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diff --git a/testing/docs/intermittent/index.rst b/testing/docs/intermittent/index.rst new file mode 100644 index 0000000000..93acfd980e --- /dev/null +++ b/testing/docs/intermittent/index.rst @@ -0,0 +1,375 @@ +Avoiding intermittent tests +=========================== + +Intermittent oranges are test failures which happen intermittently, +in a seemingly random way. Many of such failures could be avoided by +good test writing principles. This page tries to explain some of +those principles for use by people who contribute tests, and also +those who review them for inclusion into mozilla-central. + +They are also called flaky tests in other projects. + +A list of patterns which have been known to cause intermittent failures +comes next, with a description of why each one causes test failures, and +how to avoid it. + +After writing a successful test case, make sure to run it locally, +preferably in a debug build. Maybe tests depend on the state of another +test or some future test or browser operation to clean up what is left +over. This is a common problem in browser-chrome, here are things to +try: + +- debug mode, run test standalone `./mach <test> <path>/<to>/<test>/test.html|js` +- debug mode, run test standalone directory `./mach <test> <path>/<to>/<test>` +- debug mode, run test standalone larger directory `./mach <test> <path>/<to>` + + +Accessing DOM elements too soon +------------------------------- + +``data:`` URLs load asynchronously. You should wait for the load event +of an `<iframe> <https://developer.mozilla.org/docs/Web/HTML/Element/iframe>`__ that is +loading a ``data:`` URL before trying to access the DOM of the +subdocument. + +For example, the following code pattern is bad: + +.. code:: html + + <html> + <body> + <iframe id="x" src="data:text/html,<div id='y'>"></iframe> + <script> + var elem = document.getElementById("x"). + contentDocument. + getElementById("y"); // might fail + // ... + </script> + </body> + </html> + +Instead, write the code like this: + +.. code:: html + + <html> + <body> + <script> + function onLoad() { + var elem = this.contentDocument. + getElementById("y"); + // ... + }; + </script> + <iframe src="data:text/html,<div id='y'>" + onload="onLoad()"></iframe> + </body> + </html> + + +Using script functions before they're defined +--------------------------------------------- + +This may be relevant to event handlers, more than anything else. Let's +say that you have an `<iframe>` and you want to +do something after it's been loaded, so you might write code like this: + +.. code:: html + + <iframe src="..." onload="onLoad()"></iframe> + <script> + function onLoad() { // oops, too late! + // ... + } + </script> + +This is bad, because the +`<iframe>`'s load may be +completed before the script gets parsed, and therefore before the +``onLoad`` function comes into existence. This will cause you to miss +the `<iframe>` load, which +may cause your test to time out, for example. The best way to fix this +is to move the function definition before where it's used in the DOM, +like this: + +.. code:: html + + <script> + function onLoad() { + // ... + } + </script> + <iframe src="..." onload="onLoad()"></iframe> + + +Relying on the order of asynchronous operations +----------------------------------------------- + +In general, when you have two asynchronous operations, you cannot assume +any order between them. For example, let's say you have two +`<iframe>`'s like this: + +.. code:: html + + <script> + var f1Doc; + function f1Loaded() { + f1Doc = document.getElementById("f1").contentDocument; + } + function f2Loaded() { + var elem = f1Doc.getElementById("foo"); // oops, f1Doc might not be set yet! + } + </script> + <iframe id="f1" src="..." onload="f1Loaded()"></iframe> + <iframe id="f2" src="..." onload="f2Loaded()"></iframe> + +This code is implicitly assuming that ``f1`` will be loaded before +``f2``, but this assumption is incorrect. A simple fix is to just +detect when all of the asynchronous operations have been finished, and +then do what you need to do, like this: + +.. code:: html + + <script> + var f1Doc, loadCounter = 0; + function process() { + var elem = f1Doc.getElementById("foo"); + } + function f1Loaded() { + f1Doc = document.getElementById("f1").contentDocument; + if (++loadCounter == 2) process(); + } + function f2Loaded() { + if (++loadCounter == 2) process(); + } + </script> + <iframe id="f1" src="..." onload="f1Loaded()"></iframe> + <iframe id="f2" src="..." onload="f2Loaded()"></iframe> + + +Using magical timeouts to cause delays +-------------------------------------- + +Sometimes when there is an asynchronous operation going on, it may be +tempting to use a timeout to wait a while, hoping that the operation has +been finished by then and that it's then safe to continue. Such code +uses patterns like this: + +.. code:: js + + setTimeout(handler, 500); + +This should raise an alarm in your head. As soon as you see such code, +you should ask yourself: "Why 500, and not 100? Why not 1000? Why not +328, for that matter?" You can never answer this question, so you +should always avoid code like this! + +What's wrong with this code is that you're assuming that 500ms is enough +for whatever operation you're waiting for. This may stop being true +depending on the platform, whether it's a debug or optimized build of +Firefox running this code, machine load, whether the test is run on a +VM, etc. And it will start failing, sooner or later. + +Instead of code like this, you should wait for the operation to be +completed explicitly. Most of the time this can be done by listening +for an event. Some of the time there is no good event to listen for, in +which case you can add one to the code responsible for the completion of +the task at hand. + +Ideally magical timeouts are never necessary, but there are a couple +cases, in particular when writing web-platform-tests, where you might +need them. In such cases consider documenting why a timer was used so it +can be removed if in the future it turns out to be no longer needed. + + +Using objects without accounting for the possibility of their death +------------------------------------------------------------------- + +This is a very common pattern in our test suite, which was recently +discovered to be responsible for many intermittent failures: + +.. code:: js + + function runLater(func) { + var timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer); + timer.initWithCallback(func, 0, Ci.nsITimer.TYPE_ONE_SHOT); + } + +The problem with this code is that it assumes that the ``timer`` object +will live long enough for the timer to fire. That may not be the case +if a garbage collection is performed before the timer needs to fire. If +that happens, the ``timer`` object will get garbage collected and will +go away before the timer has had a chance to fire. A simple way to fix +this is to make the ``timer`` object global, so that an outstanding +reference to the object would still exist by the time that the garbage +collection code attempts to collect it. + +.. code:: js + + var timer; + function runLater(func) { + timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer); + timer.initWithCallback(func, 0, Ci.nsITimer.TYPE_ONE_SHOT); + } + +A similar problem may happen with ``nsIWebProgressListener`` objects +passed to the ``nsIWebProgress.addProgressListener()`` method, because +the web progress object stores a weak reference to the +``nsIWebProgressListener`` object, which does not prevent it from being +garbage collected. + + +Tests which require focus +------------------------- + +Some tests require the application window to be focused in order to +function properly. + +For example if you're writing a crashtest or reftest which tests an +element which is focused, you need to specify it in the manifest file, +like this: + +:: + + needs-focus load my-crashtest.html + needs-focus == my-reftest.html my-reftest-ref.html + +Also, if you're writing a mochitest which synthesizes keyboard events +using ``synthesizeKey()``, the window needs to be focused, otherwise the +test would fail intermittently on Linux. You can ensure that by using +``SimpleTest.waitForFocus()`` and start what your test does from inside +the callback for that function, as below: + +.. code:: js + + SimpleTest.waitForFocus(function() { + synthesizeKey("x", {}); + // ... + }); + +Tests which require mouse interaction, open context menus, etc. may also +require focus. Note that waitForFocus implicitly waits for a load event +as well, so it's safe to call it for a window which has not finished +loading yet. + + +Tests which take too long +------------------------- + +Sometimes what happens in a single unit test is just too much. This +will cause the test to time out in random places during its execution if +the running machine is under a heavy load, which is a sign that the test +needs to have more time to execute. This could potentially happen only +in debug builds, as they are slower in general. There are two ways to +solve this problem. One of them is to split the test into multiple +smaller tests (which might have other advantages as well, including +better readability in the test), or to ask the test runner framework to +give the test more time to finish correctly. The latter can be done +using the ``requestLongerTimeout`` function. + + +Tests that do not clean up properly +----------------------------------- + +Sometimes, tests register event handlers for various events, but they +don't clean up after themselves