refactor: update unix build tags and add plan9

resolve obscure compile errors when trying to compile on
different goos.
Use unix build tags and add plan9.

We can't implement uid/gid logic without breaks as plan9 uses
strings while the api requires int.
Use plan9.Dup to redirect stderr.

file/fileinfo_plan9.go

@@ -0,0 +1,48 @@
+// Licensed to Elasticsearch B.V. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. Elasticsearch B.V. licenses this file to you under
+// the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//go:build plan9
+
+package file
+
+import (
+	"errors"
+	"os"
+
+	"golang.org/x/sys/plan9"
+)
+
+func stat(name string, statFunc func(name string) (os.FileInfo, error)) (FileInfo, error) {
+	info, err := statFunc(name)
+	if err != nil {
+		return nil, err
+	}
+
+	return wrap(info)
+}
+
+func wrap(info os.FileInfo) (FileInfo, error) {
+	stat, ok := info.Sys().(*plan9.Dir)
+	if !ok {
+		return nil, errors.New("failed to get uid/gid")
+	}
+
+	// on plan9 uid/gid is a string so we can't cast to int
+	_ = stat.Uid
+	_ = stat.Gid
+	return fileInfo{FileInfo: info, uid: nil, gid: nil}, nil
+}

file/fileinfo_unix.go

@@ -15,14 +15,15 @@
 // specific language governing permissions and limitations
 // under the License.
 
-//go:build !windows
+//go:build unix
 
 package file
 
 import (
 	"errors"
 	"os"
-	"syscall"
+
+	"golang.org/x/sys/unix"
 )
 
 func stat(name string, statFunc func(name string) (os.FileInfo, error)) (FileInfo, error) {
@@ -35,7 +36,7 @@ func stat(name string, statFunc func(name string) (os.FileInfo, error)) (FileInf
 }
 
 func wrap(info os.FileInfo) (FileInfo, error) {
-	stat, ok := info.Sys().(*syscall.Stat_t)
+	stat, ok := info.Sys().(*unix.Stat_t)
 	if !ok {
 		return nil, errors.New("failed to get uid/gid")
 	}

file/stderr_plan9.go

@@ -0,0 +1,33 @@
+// Licensed to Elasticsearch B.V. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. Elasticsearch B.V. licenses this file to you under
+// the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//go:build plan9
+
+package file
+
+import (
+	"os"
+
+	"golang.org/x/sys/plan9"
+)
+
+// RedirectStandardError causes all standard error output to be directed to the
+// given file.
+func RedirectStandardError(toFile *os.File) error {
+	_, err := plan9.Dup(int(toFile.Fd()), 2)
+	return err
+}

file/stderr_other.go → file/stderr_unix.go

@@ -15,7 +15,7 @@
 // specific language governing permissions and limitations
 // under the License.
 
-//go:build !windows
+//go:build unix
 
 package file