jni: Allow other threads to call setLogLevel()

It is useful to allow the user to toggle PRG_TRACE logging on an active
connection for debugging purposes.  But currently this would involve
releasing the class lock held by the mainloop.  So we will use asyncLock
to protect the shared variable.

Timings conducted on a Nexus 7 (2012) show that the new
MonitorEnter/MonitorExit pair adds about 700ns to each progress_cb (which
is called on each packet).  This isn't great, but it's probably small
enough to make it worth doing things the right way (using a lock) instead
of just declaring loglevel as volatile.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>

java/src/org/infradead/libopenconnect/LibOpenConnect.java

@@ -92,6 +92,7 @@ public boolean isCanceled() {
 
 	public native void pause();
 	public native void requestStats();
+	public native void setLogLevel(int level);
 
 	/* control operations */
 
@@ -108,7 +109,6 @@ public boolean isCanceled() {
 
 	/* connection settings */
 
-	public synchronized native void setLogLevel(int level);
 	public synchronized native int passphraseFromFSID();
 	public synchronized native void setCertExpiryWarning(int seconds);
 	public synchronized native int setHTTPProxy(String proxy);

jni.c

@@ -488,10 +488,14 @@ static void progress_cb(void *privdata, int level, const char *fmt, ...)
 	va_list ap;
 	char *msg;
 	jstring jmsg;
-	int ret;
+	int ret, loglevel;
 	jmethodID mid;
 
-	if (level > ctx->loglevel)
+	(*ctx->jenv)->MonitorEnter(ctx->jenv, ctx->async_lock);
+	loglevel = ctx->loglevel;
+	(*ctx->jenv)->MonitorExit(ctx->jenv, ctx->async_lock);
+
+	if (level > loglevel)
 		return;
 
 	va_start(ap, fmt);
@@ -930,7 +934,10 @@ JNIEXPORT void JNICALL Java_org_infradead_libopenconnect_LibOpenConnect_setLogLe
 
 	if (!ctx)
 		return;
+
+	(*ctx->jenv)->MonitorEnter(ctx->jenv, ctx->async_lock);
 	ctx->loglevel = arg;
+	(*ctx->jenv)->MonitorExit(ctx->jenv, ctx->async_lock);
 }
 
 JNIEXPORT jint JNICALL Java_org_infradead_libopenconnect_LibOpenConnect_setupTunFD(