torerr: Try harder to flush raw assert messages before process termination

Some platforms (macOS, maybe others?) can swallow the last write before an
abort. This issue is probably caused by a race condition between write
buffer cache flushing, and process termination. So we write an extra
newline, to make sure that the message always gets through.

Fixes bug 31571; bugfix on 0.3.5.1-alpha.

changes/bug31571

@@ -0,0 +1,7 @@
+  o Minor bugfixes (error handling):
+    - Report the tor version whenever an assertion fails. Previously, we only
+      reported the Tor version on some crashes, and some non-fatal assertions.
+      Fixes bug 31571; bugfix on 0.3.5.1-alpha.
+    - On abort, try harder to flush the output buffers of log messages. On
+      some platforms (macOS), log messages can be discarded when the process
+      terminates. Fixes bug 31571; bugfix on 0.3.5.1-alpha.

src/lib/err/torerr.c

@@ -146,13 +146,19 @@ tor_raw_assertion_failed_msg_(const char *file, int line, const char *expr,
   format_dec_number_sigsafe(line, linebuf, sizeof(linebuf));
   tor_log_err_sigsafe("INTERNAL ERROR: Raw assertion failed in ",
                       get_tor_backtrace_version(), " at ",
-                      file, ":", linebuf, ": ", expr, NULL);
+                      file, ":", linebuf, ": ", expr, "\n", NULL);
   if (msg) {
     tor_log_err_sigsafe_write(msg);
     tor_log_err_sigsafe_write("\n");
   }
 
   dump_stack_symbols_to_error_fds();
+
+  /* Some platforms (macOS, maybe others?) can swallow the last write before an
+   * abort. This issue is probably caused by a race condition between write
+   * buffer cache flushing, and process termination. So we write an extra
+   * newline, to make sure that the message always gets through. */
+  tor_log_err_sigsafe_write("\n");
 }
 
 /* As format_{hex,dex}_number_sigsafe, but takes a <b>radix</b> argument