Initial commit

vendor/github.com/cockroachdb/errors/errutil/as.go

@@ -0,0 +1,74 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Copyright 2019 The Cockroach Authors.
+//
+// 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 errutil
+
+import (
+	"reflect"
+
+	"github.com/cockroachdb/errors/errbase"
+)
+
+// As finds the first error in err's chain that matches the type to which target
+// points, and if so, sets the target to its value and returns true. An error
+// matches a type if it is assignable to the target type, or if it has a method
+// As(interface{}) bool such that As(target) returns true. As will panic if target
+// is not a non-nil pointer to a type which implements error or is of interface type.
+//
+// The As method should set the target to its value and return true if err
+// matches the type to which target points.
+//
+// Note: this implementation differs from that of xerrors as follows:
+// - it also supports recursing through causes with Cause().
+// - if it detects an API use error, its panic object is a valid error.
+func As(err error, target interface{}) bool {
+	if target == nil {
+		panic(AssertionFailedf("errors.As: target cannot be nil"))
+	}
+
+	// We use introspection for now, of course when/if Go gets generics
+	// all this can go away.
+	val := reflect.ValueOf(target)
+	typ := val.Type()
+	if typ.Kind() != reflect.Ptr || val.IsNil() {
+		panic(AssertionFailedf("errors.As: target must be a non-nil pointer, found %T", target))
+	}
+	if e := typ.Elem(); e.Kind() != reflect.Interface && !e.Implements(errorType) {
+		panic(AssertionFailedf("errors.As: *target must be interface or implement error, found %T", target))
+	}
+
+	targetType := typ.Elem()
+	for c := err; c != nil; c = errbase.UnwrapOnce(c) {
+		if reflect.TypeOf(c).AssignableTo(targetType) {
+			val.Elem().Set(reflect.ValueOf(c))
+			return true
+		}
+		if x, ok := c.(interface{ As(interface{}) bool }); ok && x.As(target) {
+			return true
+		}
+
+		// If at any point in the single cause chain including the top,
+		// we encounter a multi-cause chain, recursively explore it.
+		for _, cause := range errbase.UnwrapMulti(c) {
+			if As(cause, target) {
+				return true
+			}
+		}
+	}
+
+	return false
+}
+
+var errorType = reflect.TypeOf((*error)(nil)).Elem()