fs: stops pre-fetching the inode of dot-dot ("..") (#1544)

Signed-off-by: Adrian Cole <adrian@tetrate.io>

RATIONALE.md

@@ -745,6 +745,33 @@ yet these are returned for `wasi_snapshot_preview1.fd_readdir`. This was a
 change specifically made to pass wasi-testsuite, and has problems known since
 late 2019.
 
+### Why don't we pre-populate an inode for the dot-dot ("..") entry?
+
+We only populate an inode for dot (".") because wasi-testsuite requires it, and
+we likely already have it (because we cache it). We could attempt to populate
+one for dot-dot (".."), but chose not to.
+
+Firstly, wasi-testsuite does not require the inode of dot-dot, possibly because
+the wasip2 adapter doesn't populate it (but we don't really know why).
+
+The only other reason to populate it would be to avoid wasi-libc's stat fanout
+when it is missing. However, the inode for dot-dot is not cached, and is also
+likely not possible to get on a pseudo file. Even if we could get it, we would
+have to use stat. In other words, pre-populating this would have the same cost
+as waiting for something to call stat instead.
+
+Fetching dot-dot's inode despite the above not only doesn't help wasi-libc, but
+it also hurts languages that don't use it, such as Go. These languages would
+pay a stat syscall penalty even if they don't need the inode. In fact, Go
+discards both dot entries!
+
+In summary, there are no significant upsides in attempting to pre-fetch
+dot-dot's inode, and there are downsides to doing it anyway.
+
+See https://github.com/WebAssembly/wasi-libc/pull/345
+See https://github.com/WebAssembly/wasi-testsuite/blob/main/tests/rust/src/bin/fd_readdir.rs#L108
+See https://github.com/bytecodealliance/preview2-prototyping/blob/e4c04bcfbd11c42c27c28984948d501a3e168121/crates/wasi-preview1-component-adapter/src/lib.rs#L1037
+
 ## Why does "wasi_snapshot_preview1" require dot entries when POSIX does not?
 
 In October 2019, WASI project knew requiring dot entries ("." and "..") was not

imports/wasi_snapshot_preview1/fs_test.go

@@ -4919,8 +4919,8 @@ func requireOpenFile(t *testing.T, tmpDir string, pathName string, data []byte,
 	return mod, fd, log, r
 }
 
-// Test_fdReaddir_dotEntriesHaveRealInodes because wasi-testsuite requires it.
-func Test_fdReaddir_dotEntriesHaveRealInodes(t *testing.T) {
+// Test_fdReaddir_dotEntryHasARealInode because wasi-testsuite requires it.
+func Test_fdReaddir_dotEntryHasARealInode(t *testing.T) {
 	if runtime.GOOS == "windows" && !platform.IsGo120 {
 		t.Skip("windows before go 1.20 has trouble reading the inode information on directories.")
 	}
@@ -4954,11 +4954,10 @@ func Test_fdReaddir_dotEntriesHaveRealInodes(t *testing.T) {
 	dirents = append(dirents, 3, 0, 0, 0)             // d_type = directory
 	dirents = append(dirents, '.')                    // name
 
-	// get the real inode of the parent directory
-	st, errno = preopen.Stat(".")
 	require.EqualErrno(t, 0, errno)
 	dirents = append(dirents, 2, 0, 0, 0, 0, 0, 0, 0) // d_next = 2
-	dirents = append(dirents, u64.LeBytes(st.Ino)...) // d_ino
+	// See /RATIONALE.md for why we don't attempt to get an inode for ".."
+	dirents = append(dirents, 0, 0, 0, 0, 0, 0, 0, 0) // d_ino
 	dirents = append(dirents, 2, 0, 0, 0)             // d_namlen = 2 characters
 	dirents = append(dirents, 3, 0, 0, 0)             // d_type = directory
 	dirents = append(dirents, '.', '.')               // name
@@ -5021,7 +5020,8 @@ func Test_fdReaddir_opened_file_written(t *testing.T) {
 	st, errno = preopen.Stat(".")
 	require.EqualErrno(t, 0, errno)
 	dirents = append(dirents, 2, 0, 0, 0, 0, 0, 0, 0) // d_next = 2
-	dirents = append(dirents, u64.LeBytes(st.Ino)...) // d_ino
+	// See /RATIONALE.md for why we don't attempt to get an inode for ".."
+	dirents = append(dirents, 0, 0, 0, 0, 0, 0, 0, 0) // d_ino
 	dirents = append(dirents, 2, 0, 0, 0)             // d_namlen = 2 characters
 	dirents = append(dirents, 3, 0, 0, 0)             // d_type = directory
 	dirents = append(dirents, '.', '.')               // name

internal/sys/fs.go

@@ -4,7 +4,6 @@ import (
 	"io"
 	"io/fs"
 	"net"
-	"path"
 	"syscall"
 
 	"github.com/tetratelabs/wazero/internal/descriptor"
@@ -160,15 +159,8 @@ func synthesizeDotEntries(f *FileEntry) (result []fsapi.Dirent, errno syscall.Er
 		return nil, errno
 	}
 	result = append(result, fsapi.Dirent{Name: ".", Ino: dotIno, Type: fs.ModeDir})
-	dotDotIno := uint64(0)
-	if !f.IsPreopen && f.Name != "." {
-		if st, errno := f.FS.Stat(path.Dir(f.Name)); errno != 0 {
-			return nil, errno
-		} else {
-			dotDotIno = st.Ino
-		}
-	}
-	result = append(result, fsapi.Dirent{Name: "..", Ino: dotDotIno, Type: fs.ModeDir})
+	// See /RATIONALE.md for why we don't attempt to get an inode for ".."
+	result = append(result, fsapi.Dirent{Name: "..", Ino: 0, Type: fs.ModeDir})
 	return result, 0
 }