feat(exp): add envutils package (#466)

Add a function to read value from the environment or from a file.

hcloud/exp/kit/envutils/env.go

@@ -0,0 +1,40 @@
+package envutils
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+// LookupEnvWithFile retrieves the value of the environment variable named by the key (e.g.
+// HCLOUD_TOKEN). If the previous environment variable is not set, it retrieves the
+// content of the file located by a second environment variable named by the key +
+// '_FILE' (.e.g HCLOUD_TOKEN_FILE).
+//
+// For both cases, the returned value may be empty.
+//
+// The value from the environment takes precedence over the value from the file.
+func LookupEnvWithFile(key string) (string, error) {
+	// Check if the value is set in the environment (e.g. HCLOUD_TOKEN)
+	value, ok := os.LookupEnv(key)
+	if ok {
+		return value, nil
+	}
+
+	key += "_FILE"
+
+	// Check if the value is set via a file (e.g. HCLOUD_TOKEN_FILE)
+	valueFile, ok := os.LookupEnv(key)
+	if !ok {
+		// Validation of the value happens outside of this function
+		return "", nil
+	}
+
+	// Read the content of the file
+	valueBytes, err := os.ReadFile(valueFile)
+	if err != nil {
+		return "", fmt.Errorf("failed to read %s: %w", key, err)
+	}
+
+	return strings.TrimSpace(string(valueBytes)), nil
+}

hcloud/exp/kit/envutils/env_test.go

@@ -0,0 +1,106 @@
+package envutils
+
+import (
+	"os"
+	"path"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// nolint:unparam
+func writeTmpFile(t *testing.T, tmpDir, filename, content string) string {
+	filepath := path.Join(tmpDir, filename)
+
+	err := os.WriteFile(filepath, []byte(content), 0644)
+	require.NoError(t, err)
+
+	return filepath
+}
+
+func TestLookupEnvWithFile(t *testing.T) {
+	testCases := []struct {
+		name  string
+		setup func(t *testing.T, tmpDir string)
+		want  func(t *testing.T, value string, err error)
+	}{
+		{
+			name:  "without any environment",
+			setup: func(_ *testing.T, _ string) {},
+			want: func(t *testing.T, value string, err error) {
+				assert.NoError(t, err)
+				assert.Equal(t, "", value)
+			},
+		},
+		{
+			name: "value from environment",
+			setup: func(t *testing.T, tmpDir string) {
+				t.Setenv("CONFIG", "value")
+
+				// Test for precedence
+				filepath := writeTmpFile(t, tmpDir, "config", "content")
+				t.Setenv("CONFIG_FILE", filepath)
+			},
+			want: func(t *testing.T, value string, err error) {
+				assert.NoError(t, err)
+				assert.Equal(t, "value", value)
+			},
+		},
+		{
+			name: "empty value from environment",
+			setup: func(t *testing.T, tmpDir string) {
+				t.Setenv("CONFIG", "")
+
+				// Test for precedence
+				filepath := writeTmpFile(t, tmpDir, "config", "content")
+				t.Setenv("CONFIG_FILE", filepath)
+			},
+			want: func(t *testing.T, value string, err error) {
+				assert.NoError(t, err)
+				assert.Equal(t, "", value)
+			},
+		},
+		{
+			name: "value from file",
+			setup: func(t *testing.T, tmpDir string) {
+				// The extra spaces ensure that the value is sanitized
+				filepath := writeTmpFile(t, tmpDir, "config", "content  ")
+				t.Setenv("CONFIG_FILE", filepath)
+			},
+			want: func(t *testing.T, value string, err error) {
+				assert.NoError(t, err)
+				assert.Equal(t, "content", value)
+			},
+		},
+		{
+			name: "empty value from file",
+			setup: func(t *testing.T, tmpDir string) {
+				filepath := writeTmpFile(t, tmpDir, "config", "")
+				t.Setenv("CONFIG_FILE", filepath)
+			},
+			want: func(t *testing.T, value string, err error) {
+				assert.NoError(t, err)
+				assert.Equal(t, "", value)
+			},
+		},
+		{
+			name: "missing file",
+			setup: func(t *testing.T, _ string) {
+				t.Setenv("CONFIG_FILE", "/tmp/this-file-does-not-exits")
+			},
+			want: func(t *testing.T, value string, err error) {
+				assert.Error(t, err, "failed to read CONFIG_FILE: open /tmp/this-file-does-not-exits: no such file or directory")
+				assert.Equal(t, "", value)
+			},
+		},
+	}
+	for _, testCase := range testCases {
+		t.Run(testCase.name, func(t *testing.T) {
+			tmpDir := t.TempDir()
+			testCase.setup(t, tmpDir)
+			value, err := LookupEnvWithFile("CONFIG")
+			testCase.want(t, value, err)
+		})
+	}
+}