WASI & darwin: support basic file io based on libc

Signed-off-by: Takeshi Yoneda <takeshi@tetrate.io>

src/os/errors.go

@@ -2,6 +2,7 @@ package os
 
 import (
 	"errors"
+	"syscall"
 )
 
 var (
@@ -17,9 +18,8 @@ var (
 	ErrExist          = errors.New("file exists")
 )
 
-func IsPermission(err error) bool {
-	return err == ErrPermission
-}
+// The following code is copied from the official implementation.
+// https://github.com/golang/go/blob/4ce6a8e89668b87dce67e2f55802903d6eb9110a/src/os/error.go#L65-L104
 
 func NewSyscallError(syscall string, err error) error {
 	if err == nil {
@@ -37,3 +37,39 @@ type SyscallError struct {
 func (e *SyscallError) Error() string { return e.Syscall + ": " + e.Err.Error() }
 
 func (e *SyscallError) Unwrap() error { return e.Err }
+
+func IsExist(err error) bool {
+	return underlyingErrorIs(err, ErrExist)
+}
+
+func IsNotExist(err error) bool {
+	return underlyingErrorIs(err, ErrNotExist)
+}
+
+func IsPermission(err error) bool {
+	return underlyingErrorIs(err, ErrPermission)
+}
+
+func underlyingErrorIs(err, target error) bool {
+	// Note that this function is not errors.Is:
+	// underlyingError only unwraps the specific error-wrapping types
+	// that it historically did, not all errors implementing Unwrap().
+	err = underlyingError(err)
+	if err == target {
+		return true
+	}
+	// To preserve prior behavior, only examine syscall errors.
+	e, ok := err.(syscall.Errno)
+	return ok && e.Is(target)
+}
+
+// underlyingError returns the underlying error for known os error types.
+func underlyingError(err error) error {
+	switch err := err.(type) {
+	case *PathError:
+		return err.Err
+	case *SyscallError:
+		return err.Err
+	}
+	return err
+}