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fmt: cannot read more than 254 characters in Windows when calling Fscanln #42551

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jimen0 opened this issue Nov 12, 2020 · 4 comments
Open

fmt: cannot read more than 254 characters in Windows when calling Fscanln #42551

jimen0 opened this issue Nov 12, 2020 · 4 comments
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NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. OS-Windows
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Backlog

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@jimen0
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jimen0 commented Nov 12, 2020 •
edited

What version of Go are you using (go version)?

$ go version
go version go1.15.4 darwin/amd64
$ gotip version
go version devel +b34b0aaf69 Thu Nov 12 15:28:43 2020 +0000 darwin/amd64

Does this issue reproduce with the latest release?

Yes. It also reproduces with current tip go version devel +b34b0aaf69 Thu Nov 12 15:28:43 2020 +0000 darwin/amd64

What operating system and processor architecture are you using (go env)?

go env Output
$ go env
N/A

What did you do?

package main

import (
	"fmt"
	"os"
)

func main() {
	var s string
	_, err := fmt.Fscanln(os.Stdin, &s)
	if err != nil {
		panic(err)
	}

	fmt.Println(s)
}
$ GOOS=windows go build -o reproducer_stdin.exe main.go
$ GOOS=windows gotip build -o reproducer_stdin_tip.exe main.go

What did you expect to see?

On the Windows host, I type A character 257 times after running the program, then press enter key. The Go program reads them correctly.

PS C:\Users\mig\Downloads> .\reproducer_stdin_tip.exe | Measure-Object -char | Select-Object -expand Characters
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
257
PS C:\Users\mig\Downloads> .\reproducer_stdin.exe | Measure-Object -char | Select-Object -expand Characters
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
257

What did you see instead?

On the Windows host, I typed A character 257 times after running the program, then presed enter key. The Go program stdin froze after 254 characters.

PS C:\Users\mig\Downloads> .\reproducer_stdin_tip.exe | Measure-Object -char | Select-Object -expand Characters
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
254
PS C:\Users\mig\Downloads> .\reproducer_stdin.exe | Measure-Object -char | Select-Object -expand Characters
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
254

NOTE: this seems to also be borken when copying&pasting text.

Note: this was tested using a Windows (Windows Server 2012 R2 Datacenter) Azure machine with default settings and environment. Just downloaded the cross-compiled binaries into it and ran them.
Edit: This is also reproducible using Windows Command Prompt using an Azure Windows (Windows 10 Pro) default machine.

I don't really know if this is related to Go at all or an issue with Powershell itself, but I thought it was worth submitting an issue. This was initially discovered by @karelorigin.

Best regards,
Miguel
@jimen0 jimen0 changed the title fmt/Fscanln: cannot read more than 254 characters in Windows Powershell fmt/Fscanln: cannot read more than 254 characters in Windows Nov 12, 2020
@cagedmantis cagedmantis changed the title fmt/Fscanln: cannot read more than 254 characters in Windows fmt: cannot read more than 254 characters in Windows when calling Fscanln Nov 13, 2020
@cagedmantis cagedmantis added OS-Windows NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. labels Nov 13, 2020
@cagedmantis cagedmantis added this to the Backlog milestone Nov 16, 2020
@networkimprov
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cc @alexbrainman @mattn

@reznik99
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reznik99 commented Jan 4, 2022

Has it been confirmed if it's a Go issue?
I have tried CMD and Powershell still having this issue.

Input is truncated to a maximum of 254 characters whether typing or pasting.
Works fine on Linux build

@sgtxn
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sgtxn commented Feb 4, 2023 •
edited

Same, stumbled upon this today when trying to paste a long authorization code into fmt.Scan input, seems to occur no matter if I'm using the old cmd.exe, the new Windows Terminal or the VSCode built-in terminal. Also occurs in cmd, Powershell and Git Bash.

Wrapping os.Stdin in bufio.NewReader solves the issue though:

var code string
in := bufio.NewReader(os.Stdin)
if _, err := fmt.Fscan(in, &code); err != nil {
    log.Fatal(err)
}

@karelorigin
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@reznik99, verifying that it is Go specific is actually quite simple. Try the following python script:

s = input("input more than 254 characters:\n")
print("Received: " + s)

You can execute this in your browser devtools to quickly generate a long string:

copy("a".repeat(254) + "b")
> python3 input.py
input more than 254 characters:
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaab
Received: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaab

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@cagedmantis @networkimprov @jimen0 @reznik99 @karelorigin @sgtxn