Problem:
1. `ctx_dirs_restore()` is the only chdir site that doesn't re-shorten
buffer names; `post_chdir()`, `update_cwd()` and `do_autochdir()` all
call `shorten_fnames(true)`. So after a temp window-context switch
that moved the CWD, every buffer's `b_fname` is still relative to the
other directory. ui2 renders messages in a float, and entering it is
such a switch, so with a `:bcd` in a `BufReadPost` handler 'statusline'
"%f" shows ".config/nvim/init.lua" while the CWD is already
`~/.config/nvim`, and :write resolves the name against it:
```
E212: Can't open file for writing: no such file or directory
```
2. `msg_multihl()` leaves `msg_ext_id` pointing at the caller's storage
when nothing was emitted (e.g. 'msg_silent'): the reset only ran on
a flush that produced chunks. The next message then ships a dead
stack frame as its msg_show id. After ":silent write" the id is
buf_write()'s `msg_id[MAXPATHL + 32]`:
```
id = "\0\0\0\0\0\0\0\0\29\0(<C6>k\24R\17p<C7><C7><C7>\1\0\0\0..."
```
Solution:
1. `shorten_fnames(true)` after restoring the CWD. Drop `cs_save_sfname`,
which was a partial workaround for the same bug.
2. Release the id in `msg_multihl()`, where it is set and the caller's
frame is still alive. Stop reading `.data.integer` out of a String
union member.
Problem:
The magic globals `it`, `describe`, etc., are more trouble than they are
worth.
- Hooking into `after_each` requires `getfenv()` hacks.
- They confuse luals/emmylua, because the top-level `.luarc.json` isn't
merged with `test/.luarc.json` (apparently a luals limitation?)
- They totally defeat discoverability because the user just has to
"know" about the various magic symbols.
So they harm DX, which means they serve no purpose at all.
Solution:
- Expose the test API from `testutil`, so tests can call `t.it()`,
`t.describe()`, etc., in the conventional way.
- Drop `getfenv()` hacks.
- Drop the `setfenv()` injection in `load_chunk`.
- Drop `test/_meta.lua`.
This is a better way to prevent parallel tests from interfering with
each other, as there are many ways files can be created and deleted in
tests, so enforcing different file names is hard.
Using $TMPDIR can also work in most cases, but 'backipskip' etc. have
special defaults for $TMPDIR.
Symlink runtime/, src/, test/ and README.md to Xtest_xdg dir to make
tests more convenient (and symlinking test/ is required for busted).
Also, use README.md instead of test/README.md in the Ex mode inccommand
test, as test/README.md no longer contains 'N' char.
- `alter_slashes` belongs in `testutil.lua`, not `testnvim.lua`.
- `alter_slashes` is an unusual name. Rename it to `fix_slashes`.
- invert its behavior, to emphasize that `/` slashes are the preferred,
pervasive convention, not `\` slashes.
Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.
Closes https://github.com/neovim/neovim/issues/27004.
Extend the capabilities of is_os to detect more platforms such as
freebsd and openbsd. Also remove `iswin()` helper function as it can be
replaced by `is_os("win")`.
