gmail.com>

1 files changed, 26 insertions, 1 deletions

diff --git a/lib/fatal-signal.c b/lib/fatal-signal.c
index ba7f5bfd3..4965c1ae8 100644
--- a/lib/fatal-signal.c
+++ b/lib/fatal-signal.c
@@ -187,7 +187,32 @@ send_backtrace_to_monitor(void) {
         dep++;
     }
 
-    ignore(write(daemonize_fd, unw_bt, dep * sizeof(struct unw_backtrace)));
+    if (monitor) {
+        ignore(write(daemonize_fd, unw_bt,
+                     dep * sizeof(struct unw_backtrace)));
+    } else {
+        /* Since there is no monitor daemon running, write backtrace
+         * in current process.  This is not asyn-signal-safe due to
+         * use of snprintf().
+         */
+        char str[] = "SIGSEGV detected, backtrace:\n";
+
+        if (vlog_get_fd() < 0) {
+            return;
+        }
+
+        ignore(write(vlog_get_fd(), str, strlen(str)));
+
+        for (int i = 0; i < dep; i++) {
+            char line[64];
+
+            snprintf(line, 64, "0x%016"PRIxPTR" <%s+0x%"PRIxPTR">\n",
+                     unw_bt[i].ip,
+                     unw_bt[i].func,
+                     unw_bt[i].offset);
+            ignore(write(vlog_get_fd(), line, strlen(line)));
+        }
+    }
 }
 #else
 static inline void