Instead of checking for absolute paths and current working directories
in various file system operations, there is one simple solution: allow
overriding `std.fs.cwd` on WASI.
os.realpath is back to causing a compile error when used on WASI. This
caused a compile error in the Sema handling of `@src()`. The compiler
should never call realpath, so the commit that made this change is
reverted (95ab942184427e7c9b840d71f4d093931e3e48fb). If this breaks
debug info, a different strategy is needed to solve it other than using
realpath.
I also removed the preopens code and replaced it with something much
simpler. There is no longer any global state in the standard library.
Additionally-
* os.openat no longer does an unnecessary fstat on WASI when O.WRONLY
is not provided.
* os.chdir is back to causing a compile error on WASI.
In general, we prefer compiler code to use relative paths based on open
directory handles because this is the most portable. However, sometimes
absolute paths are used, and sometimes relative paths are used that go
up a directory.
The recent improvements in 81d2135ca6ebd71b8c121a19957c8fbf7f87125b
regressed the use case when an absolute path is used for the zig lib
directory mixed with a relative path used for the root source file. This
could happen when, for example, running the standard library tests, like
this:
stage3/bin/zig test ../lib/std/std.zig
This happened because the zig lib dir was inferred to be an absolute
directory based on the zig executable directory, while the root source
file was detected as a relative path. There was no common prefix and so
it was not determined that the std.zig file was inside the lib
directory.
This commit adds a function for resolving paths that preserves relative
path names while allowing absolute paths, and converting relative
upwards paths (e.g. "../foo") to absolute paths. This restores the
previous functionality while remaining compatible with systems such as
WASI that cannot deal with absolute paths.
This change adds support for locating the Zig executable and the library
and global cache directories, based on looking in the fixed "/zig" and
"/cache" directories.
Since our argv[0] on WASI is just the basename (any absolute/relative
path information is deleted by the runtime), there's very limited
introspection we can do on WASI, so we rely on these fixed directories.
These can be provided on the command-line using `--mapdir`, as follows:
```
wasmtime --mapdir=/cwd::. --mapdir=/cache::"$HOME/.cache/zig" --mapdir=/zig::./zig-out/ ./zig-out/bin/zig.wasm
```
