Do not send Libevent log messages to a controller.

Doing so could make Libevent call Libevent from inside a Libevent
logging call, which is a recipe for reentrant confusion and
hard-to-debug crashes.  This would especially hurt if Libevent
debug-level logging is enabled AND the user has a controller
watching for low-severity log messages.

Fix bug 2190; fix on 0.1.0.2-rc.

changes/bug2190

@@ -0,0 +1,6 @@
+  o Minor bugfixes
+    - Prevent calls from Libevent from inside Libevent log handlers.
+      This had potential to cause a nasty set of crashes, especially if
+      running Libevent with debug logging enabled, and running Tor
+      with a controller watching for low-severity log messages.
+      Bugfix on 0.1.0.2-rc.  Fixes bug 2190.

src/common/compat_libevent.c

@@ -65,19 +65,19 @@ libevent_logging_callback(int severity, const char *msg)
   }
   switch (severity) {
     case _EVENT_LOG_DEBUG:
-      log(LOG_DEBUG, LD_NET, "Message from libevent: %s", buf);
+      log(LOG_DEBUG, LD_NOCB|LD_NET, "Message from libevent: %s", buf);
       break;
     case _EVENT_LOG_MSG:
-      log(LOG_INFO, LD_NET, "Message from libevent: %s", buf);
+      log(LOG_INFO, LD_NOCB|LD_NET, "Message from libevent: %s", buf);
       break;
     case _EVENT_LOG_WARN:
-      log(LOG_WARN, LD_GENERAL, "Warning from libevent: %s", buf);
+      log(LOG_WARN, LD_NOCB|LD_GENERAL, "Warning from libevent: %s", buf);
       break;
     case _EVENT_LOG_ERR:
-      log(LOG_ERR, LD_GENERAL, "Error from libevent: %s", buf);
+      log(LOG_ERR, LD_NOCB|LD_GENERAL, "Error from libevent: %s", buf);
       break;
     default:
-      log(LOG_WARN, LD_GENERAL, "Message [%d] from libevent: %s",
+      log(LOG_WARN, LD_NOCB|LD_GENERAL, "Message [%d] from libevent: %s",
           severity, buf);
       break;
   }

src/common/log.c

@@ -278,6 +278,10 @@ logv(int severity, log_domain_mask_t domain, const char *funcname,
       lf = lf->next;
       continue;
     }
+    if (lf->callback && (domain & LD_NOCB)) {
+      lf = lf->next;
+      continue;
+    }
     if (lf->seems_dead) {
       lf = lf->next;
       continue;

src/common/torlog.h

@@ -95,6 +95,10 @@
 /** Number of logging domains in the code. */
 #define N_LOGGING_DOMAINS 20
 
+/** This log message is not safe to send to a callback-based logger.
+ * Used as a flag, not a log domain. */
+#define LD_NOCB (1u<<31)
+
 typedef uint32_t log_domain_mask_t;
 
 /** Configures which severities are logged for each logging domain for a given