libcamera: process: Fail loudly on isolate

Add an error debug message when disassociating part of a process
execution context using unshare fails.

As this is currently used to isolate a child process which is
immediately terminated silently if unshare fails, add a debug printout
and propagate up the error code to make the failure more visible.

Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/process.cpp

@@ -306,7 +306,15 @@ void Process::closeAllFdsExcept(const std::vector<int> &fds)
 
 int Process::isolate()
 {
-	return unshare(CLONE_NEWUSER | CLONE_NEWNET);
+	int ret = unshare(CLONE_NEWUSER | CLONE_NEWNET);
+	if (ret) {
+		ret = -errno;
+		LOG(Process, Error) << "Failed to unshare execution context: "
+				    << strerror(-ret);
+		return ret;
+	}
+
+	return 0;
 }
 
 /**