Add/fix missing WASI functionality to pass libstd tests

This rather large commit adds/fixes missing WASI functionality
in `libstd` needed to pass the `libstd` tests. As such, now by
default tests targeting `wasm32-wasi` target are enabled in
`test/tests.zig` module. However, they can be disabled by passing
the `-Dskip-wasi=true` flag when invoking the `zig build test`
command. When the flag is set to `false`, i.e., when WASI tests are
included, `wasmtime` with `--dir=.` is used as the default testing
command.

Since the majority of `libstd` tests were relying on `fs.cwd()`
call to get current working directory handle wrapped in `Dir`
struct, in order to make the tests WASI-friendly, `fs.cwd()`
call was replaced with `testing.getTestDir()` function which
resolved to either `fs.cwd()` for non-WASI targets, or tries to
fetch the preopen list from the WASI runtime and extract a
preopen for '.' path.

The summary of changes introduced by this commit:
* implement `Dir.makeDir` and `Dir.openDir` targeting WASI
* implement `Dir.deleteFile` and `Dir.deleteDir` targeting WASI
* fix `os.close` and map errors in `unlinkat`
* move WASI-specific `mkdirat` and `unlinkat` from `std.fs.wasi`
  to `std.os` module
* implement `lseek_{SET, CUR, END}` targeting WASI
* implement `futimens` targeting WASI
* implement `ftruncate` targeting WASI
* implement `readv`, `writev`, `pread{v}`, `pwrite{v}` targeting WASI
* make sure ANSI escape codes are _not_ used in stderr or stdout
  in WASI, as WASI always sanitizes stderr, and sanitizes stdout if
  fd is a TTY
* fix specifying WASI rights when opening/creating files/dirs
* tweak `AtomicFile` to be WASI-compatible
* implement `os.renameatWasi` for WASI-compliant `os.renameat` function
* implement sleep() targeting WASI
* fix `process.getEnvMap` targeting WASI

1 files changed, 15 insertions, 0 deletions

diff --git a/lib/std/testing.zig b/lib/std/testing.zig
index 0f6cefb787..1592923ba8 100644
--- a/lib/std/testing.zig
+++ b/lib/std/testing.zig
@@ -14,6 +14,21 @@ pub var failing_allocator_instance = FailingAllocator.init(&base_allocator_insta
 pub var base_allocator_instance = std.heap.ThreadSafeFixedBufferAllocator.init(allocator_mem[0..]);
 var allocator_mem: [2 * 1024 * 1024]u8 = undefined;
 
+/// This function is intended to be used only in tests. It should be used in any testcase
+/// where we intend to test WASI and should be used a replacement for `std.fs.cwd()` in WASI.
+pub fn getTestDir() std.fs.Dir {
+    if (@import("builtin").os.tag == .wasi) {
+        var preopens = std.fs.wasi.PreopenList.init(allocator);
+        defer preopens.deinit();
+        preopens.populate() catch unreachable;
+
+        const preopen = preopens.find(".") orelse unreachable;
+        return std.fs.Dir{ .fd = preopen.fd };
+    } else {
+        return std.fs.cwd();
+    }
+}
+
 /// This function is intended to be used only in tests. It prints diagnostics to stderr
 /// and then aborts when actual_error_union is not expected_error.
 pub fn expectError(expected_error: anyerror, actual_error_union: var) void {