Add template support to evaluation details.

This change extends error type available for use in evaluation engines
with support for go templates. This error type supports rendering
complex templates along with the usual string rendering. Despite the
type itself being the same, a new constructor was added to avoid a
bigger refactoring. Ideally, the old constructor should be deprecated
and all places returning an `ErrEvaluationFailed` error should use the
new one instead.

Templates are internal to Minder and cannot be customized by the end
user. Also, template engine choice was arbitrary and can be changed at
any time.

Fixes #4525

internal/engine/errors/errors.go

@@ -20,17 +20,51 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
+	"strings"
+	"text/template"
 
 	"github.com/stacklok/minder/internal/db"
 )
 
 // ErrInternal is an error that occurs when there is an internal error in the minder engine.
 var ErrInternal = errors.New("internal minder error")
 
+type limitedWriter struct {
+	w io.Writer
+	n int64
+}
+
+var _ io.Writer = (*limitedWriter)(nil)
+
+func (l *limitedWriter) Write(p []byte) (int, error) {
+	if l.n < 0 {
+		return 0, io.ErrShortBuffer
+	}
+	if int64(len(p)) > l.n {
+		return 0, io.ErrShortBuffer
+	}
+	n, err := l.w.Write(p)
+	l.n -= int64(n)
+	return n, err
+}
+
+// LimitedWriter returns a writer that allows up to `n` bytes being
+// written. If more than `n` total bytes are written,
+// `io.ErrShortBuffer` is returned.
+func LimitedWriter(w io.Writer, n int64) io.Writer {
+	return &limitedWriter{
+		w: w,
+		n: n,
+	}
+}
+
 // EvaluationError is a custom error type for evaluation errors.
 type EvaluationError struct {
-	Base error
-	Msg  string
+	Base         error
+	Msg          string
+	Template     string
+	TemplateArgs any
 }
 
 // Unwrap returns the base error, allowing errors.Is to work with wrapped errors.
@@ -43,6 +77,42 @@ func (e *EvaluationError) Error() string {
 	return fmt.Sprintf("%v: %s", e.Base, e.Msg)
 }
 
+// Details returns a pretty-printed message detailing the reason of
+// the failure.
+func (e *EvaluationError) Details() string {
+	if e.Template == "" {
+		return e.Msg
+	}
+	tmpl, err := template.New("error").Parse(e.Template)
+	if err != nil {
+		return e.Error()
+	}
+
+	var buf strings.Builder
+	w := LimitedWriter(&buf, 1<<10)
+	if err := tmpl.Execute(w, e.TemplateArgs); err != nil {
+		return e.Error()
+	}
+	return buf.String()
+}
+
+// NewDetailedErrEvaluationFailed creates a new evaluation error with
+// a given error message and a templated detail message.
+func NewDetailedErrEvaluationFailed(
+	tmpl string,
+	tmplArgs any,
+	sfmt string,
+	args ...any,
+) error {
+	formatted := fmt.Sprintf(sfmt, args...)
+	return &EvaluationError{
+		Base:         ErrEvaluationFailed,
+		Msg:          formatted,
+		Template:     tmpl,
+		TemplateArgs: tmplArgs,
+	}
+}
+
 // ErrEvaluationFailed is an error that occurs during evaluation of a rule.
 var ErrEvaluationFailed = errors.New("evaluation failure")
 
@@ -131,12 +201,15 @@ func ErrorAsEvalStatus(err error) db.EvalStatusTypes {
 // ErrorAsEvalDetails returns the evaluation details for a given error
 func ErrorAsEvalDetails(err error) string {
 	var evalErr *EvaluationError
+	if errors.As(err, &evalErr) && evalErr.Template != "" {
+		return evalErr.Details()
+	}
 	if errors.As(err, &evalErr) {
 		return evalErr.Msg
-	} else if err != nil {
+	}
+	if err != nil {
 		return err.Error()
 	}
-
 	return ""
 }
 

