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If typehash (used by reflect) does not match the built-in map's hash, then problems occur. If a map is built using reflect, and then assigned to a variable of map type, the hash function can change. That causes very bad things. This issue is rare. MapOf consults a cache of all types that occur in the binary before making a new one. To make a true new map type (with a hash function derived from typehash) that map type must not occur in the binary anywhere. But to cause the bug, we need a variable of that type in order to assign to it. The only way to make that work is to use a named map type for the variable, so it is distinct from the unnamed version that MapOf looks for. Fixes #37716 Change-Id: I3537bfceca8cbfa1af84202f432f3c06953fe0ed Reviewed-on: https://go-review.googlesource.com/c/go/+/222357 Run-TryBot: Keith Randall <khr@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Cherry Zhang <cherryyz@google.com>
33 lines
872 B
Go
33 lines
872 B
Go
// run
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// Copyright 2020 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package main
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import "reflect"
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// complicated enough to require a compile-generated hash function
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type K struct {
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a, b int32 // these get merged by the compiler into a single field, something typehash doesn't do
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c float64
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}
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func main() {
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k := K{a: 1, b: 2, c: 3}
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// Make a reflect map.
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m := reflect.MakeMap(reflect.MapOf(reflect.TypeOf(K{}), reflect.TypeOf(true)))
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m.SetMapIndex(reflect.ValueOf(k), reflect.ValueOf(true))
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// The binary must not contain the type map[K]bool anywhere, or reflect.MapOf
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// will use that type instead of making a new one. So use an equivalent named type.
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type M map[K]bool
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var x M
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reflect.ValueOf(&x).Elem().Set(m)
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if !x[k] {
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panic("key not found")
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}
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}
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