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

[Shengjing Zhu]

Public bug reported:

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=[retracted]=. -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.

** Affects: dh-golang (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

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=[retracted]=. -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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