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html/template: data race with concurrent ExecuteTemplate calls #47040

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josharian opened this issue Jul 2, 2021 · 5 comments
Closed

html/template: data race with concurrent ExecuteTemplate calls #47040

josharian opened this issue Jul 2, 2021 · 5 comments
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
Milestone
Go1.17

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@josharian
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Extracted from flaky production tests.

ExecuteTemplate is documented to allow concurrent calls:

A template may be executed safely in parallel, although if parallel executions share a Writer the output may be interleaved.

https://play.golang.org/p/MHNbSuG9gOj

package main

import (
	"html/template"
	"io"
	"strings"
	"sync"
)

func main() {
	tmpl, err := template.New("a").Parse(`{{template "t"}}`)
	if err != nil {
		panic(err)
	}
	tmpl.New("b").Parse(`{{template "h"}}{{template "t"}}`)
	tmpl.New("h").Parse(`<style>{{template "s"}}</style>`)
	tmpl.New("t").Parse("")
	tmpl.New("s").Parse(strings.Repeat("🏎", 10000))
	var wg sync.WaitGroup
	wg.Add(100)
	for i := 0; i < 100; i++ {
		name := "a"
		if i&1 == 0 {
			name = "b"
		}
		go func(name string) {
			defer wg.Done()
			tmpl.ExecuteTemplate(io.Discard, name, nil)
		}(name)
	}
	wg.Wait()
}

Result:

==================
WARNING: DATA RACE
Write at 0x00c00011c540 by goroutine 35:
  runtime.mapaccess2_faststr()
      /Users/josh/go/1.16/src/runtime/map_faststr.go:107 +0x48c
  text/template.(*Template).associate()
      /Users/josh/go/1.16/src/text/template/template.go:227 +0x160
  text/template.(*Template).AddParseTree()
      /Users/josh/go/1.16/src/text/template/template.go:133 +0x218
  html/template.(*escaper).commit()
      /Users/josh/go/1.16/src/html/template/escape.go:812 +0x1fc
  html/template.escapeTemplate()
      /Users/josh/go/1.16/src/html/template/escape.go:38 +0x264
  html/template.(*Template).lookupAndEscapeTemplate()
      /Users/josh/go/1.16/src/html/template/template.go:163 +0x2a8
  html/template.(*Template).ExecuteTemplate()
      /Users/josh/go/1.16/src/html/template/template.go:135 +0x38
  main.main.func1()
      /Users/josh/grrr/x.go:28 +0x94

Previous read at 0x00c00011c540 by goroutine 34:
  runtime.evacuate_fast64()
      /Users/josh/go/1.16/src/runtime/map_fast64.go:375 +0x3dc
  text/template.(*state).walkTemplate()
      /Users/josh/go/1.16/src/text/template/exec.go:404 +0xf0
  text/template.(*state).walk()
      /Users/josh/go/1.16/src/text/template/exec.go:269 +0x214
  text/template.(*state).walk()
      /Users/josh/go/1.16/src/text/template/exec.go:264 +0x140
  text/template.(*Template).execute()
      /Users/josh/go/1.16/src/text/template/exec.go:220 +0x214
  text/template.(*Template).Execute()
      /Users/josh/go/1.16/src/text/template/exec.go:203 +0xb8
  html/template.(*Template).ExecuteTemplate()
      /Users/josh/go/1.16/src/html/template/template.go:139 +0x7c
  main.main.func1()
      /Users/josh/grrr/x.go:28 +0x94

Goroutine 35 (running) created at:
  main.main()
      /Users/josh/grrr/x.go:26 +0x21c

Goroutine 34 (finished) created at:
  main.main()
      /Users/josh/grrr/x.go:26 +0x21c
==================
==================
WARNING: DATA RACE
Write at 0x00c00011f020 by goroutine 35:
  text/template.(*Template).associate()
      /Users/josh/go/1.16/src/text/template/template.go:227 +0x174
  text/template.(*Template).AddParseTree()
      /Users/josh/go/1.16/src/text/template/template.go:133 +0x218
  html/template.(*escaper).commit()
      /Users/josh/go/1.16/src/html/template/escape.go:812 +0x1fc
  html/template.escapeTemplate()
      /Users/josh/go/1.16/src/html/template/escape.go:38 +0x264
  html/template.(*Template).lookupAndEscapeTemplate()
      /Users/josh/go/1.16/src/html/template/template.go:163 +0x2a8
  html/template.(*Template).ExecuteTemplate()
      /Users/josh/go/1.16/src/html/template/template.go:135 +0x38
  main.main.func1()
      /Users/josh/grrr/x.go:28 +0x94

Previous read at 0x00c00011f020 by goroutine 12:
  text/template.(*state).walkTemplate()
      /Users/josh/go/1.16/src/text/template/exec.go:404 +0x100
  text/template.(*state).walk()
      /Users/josh/go/1.16/src/text/template/exec.go:269 +0x214
  text/template.(*state).walk()
      /Users/josh/go/1.16/src/text/template/exec.go:264 +0x140
  text/template.(*state).walkTemplate()
      /Users/josh/go/1.16/src/text/template/exec.go:418 +0x2f4
  text/template.(*state).walk()
      /Users/josh/go/1.16/src/text/template/exec.go:269 +0x214
  text/template.(*state).walk()
      /Users/josh/go/1.16/src/text/template/exec.go:264 +0x140
  text/template.(*Template).execute()
      /Users/josh/go/1.16/src/text/template/exec.go:220 +0x214
  text/template.(*Template).Execute()
      /Users/josh/go/1.16/src/text/template/exec.go:203 +0xb8
  html/template.(*Template).ExecuteTemplate()
      /Users/josh/go/1.16/src/html/template/template.go:139 +0x7c
  main.main.func1()
      /Users/josh/grrr/x.go:28 +0x94

Goroutine 35 (running) created at:
  main.main()
      /Users/josh/grrr/x.go:26 +0x21c

Goroutine 12 (finished) created at:
  main.main()
      /Users/josh/grrr/x.go:26 +0x21c
==================
Found 2 data race(s)
exit status 66
@bradfitz
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bradfitz commented Jul 2, 2021

This is from 1.16. Did you also try 1.17beta1?

@josharian
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Yep. Doesn't reproduce on 1.17beta.

@bradfitz
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bradfitz commented Jul 2, 2021

Still might be worth tracking for a cherry pick to 1.16.

@josharian
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496d7c6 is the fix.

@gopherbot, please open backport issues.

This was referenced Jul 2, 2021
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@gopherbot
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Backport issue(s) opened: #47041 (for 1.15), #47042 (for 1.16).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://golang.org/wiki/MinorReleases.

@dmitshur dmitshur added the NeedsFix The path to resolution is known, but the work has not been done. label Jul 9, 2021
@dmitshur dmitshur added this to the Go1.17 milestone Jul 9, 2021
@golang golang locked and limited conversation to collaborators Jul 9, 2022
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