Race conditions software testing

Web applications can suffer from race condition since multiple clients are sending requests in parallel to the web server, which may spawn multiple threads or processes to serve the requests. Even for a single user session, race condition is possible. For example, a web chat application may periodically send an AJAX request to check for the latest updates.

At the same time, the user may create a new chat message. The usual solution to avoid race condition is to serialize access to the shared resource. If one process gains access first, the resource is "locked" so that other processes have to wait for the resource to become available. Even if the operating system allows other processes to execute, they will get blocked on the resource.

This allows the first process to access and update the resource safely. Once done, it will "release" the resource. One of the processes waiting on the resource will now get its chance to access the resource. Code protected this way using locks is called critical section. The idea of granting access to only one process is called mutual exclusion. Having large critical sections will affect performance. Such code may be refactored into smaller critical sections.

In real-time systems or kernel code, another solution is to turn off interrupts when entering and turn on when leaving a critical section. The first symptom is that results are unpredictable.

In fact, the random nature of race condition poses a problem for debugging since under debug mode the race condition may not appear. For this reason, code reviews are recommended to catch race condition. This really implies that best way to avoid race condition is by careful design and not by testing and debugging.

There are tools that perform dynamic analysis and flag possible race conditions that may occur. Clang Thread Safety Analysis is a static analysis tool to detect race condition. The common mechanism used to allow safe access to shared resource is called mutex , which is short for mutual exclusion.

Any process entering a critical section will first acquire the mutex. Other processes wanting access to the critical section will get blocked. The owning process will give up the mutex when leaving the critical section. What separate mutexes from semaphores is the concept of ownership. A mutex can be released only by its owner. This is not so with semaphores that can be used for signalling state change and synchronization from one thread to another. While it's possible to use semaphores to protect critical sections, they come with their own set of problems such as accidental release, recursive deadlock, task-death deadlock, and priority inversion.

Mutexes are therefore a better solution. Netzer and Miller make this distinction: General races : Programs designed to be deterministic behaving in nondeterministic manner. Data races : Failure with accessing nonatomic critical sections in nondeterministic programs. They also claim that debugging data races is easier since it's local to the critical section. General races require analysis of entire execution to understand exactly how the expected deterministic behaviour deviated.

A data race need not result in a race condition in the sense that program correctness is not compromised. A race condition that's not a data race implies a general race, for which the accompanying figure is an example. Therac was a software-controlled radiation therapy machine used in the s. Six patients were overdosed and some died.

As with many DSLs, we use marbles to improve the readability of our tests. Marbles do an excellent job at it — we can see what a test does by just glancing at it. If you want to learn more about testing, check out this video. Every time the user select a movie, we create a new independent observable. If the user clicks twice, we will have two observables, which are not synchronized in any way.

This is the source of the problem. By doing this we replaced a collection of independent observables with a single observable of observables, which we can apply synchronization operators to. The switchMap operator only subscribes to the latest invocation of backend. If another invocation happens, it will unsubscribe from the one before. With this change our test will pass. You can find the source code in this repo. In this article we have looked at an example of a bug caused by a race condition.

We used marbles, a powerful way to test async code, to expose this bug in a unit test. We then fixed the bug by refactoring our code to use a single observable of observables, which we applied switchMap to. Follow victorsavkin to read more about monorepos, Nx, Angular, and React. At Nrwl we build dev tools for monorepos and help companies…. Nrwlio co-founder, Xoogler, Xangular. NxDevTools and Nx Cloud architect.

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