2. bcachefs private error codes

In bcachefs, as a hard rule we do not throw or directly use standard error codes (-EINVAL, -EBUSY, etc.). Instead, we define private error codes as needed in fs/bcachefs/errcode.h.

This gives us much better error messages and makes debugging much easier. Any direct uses of standard error codes you see in the source code are simply old code that has yet to be converted - feel free to clean it up!

Private error codes may subtype another error code, this allows for grouping of related errors that should be handled similarly (e.g. transaction restart errors), as well as specifying which standard error code should be returned at the bcachefs module boundary.

At the module boundary, we use bch2_err_class() to convert to a standard error code; this also emits a trace event so that the original error code be recovered even if it wasn’t logged.

Do not reuse error codes! Generally speaking, a private error code should only be thrown in one place. That means that when we see it in a log message we can see, unambiguously, exactly which file and line number it was returned from.

Try to give error codes names that are as reasonably descriptive of the error as possible. Frequently, the error will be logged at a place far removed from where the error was generated; good names for error codes mean much more descriptive and useful error messages.