github.com/cockroachdb/errors@v1.11.1/errutil/as.go (about)

     1  // Copyright 2018 The Go Authors. All rights reserved.
     2  // Copyright 2019 The Cockroach Authors.
     3  //
     4  // Licensed under the Apache License, Version 2.0 (the "License");
     5  // you may not use this file except in compliance with the License.
     6  // You may obtain a copy of the License at
     7  //
     8  //     http://www.apache.org/licenses/LICENSE-2.0
     9  //
    10  // Unless required by applicable law or agreed to in writing, software
    11  // distributed under the License is distributed on an "AS IS" BASIS,
    12  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
    13  // implied. See the License for the specific language governing
    14  // permissions and limitations under the License.
    15  
    16  package errutil
    17  
    18  import (
    19  	"reflect"
    20  
    21  	"github.com/cockroachdb/errors/errbase"
    22  )
    23  
    24  // As finds the first error in err's chain that matches the type to which target
    25  // points, and if so, sets the target to its value and returns true. An error
    26  // matches a type if it is assignable to the target type, or if it has a method
    27  // As(interface{}) bool such that As(target) returns true. As will panic if target
    28  // is not a non-nil pointer to a type which implements error or is of interface type.
    29  //
    30  // The As method should set the target to its value and return true if err
    31  // matches the type to which target points.
    32  //
    33  // Note: this implementation differs from that of xerrors as follows:
    34  // - it also supports recursing through causes with Cause().
    35  // - if it detects an API use error, its panic object is a valid error.
    36  func As(err error, target interface{}) bool {
    37  	if target == nil {
    38  		panic(AssertionFailedf("errors.As: target cannot be nil"))
    39  	}
    40  
    41  	// We use introspection for now, of course when/if Go gets generics
    42  	// all this can go away.
    43  	val := reflect.ValueOf(target)
    44  	typ := val.Type()
    45  	if typ.Kind() != reflect.Ptr || val.IsNil() {
    46  		panic(AssertionFailedf("errors.As: target must be a non-nil pointer, found %T", target))
    47  	}
    48  	if e := typ.Elem(); e.Kind() != reflect.Interface && !e.Implements(errorType) {
    49  		panic(AssertionFailedf("errors.As: *target must be interface or implement error, found %T", target))
    50  	}
    51  
    52  	targetType := typ.Elem()
    53  	for c := err; c != nil; c = errbase.UnwrapOnce(c) {
    54  		if reflect.TypeOf(c).AssignableTo(targetType) {
    55  			val.Elem().Set(reflect.ValueOf(c))
    56  			return true
    57  		}
    58  		if x, ok := c.(interface{ As(interface{}) bool }); ok && x.As(target) {
    59  			return true
    60  		}
    61  
    62  		// If at any point in the single cause chain including the top,
    63  		// we encounter a multi-cause chain, recursively explore it.
    64  		for _, cause := range errbase.UnwrapMulti(c) {
    65  			if As(cause, target) {
    66  				return true
    67  			}
    68  		}
    69  	}
    70  
    71  	return false
    72  }
    73  
    74  var errorType = reflect.TypeOf((*error)(nil)).Elem()