Make std.fs.cwd() return preopen for "." if exists

This commit adds WASI specific impl of `std.fs.cwd()` in which we
emulate the `cwd` behaviour by inquiring the runtime for a "."
preopen if available. This is OK for simple relative ops, but will
not work for any ops which require absolute paths.

2 files changed, 21 insertions, 8 deletions

diff --git a/lib/std/fs.zig b/lib/std/fs.zig
index a175ce4935..be729eb295 100644
--- a/lib/std/fs.zig
+++ b/lib/std/fs.zig
@@ -591,7 +591,7 @@ pub const Dir = struct {
         return self.openFileZ(&path_c, flags);
     }
 
-    pub fn openFileWasi(self: Dir, sub_path: []const u8, flags: File.OpenFlags) File.OpenError!File {
+    fn openFileWasi(self: Dir, sub_path: []const u8, flags: File.OpenFlags) File.OpenError!File {
         var fdflags: wasi.fdflag_t = 0x0;
         var rights: wasi.rights_t = 0x0;
         if (flags.read) {
@@ -703,11 +703,12 @@ pub const Dir = struct {
 
     pub const createFileC = @compileError("deprecated: renamed to createFileZ");
 
-    pub fn createFileWasi(self: Dir, sub_path: []const u8, flags: File.CreateFlags) File.OpenError!File {
-        var oflags: wasi.oflags_t = 0x0;
-        var rights: wasi.rights_t = wasi.FD_WRITE | wasi.FD_DATASYNC | wasi.FD_SEEK | wasi.FD_FDSTAT_SET_FLAGS | wasi.FD_SYNC | wasi.FD_ALLOCATE | wasi.FD_ADVISE | wasi.FD_FILESTAT_SET_TIMES | wasi.FD_FILESTAT_SET_SIZE;
+    fn createFileWasi(self: Dir, sub_path: []const u8, flags: File.CreateFlags) File.OpenError!File {
+        const wasi = os.wasi;
+        var oflags: wasi.oflags_t = wasi.O_CREAT;
+        var rights: wasi.rights_t = wasi.RIGHT_FD_WRITE | wasi.RIGHT_FD_DATASYNC | wasi.RIGHT_FD_SEEK | wasi.RIGHT_FD_FDSTAT_SET_FLAGS | wasi.RIGHT_FD_SYNC | wasi.RIGHT_FD_ALLOCATE | wasi.RIGHT_FD_ADVISE | wasi.RIGHT_FD_FILESTAT_SET_TIMES | wasi.RIGHT_FD_FILESTAT_SET_SIZE;
         if (flags.read) {
-            rights |= wasi.FD_READ | wasi.FD_TELL | wasi.FD_FILESTAT_GET;
+            rights |= wasi.RIGHT_FD_READ | wasi.RIGHT_FD_TELL | wasi.RIGHT_FD_FILESTAT_GET;
         }
         if (flags.truncate) {
             oflags |= wasi.O_TRUNC;
@@ -1438,6 +1439,18 @@ pub const Dir = struct {
 pub fn cwd() Dir {
     if (builtin.os.tag == .windows) {
         return Dir{ .fd = os.windows.peb().ProcessParameters.CurrentDirectory.Handle };
+    } else if (builtin.os.tag == .wasi) {
+        const wasi = os.wasi;
+        // On WASI we indeed don't have a concept of cwd; however, we can approximate it with
+        // trying to obtain a preopen for ".".
+        // TODO `cwd()` should be a fallible operation if we're going to support WASI this way.
+        var fd: wasi.fd_t = undefined;
+        var prefix: usize = undefined;
+        switch (wasi.resolve_preopen(".", &fd, &prefix)) {
+            0 => {},
+            else => {},
+        }
+        return Dir{ .fd = fd };
     } else {
         return Dir{ .fd = os.AT_FDCWD };
     }
diff --git a/lib/std/os.zig b/lib/std/os.zig
index eab623d93f..c3f5bc55ea 100644
--- a/lib/std/os.zig
+++ b/lib/std/os.zig
@@ -943,13 +943,13 @@ pub fn openat(dir_fd: fd_t, file_path: []const u8, flags: u32, mode: mode_t) Ope
     return openatZ(dir_fd, &file_path_c, flags, mode);
 }
 
-pub fn openatWasi(dir_fd: fd_t, file_path: []const u8, oflags: wasi.oflag_t, fdflags: wasi.fdflag_t, rights: wasi.rights_t) OpenError!fd_t {
-    switch (wasi.path_open(dirfd, 0x0, &file_path, file_path.len, oflags, rights, 0x0, fdflags, &fd)) {
+pub fn openatWasi(dir_fd: fd_t, file_path: []const u8, oflags: wasi.oflags_t, fdflags: wasi.fdflags_t, rights: wasi.rights_t) OpenError!fd_t {
+    var fd: fd_t = undefined;
+    switch (wasi.path_open(dir_fd, 0x0, file_path.ptr, file_path.len, oflags, rights, 0x0, fdflags, &fd)) {
         0 => {},
         // TODO map errors
         else => |err| return unexpectedErrno(err),
     }
-
     return fd;
 }