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slices: Replace does not zero trailing elements if if j == len(s) and i > 0 #65669

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joeycumines opened this issue Feb 12, 2024 · 3 comments
Closed

slices: Replace does not zero trailing elements if if j == len(s) and i > 0 #65669

joeycumines opened this issue Feb 12, 2024 · 3 comments
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NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
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Unreleased

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@joeycumines
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joeycumines commented Feb 12, 2024 •
edited

Go version

go version go1.22.0 darwin/arm64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/joeyc/Library/Caches/go-build'
GOENV='/Users/joeyc/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/joeyc/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/joeyc/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/Users/joeyc/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.22.0.darwin-arm64'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/Users/joeyc/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.22.0.darwin-arm64/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.22.0'
GCCGO='gccgo'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/Users/joeyc/dev/boi/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/_r/v0qs308n49952w5gddyqznbw0000gn/T/go-build2791652445=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

Using golang.org/x/exp v0.0.0-20240205201215-2c58cdc269a3, I calledslices.Replace, to shift elements from the end of the slice, and noticed it didn't clear as documented.

For context, I was using Replace like:

noFilterAfterIdx := // ...
filtered := slices.DeleteFunc(data[:noFilterAfterIdx], /* .. */)
if len(filtered) != noFilterAfterIdx {
  data = slices.Replace(data, len(filtered), len(data), data[noFilterAfterIdx:]...)
}

But I found that it wasn't behaving per the docs:

When len(v) < (j-i), Replace zeroes the elements between the new length and the original length.

Debugging the code shows it hit the second guard in the current implementation:

	if j == len(s) {
		return append(s[:i], v...)
	}

And I've confirmed it only appears to be an issue for that one edge case, so it's probably just an oversight / easy to fix bug.

package main

import (
	"fmt"
	"golang.org/x/exp/slices"
)

const l = 10

func input() []int {
	v := make([]int, l)
	for i := range v {
		v[i] = i + 1
	}
	return v
}

func main() {
	fmt.Println("input:", input())
	for i := 0; i < l; i++ {
		for j := i; j <= l; j++ {
			for k := i; k <= l; k++ {
				s := input()
				v := s[k:]
				n := len(slices.Replace(s, i, j, v...))
				// "When len(v) < (j-i), Replace zeroes the elements between the new length and the original length."
				if len(v) < (j - i) {
					expected := len(s) - n
					var actual int
					for _, v := range s[n:] {
						if v == 0 {
							actual++
						}
					}
					if actual != expected {
						fmt.Printf("replaced %d:%d with %d:%d but failed to zero %d/%d: %v\n", i, j, k, l, expected-actual, expected, s)
					}
				}
			}
		}
	}
}

What did you see happen?

Elements in the range (specified by i and j) weren't one of: overwritten, zeroed. Specifically, they weren't zeroed.

From the runnable example:

