The `OnSystem.os_condition_met?` method only handles Linux and macOS
values for the current OS, so this fails when testing with a generic
OS. This shortcoming is only being surfaced now because there weren't
any tests for `OnSystem` before.
This addresses the issue by accounting for macOS values (`:macos` or
a symbol from `MacOSVersion::SYMBOLS`) and returning `false` for any
other values (`:linux`, `:generic`, etc.).
When determining macOS requirements for a cask, we may need to
reference requirements from related on_system blocks (e.g.,
`on_monterey :or_older`, `on_ventura`, `on_sonoma :or_newer`) when
`depends_on macos` isn't adequate.
Sometimes casks specify different `depends_on macos` values in macOS
on_system blocks but that value is only set when the cask is loaded
in an environment that satisfies the on_system block's requirements.
There are other casks that contain macOS on_system blocks and use
`depends_on macos` outside of the on_system blocks but it may only
use the macOS version of the lowest on_system block (e.g.,
`>= :monterey`), which isn't sufficient if the cask's values vary
based on macOS version.
To be able to simulate macOS versions that meet the requirements of
all the on_system blocks in a cask, we need to collect the macOS
requirements in a way that doesn't require OS simulation. This is also
something that's easy to do in on_system methods, so this adds a
`macos_requirements` array to `UsesOnSystem`, containing
`MacOSRequirement` objects created from the cask's macOS on_system
block conditions.
This adds a `UsesOnSystem` class to `OnSystem`, containing boolean
instance variables to indicate which types of on_system methods are
used in a formula or cask. This is intended as a replacement for
`@on_system_blocks_exist`, which doesn't allow us to determine what
kinds of on_system calls were used. This provides more granularity
but we can still use `@uses_on_system.present?` to determine whether
any on_system calls were used (and this doubles as a `nil` check in
`Formula`, as the `self.class` instance variable has to use a nilable
type).
The `UsesOnSystem` instance variables cover the current
`ARCH_OPTIONS` and `BASE_OS_OPTIONS`. At the moment, we mostly need
to tell whether there are macOS/Linux or Intel/ARM on_system calls,
so I've omitted instance variables for specific macOS version until
we have a need for them.
As a practical example, if you wanted to determine whether a cask
uses Linux on_system calls, you can call
`cask.uses_on_system.linux?`. The `linux` boolean will be `true` if
the cask has an `on_linux` block, an `on_system` block (which requires
Linux), or uses `os linux: ...`. This is something that would be
challenging to determine from outside of `OnSystem` but it's
relatively easy to collect the information in `OnSystem` methods and
make it available like this.
`OnSystem` is exercised by other tests that include its modules but
this adds some baseline tests to ensure some of these methods work as
expected when tested in isolation. Between these added tests and
existing tests, we should have 100% coverage when run on Homebrew/brew
CI.
