Fix empty host.id (#4317)

CHANGELOG.md

@@ -36,6 +36,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 - Correctly format log messages from the `go.opentelemetry.io/otel/exporters/zipkin` exporter. (#4143)
 - Log an error for calls to `NewView` in `go.opentelemetry.io/otel/sdk/metric` that have empty criteria. (#4307)
+- Fix `resource.WithHostID()` to not set an empty `host.id`. (#4317)
 
 ## [1.16.0/0.39.0] 2023-05-18
 

sdk/resource/host_id_readfile.go

@@ -21,7 +21,7 @@ import "os"
 func readFile(filename string) (string, error) {
 	b, err := os.ReadFile(filename)
 	if err != nil {
-		return "", nil
+		return "", err
 	}
 
 	return string(b), nil

sdk/resource/host_id_readfile_test.go

@@ -0,0 +1,53 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed 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 linux || dragonfly || freebsd || netbsd || openbsd || solaris
+
+package resource
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestReadFileExistent(t *testing.T) {
+	fileContents := "foo"
+
+	f, err := os.CreateTemp("", "readfile_")
+	require.NoError(t, err)
+
+	defer os.Remove(f.Name())
+
+	_, err = f.WriteString(fileContents)
+	require.NoError(t, err)
+	require.NoError(t, f.Close())
+
+	result, err := readFile(f.Name())
+	require.NoError(t, err)
+	require.Equal(t, result, fileContents)
+}
+
+func TestReadFileNonExistent(t *testing.T) {
+	// create unique filename
+	f, err := os.CreateTemp("", "readfile_")
+	require.NoError(t, err)
+
+	// make file non-existent
+	require.NoError(t, os.Remove(f.Name()))
+
+	_, err = readFile(f.Name())
+	require.ErrorIs(t, err, os.ErrNotExist)
+}