Allow raw_assert() to dump stack traces.

It doesn't do this as beautifully as tor_assert(), but it doesn't
depend on any higher-level code.

src/common/backtrace.c

@@ -167,6 +167,21 @@ crash_handler(int sig, siginfo_t *si, void *ctx_)
   abort();
 }
 
+/** Write a backtrace to all of the emergency-error fds. */
+void
+dump_stack_symbols_to_error_fds(void)
+{
+  int n_fds, i;
+  const int *fds = NULL;
+  size_t depth;
+
+  depth = backtrace(cb_buf, MAX_DEPTH);
+
+  n_fds = tor_log_get_sigsafe_err_fds(&fds);
+  for (i=0; i < n_fds; ++i)
+    backtrace_symbols_fd(cb_buf, (int)depth, fds[i]);
+}
+
 /** Install signal handlers as needed so that when we crash, we produce a
  * useful stack trace. Return 0 on success, -errno on failure. */
 static int
@@ -234,6 +249,11 @@ static void
 remove_bt_handler(void)
 {
 }
+
+void
+dump_stack_symbols_to_error_fds(void)
+{
+}
 #endif /* defined(NO_BACKTRACE_IMPL) */
 
 /** Set up code to handle generating error messages on crashes. */

src/common/backtrace.h

@@ -14,6 +14,7 @@ void log_backtrace_impl(int severity, int domain, const char *msg,
                         tor_log_fn logger);
 int configure_backtrace_handler(const char *tor_version);
 void clean_up_backtrace_handler(void);
+void dump_stack_symbols_to_error_fds(void);
 
 #define log_backtrace(sev, dom, msg) \
   log_backtrace_impl((sev), (dom), (msg), tor_log)

src/common/torerr.c

@@ -29,6 +29,7 @@
 #endif
 
 #include "common/torerr.h"
+#include "common/backtrace.h"
 
 /** Array of fds to log crash-style warnings to. */
 static int sigsafe_log_fds[TOR_SIGSAFE_LOG_MAX_FDS] = { STDERR_FILENO };
@@ -140,6 +141,8 @@ tor_raw_assertion_failed_msg_(const char *file, int line, const char *expr,
     tor_log_err_sigsafe_write(msg);
     tor_log_err_sigsafe_write("\n");
   }
+
+  dump_stack_symbols_to_error_fds();
 }
 
 /* As format_{hex,dex}_number_sigsafe, but takes a <b>radix</b> argument