libcamera: process: Return error if already running

Returning 0 when a running process is already managed can be confusing
since the parameters might be completely different, causing the caller
to mistakenly assume that the program it specified has been started.

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/libcamera/process.cpp

@@ -241,7 +241,7 @@ int Process::start(const std::string &path,
 	int ret;
 
 	if (running_)
-		return 0;
+		return -EBUSY;
 
 	int childPid = fork();
 	if (childPid == -1) {