Just: add README.md

Author: redsun82

justfile

@@ -1,2 +1,4 @@
-import 'misc/just/lib.just'
+# see misc/just/README.md for an overview
+
+import 'lib.just'
 import 'misc/just/forward.just'

misc/just/README.md

@@ -0,0 +1,44 @@
+This directory contains an infrastructure for [`just`](https://github.com/casey/just)
+recipes that can be used throughout this and the internal repository. In particular we
+have common verbs (`build`, `test`, `format`, `lint`, `generate`) that individual parts
+of the project can implement, and some common functionality that can be used to that
+effect.
+
+# Forwarding
+
+The core of the functionality is given by forwarding. The idea is that:
+
+- if you are in the directory where a verb is implemented, you will get that as per
+  standard `just` behaviour (possibly using fallback).
+- if on the other hand you are beneath it, and you run something like
+  `just test ql/rust/ql/test/{a,b}`, then a forwarder script finds a common justfile
+  implementing the verb for all the positional arguments passed there, and then retries
+  calling `just test` from there. So if `test` is implemented beneath that (in that case,
+  it is in `rust/ql/test`), it uses that recipe.
+- even if there isn't a recipe that is common to all the positional arguments, the
+  forwarder will still group the arguments in batches using the same recipe. So
+  `just build ql/rust ql/java`, or
+  `just test ql/rust/ql/test/some/language/test ql/rust/ql/integration-test/some/integration/test`
+  will also work, with corresponding recipes run sequentially.
+
+Another point is how launching QL tests can be tweaked:
+
+- by default, the corresponding CLI is built from the internal repo (nothing is done if
+  working in `codeql` standalone), and no additional database or consistency checks are
+  made
+- `--codeql=built` can be passed to skip the build step (if no changes were made to the
+  CLI/extractors). This is consistent with the same pytest option
+- you can add the additional checks that CI does with `--all-checks` or the `+`
+  abbreviation. These additional checks are configured in justfiles per language, and
+  correspond to all the additional checks that CI adds (but that a dev might not want to
+  run by default).
+
+Some caveats:
+
+- passing arguments with spaces generally doesn't work, although setting arguments with
+  spaces in `justfile`s (for the base arguments) is supported using escaping as in `\\`.
+  This is a known limitation of just (see
+  <https://github.com/casey/just/issues/1988>)
+- when running different recipes for the same verb, non-positional arguments need to be
+  supported by all recipes involved. For example, this will work ok for `--learn` or
+  `--codeql` options in language and integration tests