Add CheckDisallowed (kelseyhightower#106)

envconfig.go

@@ -8,6 +8,7 @@ import (
 	"encoding"
 	"errors"
 	"fmt"
+	"os"
 	"reflect"
 	"regexp"
 	"strconv"
@@ -142,6 +143,37 @@ func gatherInfo(prefix string, spec interface{}) ([]varInfo, error) {
 	return infos, nil
 }
 
+// CheckDisallowed checks that no environment variables with the prefix are set
+// that we don't know how or want to parse. This is likely only meaningful with
+// a non-empty prefix.
+func CheckDisallowed(prefix string, spec interface{}) error {
+	infos, err := gatherInfo(prefix, spec)
+	if err != nil {
+		return err
+	}
+
+	vars := make(map[string]struct{})
+	for _, info := range infos {
+		vars[info.Key] = struct{}{}
+	}
+
+	if prefix != "" {
+		prefix = strings.ToUpper(prefix) + "_"
+	}
+
+	for _, env := range os.Environ() {
+		if !strings.HasPrefix(env, prefix) {
+			continue
+		}
+		v := strings.SplitN(env, "=", 2)[0]
+		if _, found := vars[v]; !found {
+			return fmt.Errorf("unknown environment variable %s", v)
+		}
+	}
+
+	return nil
+}
+
 // Process populates the specified struct based on environment variables
 func Process(prefix string, spec interface{}) error {
 	infos, err := gatherInfo(prefix, spec)

envconfig_test.go

@@ -741,6 +741,39 @@ func TestBinaryUnmarshalerError(t *testing.T) {
 	}
 }
 
+func TestCheckDisallowedOnlyAllowed(t *testing.T) {
+	var s Specification
+	os.Clearenv()
+	os.Setenv("ENV_CONFIG_DEBUG", "true")
+	os.Setenv("UNRELATED_ENV_VAR", "true")
+	err := CheckDisallowed("env_config", &s)
+	if err != nil {
+		t.Errorf("expected no error, got %s", err)
+	}
+}
+
+func TestCheckDisallowedMispelled(t *testing.T) {
+	var s Specification
+	os.Clearenv()
+	os.Setenv("ENV_CONFIG_DEBUG", "true")
+	os.Setenv("ENV_CONFIG_ZEBUG", "false")
+	err := CheckDisallowed("env_config", &s)
+	if experr := "unknown environment variable ENV_CONFIG_ZEBUG"; err.Error() != experr {
+		t.Errorf("expected %s, got %s", experr, err)
+	}
+}
+
+func TestCheckDisallowedIgnored(t *testing.T) {
+	var s Specification
+	os.Clearenv()
+	os.Setenv("ENV_CONFIG_DEBUG", "true")
+	os.Setenv("ENV_CONFIG_IGNORED", "false")
+	err := CheckDisallowed("env_config", &s)
+	if experr := "unknown environment variable ENV_CONFIG_IGNORED"; err.Error() != experr {
+		t.Errorf("expected %s, got %s", experr, err)
+	}
+}
+
 type bracketed string
 
 func (b *bracketed) Set(value string) error {