build: add errors.Join workaround for go <1.20

Signed-off-by: Stephan Renatus <stephan@styra.com>

internal/errors/join.go

@@ -0,0 +1,53 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !go1.20
+
+package errors
+
+// Join returns an error that wraps the given errors.
+// Any nil error values are discarded.
+// Join returns nil if errs contains no non-nil values.
+// The error formats as the concatenation of the strings obtained
+// by calling the Error method of each element of errs, with a newline
+// between each string.
+func Join(errs ...error) error {
+	n := 0
+	for _, err := range errs {
+		if err != nil {
+			n++
+		}
+	}
+	if n == 0 {
+		return nil
+	}
+	e := &joinError{
+		errs: make([]error, 0, n),
+	}
+	for _, err := range errs {
+		if err != nil {
+			e.errs = append(e.errs, err)
+		}
+	}
+	return e
+}
+
+type joinError struct {
+	errs []error
+}
+
+func (e *joinError) Error() string {
+	var b []byte
+	for i, err := range e.errs {
+		if i > 0 {
+			b = append(b, '\n')
+		}
+		b = append(b, err.Error()...)
+	}
+	return string(b)
+}
+
+func (e *joinError) Unwrap() []error {
+	return e.errs
+}

plugins/discovery/discovery.go

@@ -8,7 +8,6 @@ package discovery
 import (
 	"context"
 	"encoding/json"
-	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -23,6 +22,7 @@ import (
 	"github.com/open-policy-agent/opa/hooks"
 	bundleUtils "github.com/open-policy-agent/opa/internal/bundle"
 	cfg "github.com/open-policy-agent/opa/internal/config"
+	"github.com/open-policy-agent/opa/internal/errors"
 	"github.com/open-policy-agent/opa/keys"
 	"github.com/open-policy-agent/opa/logging"
 	"github.com/open-policy-agent/opa/metrics"

plugins/plugins.go

@@ -7,7 +7,6 @@ package plugins
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"sync"
 	"time"
@@ -22,6 +21,7 @@ import (
 	"github.com/open-policy-agent/opa/hooks"
 	bundleUtils "github.com/open-policy-agent/opa/internal/bundle"
 	cfg "github.com/open-policy-agent/opa/internal/config"
+	"github.com/open-policy-agent/opa/internal/errors"
 	initload "github.com/open-policy-agent/opa/internal/runtime/init"
 	"github.com/open-policy-agent/opa/keys"
 	"github.com/open-policy-agent/opa/loader"