Using a lock in C++ across multiple tasks - c++

Question

I am not really seeking code examples, but I'm hoping someone can review my program design and provide feedback. I am trying to figure out how do I ensure I have one instance of my "workflow" running at a time.
I am working in C++.
This is my workflow:
I read rows off of a Postgres database.
If the table has any records, I want to do these instructions:
Read the records and transform them to JSON
Send the JSON document to a remote Web service
Parse the response from the service. The service tells me which records were saved or not saved, based on their primary key.
I delete the successfully saved records
I log the unsuccessful records (there's another process that consumes the logs and so my work is done).
I want to perform all of this threads using a separate thread (or "task", whatever higher-level abstraction is available in C++), and I want to make sure that if my function for [1] gets called multiple times, the additional calls basically get "dropped" if step 1 is already in flight.
In C++, I believe I can use a flag and a mutex. I use a something like std::lock_guard<std::mutex> at the top of my method. Then the next line checks for a flag.
// MyWorkflow.cpp
std::mutex myMutex;
int inFlight = 0;
void process() {
std::lock_guard<std::mutex> guard(myMutex);
if (inflight) {
return;
}
inflight = 1;
std::vector<Widget> widgets = readFromMyTable();
std::string json = getJson(&widgets);
... // Send the json to the remote service and handle the response
}
Okay, let me explain my confusion. I want to use Curl to perform the HTTP request. But Curl works asynchronously. And so if I make the asynchronous HTTP call via Curl, my update function will just return and myMutex will be released, right?
I think in my asynchronous response handler, I need to call a second function that's in MyWorkflow.cpp
void markCompletion() {
std::lock_guard<std::mutex> guard(myMutex);
inFlight = 0; // Reset the inflight flag here
}
Is this the right approach? I am worried that if an exception is thrown anywhere before I call markCompletion(), I will block all future callers. I think I need to ensure I have proper exception handling and always call markCompletion().
I am terribly sorry for asking such a noob question, but I really want to learn to do this the right way.