Testing mtail programmes


By default any compile errors are logged to the standard log /tmp/mtail.INFO unless otherwise redirected. (You can emit to standard out with --logtostderr flag.) Program errors are also printed on the HTTP status handler, by default at porrt 3903.

If you want more debugging information, mtail provides a few flags to assist with testing your program in standalone mode.


Compilation errors

The compile_only flag will run the mtail compiler, print any error messages, and then exit.

You can use this to check your programs are syntactically valid during the development process.

mtail --compile_only --progs ./progs

This could be added as a pre-commit hook to your source code repository.

Testing programs

The one_shot flag will compile and run the mtail programs, then feed in any logs specified from the beginning of the file (instead of tailing them), then print to the log all metrics collected.

You can use this to check that your programs are giving the expected output against some gold standard log file samples.

mtail --one_shot --progs ./progs --logs testdata/foo.log

Continuous Testing

If you wish, send a PR containing your program, some sample input, and a golden output to be run as a test in to ensure that mtail never breaks your program (or that your program gets any updates if the language changes.)

To have a syntax-only compile test, merely send in a PR with the program in the examples directory.

The TestExamplePrograms behaves like the one_shot flag, and TestCompileExamplePrograms tests that program syntax is correct.

Test writing

Use the testutil module where possible.

Do not use time.Sleep; poll for events. The TestServer provides a PollWatched() method for this purpose. Even integration tests which write to disk can be fast and not require sleeps to synchronise.

Use the if testing.Short() signal in tests with disk access so that the make smoke command is fast.

Do not comment out tests, prefer to use the t.Skip() method indicating why it’s not working if a test needs to be disabled. This keeps them visible and compilable.


For more information about debugging mtail programs, see the tips under Troubleshooting