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For a failed interface conversion not in ",ok" form, getitab calls itab.init to get the name of the missing method for the panic message. itab.init will try to find the methods, populate the method table as it goes. When some method is missing, it sets itab.fun[0] to 0 before return. There is a small window that itab.fun[0] could be non-zero. If concurrently, another goroutine tries to do the same interface conversion, it will read the same itab's fun[0]. If this happens in the small window, it sees a non-zero fun[0] and thinks the conversion succeeded, which is bad. Fix the race by setting fun[0] to non-zero only when we know the conversion succeeds. While here, also simplify the syntax slightly. Fixes #31419. Change-Id: Ied34d3043079eb933e330c5877b85e13f98f1916 Reviewed-on: https://go-review.googlesource.com/c/go/+/171759 Run-TryBot: Cherry Zhang <cherryyz@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Keith Randall <khr@golang.org>
59 lines
948 B
Go
59 lines
948 B
Go
// run
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// Copyright 2019 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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// Issue 31419: race in getitab when two goroutines try
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// to do the same failed interface conversion.
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package main
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type T int
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func (t T) M() {}
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type I interface {
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M()
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M2()
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}
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var t T
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var e interface{} = &t
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var ok = false
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var ch = make(chan int)
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func main() {
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_, ok = e.(I) // populate itab cache with a false result
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go f() // get itab in a loop
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var i I
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for k := 0; k < 10000; k++ {
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i, ok = e.(I) // read the cached itab
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if ok {
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println("iteration", k, "i =", i, "&t =", &t)
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panic("conversion succeeded")
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}
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}
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<-ch
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}
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func f() {
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for i := 0; i < 10000; i++ {
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f1()
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}
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ch <- 1
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}
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func f1() {
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defer func() {
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err := recover()
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if err == nil {
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panic("did not panic")
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}
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}()
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i := e.(I) // triggers itab.init, for getting the panic string
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_ = i
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}
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