osx: handle run loop firing report_callback() multiple times

One invocation of CFRunLoopRunInMode() may fire the report callback
multiple times. When that happens, the next call to fido_hid_read() may
block for the full timeout if the authenticator has already sent its
response.

The pipe is already configured to be non-blocking. This means we can
attempt to read from the pipe on entering fido_hid_read() and only
execute the run loop if there is no data already available.

src/hid_osx.c

@@ -537,20 +537,28 @@ fido_hid_read(void *handle, unsigned char *buf, size_t len, int ms)
 		return (-1);
 	}
 
-	IOHIDDeviceScheduleWithRunLoop(ctx->ref, CFRunLoopGetCurrent(),
-	    ctx->loop_id);
+	/* check for pending frame */
+	if ((r = read(ctx->report_pipe[0], buf, len)) == -1) {
+		if (errno != EAGAIN && errno != EWOULDBLOCK) {
+			fido_log_error(errno, "%s: read", __func__);
+			return (-1);
+		}
 
-	if (ms == -1)
-		ms = 5000; /* wait 5 seconds by default */
+		IOHIDDeviceScheduleWithRunLoop(ctx->ref, CFRunLoopGetCurrent(),
+		    ctx->loop_id);
 
-	CFRunLoopRunInMode(ctx->loop_id, (double)ms/1000.0, true);
+		if (ms == -1)
+			ms = 5000; /* wait 5 seconds by default */
 
-	IOHIDDeviceUnscheduleFromRunLoop(ctx->ref, CFRunLoopGetCurrent(),
-	    ctx->loop_id);
+		CFRunLoopRunInMode(ctx->loop_id, (double)ms/1000.0, true);
 
-	if ((r = read(ctx->report_pipe[0], buf, len)) == -1) {
-		fido_log_error(errno, "%s: read", __func__);
-		return (-1);
+		IOHIDDeviceUnscheduleFromRunLoop(ctx->ref,
+		    CFRunLoopGetCurrent(), ctx->loop_id);
+
+		if ((r = read(ctx->report_pipe[0], buf, len)) == -1) {
+			fido_log_error(errno, "%s: read", __func__);
+			return (-1);
+		}
 	}
 
 	if (r < 0 || (size_t)r != len) {