correctly. Alternatively, sometimes +tests do things which have persistent effects in the browser running the +test suite. Examples include opening a new window, adding a bookmark, +changing the value of a preference, etc. + +In these situations, sometimes the problem is caught as soon as the test +is checked into the tree. But it's also possible for the thing which +was not cleaned up properly to have an intermittent effect on future +(and perhaps seemingly unrelated) tests. These types of intermittent +failures may be extremely hard to debug, and not obvious at first +because most people only look at the test in which the failure happens +instead of previous tests. How the failure would look varies on a case +by case basis, but one example is `bug +612625 <https://bugzilla.mozilla.org/show_bug.cgi?id=612625>`__. + + +Not waiting on the specific event that you need +----------------------------------------------- + +Sometimes, instead of waiting for event A, tests wait on event B, +implicitly hoping that B occurring means that A has occurred too. `Bug +626168 <https://bugzilla.mozilla.org/show_bug.cgi?id=626168>`__ was an +example of this. The test really needed to wait for a paint in the +middle of its execution, but instead it would wait for an event loop +hit, hoping that by the time that we hit the event loop, a paint has +also occurred. While these types of assumptions may hold true when +developing the test, they're not guaranteed to be true every time that +the test is run. When writing a test, if you have to wait for an event, +you need to take note of why you're waiting for the event, and what +exactly you're waiting on, and then make sure that you're really waiting +on the correct event. + + +Tests that rely on external sites +--------------------------------- + +Even if the external site is not actually down, variable performance of +the external site, and external networks can add enough variation to +test duration that it can easily cause a test to fail intermittently. + +External sites should NOT be used for testing. + + +Tests that rely on Math.random() to create unique values +-------------------------------------------------------- + +Sometimes you need unique values in your test. Using ``Math.random()`` +to get unique values works most of the time, but this function actually +doesn't guarantee that its return values are unique, so your test might +get repeated values from this function, which means that it may fail +intermittently. You can use the following pattern instead of calling +``Math.random()`` if you need values that have to be unique for your +test: + +.. code:: js + + var gUniqueCounter = 0; + function generateUniqueValues() { + return Date.now() + "-" + (++gUniqueCounter); + } + +Tests that depend on the current time +------------------------------------- + +When writing a test which depends on the current time, extra attention +should be paid to different types of behavior depending on when a test +runs. For example, how does your test handle the case where the +daylight saving (DST) settings change while it's running? If you're +testing for a time concept relative to now (like today, yesterday, +tomorrow, etc) does your test handle the case where these concepts +change their meaning at the middle of the test (for example, what if +your test starts at 23:59:36 on a given day and finishes at 00:01:13)? + + +Tests that depend on time differences or comparison +--------------------------------------------------- + +When doing time differences the operating system timers resolution +should be taken into account. For example consecutive calls to +`Date() <https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Date>`__ don't +guarantee to get different values. Also when crossing XPCOM different +time implementations can give surprising results. For example when +comparing a timestamp got through :ref:`PR_Now` with one +got though a JavaScript date, the last call could result in the past of +the first call! These differences are more pronounced on Windows, where +the skew can be up to 16ms. Globally, the timers' resolutions are +guesses that are not guaranteed (also due to bogus resolutions on +virtual machines), so it's better to use larger brackets when the +comparison is really needed. + + +Tests that destroy the original tab +----------------------------------- + +Tests that remove the original tab from the browser chrome test window +can cause intermittent oranges or can, and of themselves, be +intermittent oranges. Obviously, both of these outcomes are undesirable. +You should neither write tests that do this, or r+ tests that do this. +As a general rule, if you call ``addTab`` or other tab-opening methods +in your test cleanup code, you're probably doing something you shouldn't +be. |