internal/engine/errors/errors_test.go

@@ -0,0 +1,147 @@
+// Copyright 2024 Stacklok, Inc.
+//
+// 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.
+
+package errors
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/stacklok/minder/internal/engine/eval/templates"
+)
+
+func TestLegacyEvaluationDetailRendering(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		msg     string
+		args    []any
+		error   string
+		details string
+	}{
+		{
+			name:    "legacy",
+			msg:     "format: %s",
+			args:    []any{"this is the message"},
+			error:   "evaluation failure: format: this is the message",
+			details: "format: this is the message",
+		},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			err := NewErrEvaluationFailed(
+				tt.msg,
+				tt.args...,
+			)
+
+			require.Equal(t, tt.error, err.Error())
+			evalErr, ok := err.(*EvaluationError)
+			require.True(t, ok)
+			require.Equal(t, tt.details, evalErr.Msg)
+		})
+	}
+}
+
+func TestEvaluationDetailRendering(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		msg     string
+		msgArgs []any
+		tmpl    string
+		args    any
+		error   string
+		details string
+	}{
+		{
+			name:    "legacy",
+			msg:     "this is the message",
+			tmpl:    "",
+			args:    nil,
+			error:   "evaluation failure: this is the message",
+			details: "this is the message",
+		},
+		{
+			name:    "empty template",
+			msg:     "this is the message",
+			tmpl:    "",
+			args:    nil,
+			error:   "evaluation failure: this is the message",
+			details: "this is the message",
+		},
+		{
+			name:    "simple template",
+			msg:     "this is the message",
+			tmpl:    "fancy template with {{ . }}",
+			args:    "fancy message",
+			error:   "evaluation failure: this is the message",
+			details: "fancy template with fancy message",
+		},
+		{
+			name:    "complex template",
+			msg:     "this is the message",
+			tmpl:    "fancy template with {{ range $idx, $val := . }}{{ if $idx }}, {{ end }}{{ . }}{{ end }}",
+			args:    []any{"many", "many", "many messages"},
+			error:   "evaluation failure: this is the message",
+			details: "fancy template with many, many, many messages",
+		},
+		{
+			name:    "enforced limit",
+			msg:     "this is the message",
+			tmpl:    "fancy template with {{ . }}",
+			args:    strings.Repeat("A", 1025),
+			error:   "evaluation failure: this is the message",
+			details: "evaluation failure: this is the message",
+		},
+		// vulncheck template
+		{
+			name:    "vulncheck template",
+			msg:     "this is the message",
+			tmpl:    templates.VulncheckTemplate,
+			args:    map[string]any{"packages": []string{"boto3", "urllib3", "python-oauth2"}},
+			error:   "evaluation failure: this is the message",
+			details: "Vulnerable packages found:\n* `boto3`\n* `urllib3`\n* `python-oauth2`\n",
+		},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			err := NewDetailedErrEvaluationFailed(
+				tt.tmpl,
+				tt.args,
+				tt.msg,
+				tt.msgArgs...,
+			)
+
+			require.Equal(t, tt.error, err.Error())
+			evalErr, ok := err.(*EvaluationError)
+			require.True(t, ok)
+			require.Equal(t, tt.details, evalErr.Details())
+			require.LessOrEqual(t, len(evalErr.Details()), 1024)
+		})
+	}
+}

internal/engine/eval/templates/templates.go

@@ -0,0 +1,27 @@
+// Copyright 2024 Stacklok, Inc.
+//
+// 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.
+
+// Package templates contains template strings for evaluation details.
+package templates
+
+import (
+	// This comment makes the linter happy.
+	_ "embed"
+)
+
+// VulncheckTemplate is the template for evaluation details of the
+// `vulncheck` evaluation engine.
+//
+//go:embed vulncheckTemplate.tmpl
+var VulncheckTemplate string

internal/engine/eval/templates/vulncheckTemplate.tmpl

@@ -0,0 +1,4 @@
+Vulnerable packages found:
+{{- range .packages }}
+* `{{- . -}}`
+{{- end }}

internal/engine/eval/vulncheck/vulncheck.go

@@ -25,6 +25,7 @@ import (
 	"github.com/rs/zerolog"
 
 	evalerrors "github.com/stacklok/minder/internal/engine/errors"
+	"github.com/stacklok/minder/internal/engine/eval/templates"
 	engif "github.com/stacklok/minder/internal/engine/interfaces"
 	pbinternal "github.com/stacklok/minder/internal/proto"
 	provifv1 "github.com/stacklok/minder/pkg/providers/v1"
@@ -59,7 +60,12 @@ func (e *Evaluator) Eval(ctx context.Context, pol map[string]any, res *engif.Res
 	}
 
 	if len(vulnerablePackages) > 0 {
-		return evalerrors.NewErrEvaluationFailed("vulnerable packages: %s", strings.Join(vulnerablePackages, ","))
+		return evalerrors.NewDetailedErrEvaluationFailed(
+			templates.VulncheckTemplate,
+			vulnerablePackages,
+			"vulnerable packages: %s",
+			strings.Join(vulnerablePackages, ","),
+		)
 	}
 
 	return nil

internal/engine/eval/vulncheck/vulncheck_test.go

@@ -0,0 +1,68 @@
+// Copyright 2024 Stacklok, Inc.
+//
+// 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.
+
+package vulncheck
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	evalerrors "github.com/stacklok/minder/internal/engine/errors"
+	"github.com/stacklok/minder/internal/engine/eval/templates"
+)
+
+func TestEvaluationDetailRendering(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		msg     string
+		msgArgs []any
+		tmpl    string
+		args    any
+		error   string
+		details string
+	}{
+		// vulncheck template
+		{
+			name:    "vulncheck template",
+			msg:     "this is the message",
+			tmpl:    templates.VulncheckTemplate,
+			args:    map[string]any{"packages": []string{"boto3", "urllib3", "python-oauth2"}},
+			error:   "evaluation failure: this is the message",
+			details: "Vulnerable packages found:\n* `boto3`\n* `urllib3`\n* `python-oauth2`\n",
+		},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			err := evalerrors.NewDetailedErrEvaluationFailed(
+				tt.tmpl,
+				tt.args,
+				tt.msg,
+				tt.msgArgs...,
+			)
+
+			require.Equal(t, tt.error, err.Error())
+			evalErr, ok := err.(*evalerrors.EvaluationError)
+			require.True(t, ok)
+			require.Equal(t, tt.details, evalErr.Details())
+		})
+	}
+}