slogassert

import "github.com/thejerf/slogassert"

About Slogassert

slogassert implements a testing handler for slog.

All important observable results of code should be tested. While commonly ignored by people writing tests, log messages are often important things to test for, for several reasons:

• Because they are not tested for, it is suprisingly easy for them to break without anyone realizing.
• Log messages can have security impact. Any log messages that may be used to reconstruct a security incident should be tested to ensure they contain the data they are supposed to contain.
• Even when you don't otherwise deeply care about log messages, log messages can often still double as a way of asserting that certain code was actually reached, and the value of variables at the time it was reached is as expected.

While I wouldn't pull in slogassert just for that third point, if the first two are in play for your code base, slog's strong focus on structured attributes means that log messages can also double as testing probe points within your code that may be otherwise unreachable. I don't use this a lot but when you need it it's very useful. (You may find it helpful to create a testing log level even above Debug for this.)

This implements a handler for slog that more-or-less records the incoming messages, then provides a mechanism for testing for the presence and number of log messages at a variety of different detail levels. As log messages are tested for and matched by assertions, they are removed from the record. If an assertion is made and no log messages match, a testing error will be thrown.

Once all the assertions are complete, an assertion should be made that all log messages are accounted for. If there are unaccounted log messages, the test will fail at the end.

Because this is a test logger, some things normally too expensive to be done in normal logging are done, like taking a full stack trace at the location of all log messages that are recorded. This assists in diagnosing where any unasserted log messages are coming from.

See usage in the godoc.

This also includes a null logger, which I am surprised is not in the library itself as I write this since a nil slog.Logger is invalid.

Release Status

slogassert is beta. I consider the package as it nows stands to be a 1.0 release candidate, but it is not yet officially 1.0.

The code is covered as much as possible, however it is not possible to directly test covering the code that calls fatal errors, so that code can not be covered or directly tested.

Version Numbering

This repository will use semantic versioning.

I will be signing this repository with the "jerf" keybase account. If you are viewing this repository through GitHub, you should see the commits as showing as "verified" in the commit view.

(Bear in mind that due to the nature of how git commit signing works, there may be runs of unverified commits; what matters is that the top one is signed.)

Version History

• v0.3.4:
  • Create a custom interface for just the components of testing.TB
  • slogassert actually uses.
• v0.3.3:
  • Export LogMessageMatch.Matches for external use.
  • Add utility function for testing.
  • Take the testing.TB interface rather than a constant *testing.T.
• v0.3.2:
  • A LogValuer being used for an attribute match would fail to match because slogassert wouldn't resolve the value, but try to match the value against the slog.Value. If the LogValuer did something like change types or something, it would never match. Now values that implement LogValuer can be used directly in attribute matches. See the ValueAsString in assertions_test.go if you don't know what I mean.
• v0.3.1:
  • Annotate the internal .Assert* functions as t.Helper()s to improve error messages when an assert fails.
• v0.3.0 more BREAKING CHANGES:
  • Significant API rewrite. This:

    • Exposes Assert directly, for functional-matching based assertions, which enables a lot of more complicated scenarios.

    • Now that the general power is available to end users, the package doesn't need to offer every marginal match method, so I'm removing the built-in message + level assertion. It's easy now to implement yourself if it's useful, and I haven't used it yet in my own code so I question its general utility, sandwiched between the very useful "please assert this message" (useful because it is agnostic about levels) and "please assert this exact match" methods.

    • If you wrap another slog Handler with this, we need to properly pass WithGroup and WithAttrs down to that wrapped handler too.

      I like slog overall but I will say writing a correct Handler wrapper is distinctly nontrivial.

• v0.2.0 BREAKING CHANGE:

  • If test code panics, and a *testing.T.Cleanup function itself has some sort of .Fatal call, the result is that the panic is eaten. Due to Golang issue #49929, there is no way to detect this in the cleanup function because the cleanup function is run in the wrong place to detect the panic and the t.Failed() method will return false.

    Practice has revealed that this is way too confusing, so I'm removing the automatic cleanup function. Therefore, any slogassert.New calls need to have a defer handler.AssertEmpty() manually added to retain the original behavior. The AssertEmpty function has code added to see whether it is in the middle of a panic, and if so, it will not fatally error. (This is in practice good anyhow because panics frequently result in the log messages being logged but the assertions not running, so it frequently produced spurious and confusing messages anyhow.)

    This still mangles the panic a bit, unfortunately, but the necessary data is still there.

  • I was doing a lot of work on my work laptop and had my work email rather than my personal email, but my signing key is my personal email. This commit signs the top of the repository correctly.

    As I mention in the version numbering section, that's what matters; a signed commit at the top of a repository is essentially signing the whole thing, not just that commit. So it is not necessary to rewrite the whole repo to fix all the previous commits.

• v0.1.3:
  • Add a return value to the *Some* methods that return how many messages they consumed as being asserted.
• v0.1.2:

  • Allow use of ints to compare against Int64, Float64, and Uint64.

    This resolves an issue where you write an AssertPrecise and use a bare int in the source code, which the Go compiler decides is an int, and then that didn't match any of the numeric types. This adds the relevant clauses to the matchers.

• v0.1.1:
  • No code changes, just screwed up tagging.
• v0.1.0:

  • Fixed a major error: Somehow I completely overlooked adding the params on a sublogger added with .With to the resulting log messages. I guess I thought slog would do that for me. And this is why v0.0.9 was only a "release candidate".

    That said, I am advancing this up the semver chain to a release candidate.

• v0.0.9:
  • Fix a locking issue in Unasserted, which should make this all completely thread-safe.
  • I consider this a v1.0.0. release candidate.
• v0.0.8:
  • Make Unasserted return a fully independent copy of the LogMessage so the user can't accidentally corrupt it.
• v0.0.7:
  • Add Unasserted call. I've resisted this because it's kind of a trap, but sometimes you just need it.
• v0.0.6:

  • BREAKING RELEASE: The ability to wrap a handler is added. This is useful for things like recording all the logs in a test into a wrapped handler, then if the test fails, printing out the logs as part of the test failure message. This changes the signature on New and NewWithoutCleanup.

    To recover previous behavior, add a nil on the end of all such calls.

• v0.0.5:
  • Add a NullHandler and NullLogger. This is not 100% on point for the package, but pretty useful for when you need a logger but don't need the logs, which comes up in testing a lot.
• v0.0.4:
  • Handlers are responsible for resolving LogValuer values.
    • This is why you don't promise no bugs.
• v0.0.3:
  • Bugs! Bugs everywhere! Fewer now, but still no promises.
• v0.0.2
  • README fixup.
• v0.0.1
  • Initial release.