"tailscale debug capture" panics on access denied error

[darkrain42]

What is the issue?

Running "tailscale debug capture" as an unprivileged user in tailscale 1.38.1 immediately crashes ("panic: runtime error: invalid memory address or nil pointer dereference").

Works fine run as root (sudo !!)

tailscale 1.38.1 on Ubuntu 22.10

% tailscale debug capture
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x40 pc=0x7c352b]

goroutine 1 [running]:
tailscale.com/client/tailscale.(*LocalClient).StreamDebugCapture(0x968b60?, {0xb1f6e8?, 0xc0000b6000?})
        tailscale.com@v1.38.1/client/tailscale/localclient.go:1104 +0x12b
tailscale.com/cmd/tailscale/cli.runCapture({0xb1f6e8, 0xc0000b6000}, {0xa8d208?, 0x7fe5dcfe25b8?, 0x18?})
        tailscale.com@v1.38.1/cmd/tailscale/cli/debug.go:771 +0x54
github.com/peterbourgon/ff/v3/ffcli.(*Command).Run(0x968b60?, {0xb1f6e8?, 0xc0000b6000?})
        github.com/peterbourgon/ff/v3@v3.1.2/ffcli/command.go:153 +0x16c
github.com/peterbourgon/ff/v3/ffcli.(*Command).Run(0x0?, {0xb1f6e8?, 0xc0000b6000?})
        github.com/peterbourgon/ff/v3@v3.1.2/ffcli/command.go:157 +0x113
github.com/peterbourgon/ff/v3/ffcli.(*Command).Run(0xc0000da630?, {0xb1f6e8?, 0xc0000b6000?})
        github.com/peterbourgon/ff/v3@v3.1.2/ffcli/command.go:157 +0x113
tailscale.com/cmd/tailscale/cli.Run({0xc0000a0160?, 0xc000094000?, 0x0?})
        tailscale.com@v1.38.1/cmd/tailscale/cli/cli.go:175 +0x9ab
main.main()
        tailscale.com@v1.38.1/cmd/tailscale/tailscale.go:22 +0xfb

% gdb --args tailscale debug capture
...
(gdb) run
...
Thread 1 "tailscale" received signal SIGSEGV, Segmentation fault.
tailscale.com/client/tailscale.(*LocalClient).StreamDebugCapture (lc=0xea2e00 <tailscale.com/cmd/tailscale/cli[localClient]>, ctx=..., ~r0=..., ~r1=...) at tailscale.com@v1.38.1/client/tailscale/localclient.go:1104
1104    tailscale.com@v1.38.1/client/tailscale/localclient.go: No such file or directory.
(gdb) p res
$1 = <optimized out>
(gdb) p err
$2 = {
  tab = 0xb1ba60 <go:itab.*tailscale.com/client/tailscale.AccessDeniedError,error>,
  data = 0xc000068ca0
}
(gdb) 

Trivial patch (untested)

diff --git a/client/tailscale/localclient.go b/client/tailscale/localclient.go
index e35846e6..9224f084 100644
--- a/client/tailscale/localclient.go
+++ b/client/tailscale/localclient.go
@@ -1099,11 +1099,10 @@ func (lc *LocalClient) StreamDebugCapture(ctx context.Context) (io.ReadCloser, e
    if err != nil {
        return nil, err
    }
    res, err := lc.doLocalRequestNiceError(req)
    if err != nil {
-       res.Body.Close()
        return nil, err
    }
    if res.StatusCode != 200 {
        res.Body.Close()
        return nil, errors.New(res.Status)

I don't see a path through doLocalRequestNiceError where res is ever non-nil when err is non-nil.

Steps to reproduce

As a non-privileged user (i.e. not root, and have not run "tailscale up --operator $USER"), run "tailscale debug capture".

Are there any recent changes that introduced the issue?

No response

OS

Linux

OS version

Ubuntu 22.10, Debian 11,

Tailscale version

1.38.1

Other software

No response

Bug report

No response