[Bug 2002076] Re: stripping lto flags doesn't work as expected

[Shengjing Zhu]

$bf->strip first split its input (here is `-ffat-lto-objects
-flto=auto`), then use for loop to strip every flag.

```
    my ($self, $flag, $value, $src, $maint) = @_;
    foreach my $tostrip (split(/\s+/, $value)) {
        next unless length $tostrip;
        $self->{flags}->{$flag} =~ s/(^|\s+)\Q$tostrip\E(\s+|$)/ /g;
    }

```

It does use `g` flag in regex, (maybe want to strip all duplicated
flag?)

In first loop, for `-ffat-lto-objects`, it does strip two `-ffat-lto-
objects`. Then the flag becomes `-flto=auto -flto=auto`.

Then in second loop, it stops working. Because there's only one space
between two `-flto=auto`. However the regex expects space before and
after one token. So it only strips one `-flto=auto`.

See https://regex101.com/r/AgDMzv/1 for regex debug.

** Also affects: dpkg (Ubuntu)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/2002076

Title:
  stripping lto flags doesn't work as expected

Status in dh-golang package in Ubuntu:
  New
Status in dpkg package in Ubuntu:
  New

Bug description:
  In dh-golang, we use dpkg's C flags as Go's CGO flags. When dpkg's LTO
  feature is enabled, the LTO C flags may pass to CGO as well. But Go's
  linker doesn't support LTO. So in dh-golang, we implemented a
  workround to strip LTO flag[1].

  [1] https://salsa.debian.org/go-team/packages/dh-
  golang/-/commit/501517bca24c155bdd3ed706ad413a22c625c7a8

  ```
  $bf->strip($flag, "-ffat-lto-objects -flto=auto");
  ```

  This works well in Debian, but it turns out not robust for Ubuntu.

  In Ubuntu, we get such C flags:

  ```
  $ dpkg-buildflags --get CFLAGS
  -g -O2 -ffile-prefix-map=[redacted]=. -flto=auto -ffat-lto-objects -flto=auto -ffat-lto-objects -fstack-protector-strong -Wformat -Werror=format-security

  ```

  It has two duplicated `-flto=auto -ffat-lto-objects`. So dh-golang's
  code doesn't handle it robustly. One `-flto=auto -ffat-lto-objects`
  set is still passed to CGO. This causes link error.

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