![]() Instead, let’s start with the beginning of our test function: func LogMount(ctx context.Context, s *testing.State) ,įunc LogMount(ctx context.Context, s *testing. #Dmesh full picture too big full#We‘ll skip over boilerplate like import statements and the test metadata since those are already covered by Codelab #1, but you can see these portions of the test in the full listing at the bottom of this document. Now that we‘ve thought through how the test should work, let’s start writing some code. select then lets us to perform non-blocking operations on channels. ![]() (There are more helpful links about concurrency in Go in the Concurrency section of the Writing Tests doc.) We can start a goroutine that reads the output from the dmesg process as long as it’s running, and writes each line to a channel that's read by the main goroutine. #Dmesh full picture too big code#Go doesn’t provide much standard support for non-blocking I/O, but it does make it easy to run code concurrently in a separate goroutine and pass data between goroutines using channels. The first long line will give the size of the entire hard drive. We also need to think about how we‘re going to consume dmesg -follow‘s output in our test. Ghosting Systems 23 is to go back into /stand/sysinstall and choose Configure then Fdisk. For the third concern, we can use dmesg -clear to clear the buffer beforehand - that doesn't completely eliminate the risk of seeing a stale message, but it at least makes it less likely. If we start dmesg -follow before mounting the filesystem, that resolves the first two concerns above. Luckily, in Linux 3.5.0 and later, dmesg -follow can be used to tail new log messages. The ring buffer may already contain a mounted filesystem message from an earlier mount, resulting in our test not detecting failures.If other messages are being logged quickly while our test is running, the mounted filesystem message might be pushed out of the buffer before we check it, resulting in our test being flaky.The kernel might take longer than expected to log the message, resulting in our test being flaky.One option would be to mount the filesystem, sleep a bit, and then run dmesg and look for the expected message, but that approach is problematic for multiple reasons: The kernel ring buffer can be viewed using the dmesg command. When the filesystem is mounted, the kernel will asynchronously write a message like this to its ring buffer: EXT4-fs (loop4): mounted filesystem without journal. We can create a filesystem within an on-disk file and then use a loopback mount. using channels to communicate between goroutinesīefore writing any code, let's think about the basic approach that we should use to test this.deciding when to run external commands vs. ![]() ![]() creating new contexts to limit the running time of operations.In doing so, we'll learn about the following: This codelab follows the creation of a Tast test that verifies that the kernel logs a message to its ring buffer when a filesystem is mounted. This document assumes that you've already gone through Codelab #1. Tast Codelab #2: kernel.LogMount (go/tast-codelab-2) ![]()
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