input: [1 2 3 4 5 6 7 8 9 10]
replaced 0:10 with 1:10 but failed to zero 1/1: [2 3 4 5 6 7 8 9 10 10]
replaced 0:10 with 2:10 but failed to zero 2/2: [3 4 5 6 7 8 9 10 9 10]
replaced 0:10 with 3:10 but failed to zero 3/3: [4 5 6 7 8 9 10 8 9 10]
replaced 0:10 with 4:10 but failed to zero 4/4: [5 6 7 8 9 10 7 8 9 10]
replaced 0:10 with 5:10 but failed to zero 5/5: [6 7 8 9 10 6 7 8 9 10]
replaced 0:10 with 6:10 but failed to zero 6/6: [7 8 9 10 5 6 7 8 9 10]
replaced 0:10 with 7:10 but failed to zero 7/7: [8 9 10 4 5 6 7 8 9 10]
replaced 0:10 with 8:10 but failed to zero 8/8: [9 10 3 4 5 6 7 8 9 10]
replaced 0:10 with 9:10 but failed to zero 9/9: [10 2 3 4 5 6 7 8 9 10]
replaced 0:10 with 10:10 but failed to zero 10/10: [1 2 3 4 5 6 7 8 9 10]
replaced 1:10 with 2:10 but failed to zero 1/1: [1 3 4 5 6 7 8 9 10 10]
replaced 1:10 with 3:10 but failed to zero 2/2: [1 4 5 6 7 8 9 10 9 10]
replaced 1:10 with 4:10 but failed to zero 3/3: [1 5 6 7 8 9 10 8 9 10]
replaced 1:10 with 5:10 but failed to zero 4/4: [1 6 7 8 9 10 7 8 9 10]
replaced 1:10 with 6:10 but failed to zero 5/5: [1 7 8 9 10 6 7 8 9 10]
replaced 1:10 with 7:10 but failed to zero 6/6: [1 8 9 10 5 6 7 8 9 10]
replaced 1:10 with 8:10 but failed to zero 7/7: [1 9 10 4 5 6 7 8 9 10]
replaced 1:10 with 9:10 but failed to zero 8/8: [1 10 3 4 5 6 7 8 9 10]
replaced 1:10 with 10:10 but failed to zero 9/9: [1 2 3 4 5 6 7 8 9 10]
replaced 2:10 with 3:10 but failed to zero 1/1: [1 2 4 5 6 7 8 9 10 10]
replaced 2:10 with 4:10 but failed to zero 2/2: [1 2 5 6 7 8 9 10 9 10]
replaced 2:10 with 5:10 but failed to zero 3/3: [1 2 6 7 8 9 10 8 9 10]
replaced 2:10 with 6:10 but failed to zero 4/4: [1 2 7 8 9 10 7 8 9 10]
replaced 2:10 with 7:10 but failed to zero 5/5: [1 2 8 9 10 6 7 8 9 10]
replaced 2:10 with 8:10 but failed to zero 6/6: [1 2 9 10 5 6 7 8 9 10]
replaced 2:10 with 9:10 but failed to zero 7/7: [1 2 10 4 5 6 7 8 9 10]
replaced 2:10 with 10:10 but failed to zero 8/8: [1 2 3 4 5 6 7 8 9 10]
replaced 3:10 with 4:10 but failed to zero 1/1: [1 2 3 5 6 7 8 9 10 10]
replaced 3:10 with 5:10 but failed to zero 2/2: [1 2 3 6 7 8 9 10 9 10]
replaced 3:10 with 6:10 but failed to zero 3/3: [1 2 3 7 8 9 10 8 9 10]
replaced 3:10 with 7:10 but failed to zero 4/4: [1 2 3 8 9 10 7 8 9 10]
replaced 3:10 with 8:10 but failed to zero 5/5: [1 2 3 9 10 6 7 8 9 10]
replaced 3:10 with 9:10 but failed to zero 6/6: [1 2 3 10 5 6 7 8 9 10]
replaced 3:10 with 10:10 but failed to zero 7/7: [1 2 3 4 5 6 7 8 9 10]
replaced 4:10 with 5:10 but failed to zero 1/1: [1 2 3 4 6 7 8 9 10 10]
replaced 4:10 with 6:10 but failed to zero 2/2: [1 2 3 4 7 8 9 10 9 10]
replaced 4:10 with 7:10 but failed to zero 3/3: [1 2 3 4 8 9 10 8 9 10]
replaced 4:10 with 8:10 but failed to zero 4/4: [1 2 3 4 9 10 7 8 9 10]
replaced 4:10 with 9:10 but failed to zero 5/5: [1 2 3 4 10 6 7 8 9 10]
replaced 4:10 with 10:10 but failed to zero 6/6: [1 2 3 4 5 6 7 8 9 10]
replaced 5:10 with 6:10 but failed to zero 1/1: [1 2 3 4 5 7 8 9 10 10]
replaced 5:10 with 7:10 but failed to zero 2/2: [1 2 3 4 5 8 9 10 9 10]
replaced 5:10 with 8:10 but failed to zero 3/3: [1 2 3 4 5 9 10 8 9 10]
replaced 5:10 with 9:10 but failed to zero 4/4: [1 2 3 4 5 10 7 8 9 10]
replaced 5:10 with 10:10 but failed to zero 5/5: [1 2 3 4 5 6 7 8 9 10]
replaced 6:10 with 7:10 but failed to zero 1/1: [1 2 3 4 5 6 8 9 10 10]
replaced 6:10 with 8:10 but failed to zero 2/2: [1 2 3 4 5 6 9 10 9 10]
replaced 6:10 with 9:10 but failed to zero 3/3: [1 2 3 4 5 6 10 8 9 10]
replaced 6:10 with 10:10 but failed to zero 4/4: [1 2 3 4 5 6 7 8 9 10]
replaced 7:10 with 8:10 but failed to zero 1/1: [1 2 3 4 5 6 7 9 10 10]
replaced 7:10 with 9:10 but failed to zero 2/2: [1 2 3 4 5 6 7 10 9 10]
replaced 7:10 with 10:10 but failed to zero 3/3: [1 2 3 4 5 6 7 8 9 10]
replaced 8:10 with 9:10 but failed to zero 1/1: [1 2 3 4 5 6 7 8 10 10]
replaced 8:10 with 10:10 but failed to zero 2/2: [1 2 3 4 5 6 7 8 9 10]
replaced 9:10 with 10:10 but failed to zero 1/1: [1 2 3 4 5 6 7 8 9 10]

What did you expect to see?

When len(v) < (j-i), Replace zeroes the elements between the new length and the original length.

The runnable example should have only printed input: [1 2 3 4 5 6 7 8 9 10].
@gopherbot gopherbot added this to the Unreleased milestone Feb 12, 2024
@ianlancetaylor
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CC @Deleplace

@Deleplace
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Deleplace commented Feb 12, 2024 •
edited

Good catch!

This does look like a bug. I could confirm and wrote a small repro: https://go.dev/play/p/FQKUFDUD4ox

Thank you @joeycumines for writing this issue with sample code, and for pinpointing the culprit in the stdlib source. It does indeed look like the second guard is taking a shortcut that incorrectly ignores the recent zeroing policy.

@seankhliao seankhliao changed the title x/exp/slices: Replace does not zero trailing elements if if j == len(s) and i > 0 slices: Replace does not zero trailing elements if if j == len(s) and i > 0 Feb 12, 2024
@thanm thanm added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Feb 12, 2024
@gopherbot
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Change https://go.dev/cl/566237 mentions this issue: slices: Delete clears the tail when j == len(s)

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