specerror: Add SplitLevel helper

This doesn't save much time for folks who are iterating over the
removed errors (e.g. to print warnings about them), but Aleksa asked
for it [1] and Ma suggested the helper struct [2].

I haven't used it in runtimetest.  I'm waiting for the in-flight
7aa3987 (cmd/runtimetest/main: Use TAP diagnostics for errors,
2017-07-28, opencontainers#439) to settle, since that commit cleans up some of the
runtimetest error-processing code with:

  validations := defaultValidations
  if platform == "linux" {
    validations = append(validations, linuxValidations...)
  }

and a single loop over the constructed validations array.

[1]: opencontainers#492 (comment)
[2]: opencontainers#492 (comment)

Signed-off-by: W. Trevor King <wking@tremily.us>

Author: wking

cmd/oci-runtime-tool/validate.go

@@ -3,7 +3,6 @@ package main
 import (
 	"fmt"
 
-	"github.com/hashicorp/go-multierror"
 	rfc2119 "github.com/opencontainers/runtime-tools/error"
 	"github.com/opencontainers/runtime-tools/specerror"
 	"github.com/opencontainers/runtime-tools/validate"
@@ -37,20 +36,14 @@ var bundleValidateCommand = cli.Command{
 		}
 
 		if err := v.CheckAll(); err != nil {
-			merr, ok := err.(*multierror.Error)
-			if !ok {
+			levelErrors, err := specerror.SplitLevel(err, complianceLevel)
+			if err != nil {
 				return err
 			}
-			var validationErrors error
-			for _, err = range merr.Errors {
-				e, ok := err.(*specerror.Error)
-				if ok && e.Err.Level < complianceLevel {
-					logrus.Warn(e)
-					continue
-				}
-				validationErrors = multierror.Append(validationErrors, err)
+			for _, e := range levelErrors.Warnings {
+				logrus.Warn(e)
 			}
-			return validationErrors
+			return levelErrors.Error
 		}
 		fmt.Println("Bundle validation succeeded.")
 		return nil

specerror/error.go

@@ -69,6 +69,16 @@ type Error struct {
 	Code Code
 }
 
+// LevelErrors represents Errors filtered into fatal and warnings.
+type LevelErrors struct {
+	// Warnings holds Errors that were below a compliance-level threshold.
+	Warnings []*Error
+
+	// Error holds errors that were at or above a compliance-level
+	// threshold, as well as errors that are not Errors.
+	Error *multierror.Error
+}
+
 var (
 	containerFormatRef = func(version string) (reference string, err error) {
 		return fmt.Sprintf(referenceTemplate, version, "bundle.md#container-format"), nil
@@ -168,3 +178,23 @@ func FindError(err error, code Code) Code {
 	}
 	return NonRFCError
 }
+
+// SplitLevel removes RFC 2119 errors with a level less than 'level'
+// from the source error.  If the source error is not a multierror, it
+// is returned unchanged.
+func SplitLevel(errIn error, level rfc2119.Level) (levelErrors LevelErrors, errOut error) {
+	merr, ok := errIn.(*multierror.Error)
+	if !ok {
+		return levelErrors, errIn
+	}
+	for _, err := range merr.Errors {
+		e, ok := err.(*Error)
+		if ok && e.Err.Level < level {
+			fmt.Println(e)
+			levelErrors.Warnings = append(levelErrors.Warnings, e)
+			continue
+		}
+		levelErrors.Error = multierror.Append(levelErrors.Error, err)
+	}
+	return levelErrors, nil
+}