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

[Shengjing Zhu]

** Changed in: dpkg (Ubuntu)
       Status: New => Fix Committed

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Title:
  stripping lto flags doesn't work as expected

Status in dh-golang package in Ubuntu:
  Invalid
Status in dpkg package in Ubuntu:
  Fix Committed

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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