stripping lto flags doesn't work as expected

[Shengjing Zhu]

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.

[Shengjing Zhu (zhsj)]

$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.

[Shengjing Zhu (zhsj)]

The best is to write a smart regexp in dpkg. Or just workaround(yeah, workaround on workarund, the lto strip is already a workaround) like:

```
diff -Nru dh-golang-1.59/lib/Debian/Debhelper/Buildsystem/golang.pm dh-golang-1.59ubuntu1/lib/Debian/Debhelper/Buildsystem/golang.pm
--- dh-golang-1.59/lib/Debian/Debhelper/Buildsystem/golang.pm 2022-09-25 11:28:07.000000000 +0000
+++ dh-golang-1.59ubuntu1/lib/Debian/Debhelper/Buildsystem/golang.pm 2023-01-06 09:59:50.000000000 +0000
@@ -380,6 +380,8 @@
     foreach my $flag (@flags) {
         if (! exists $ENV{"CGO_" . $flag}) {
             $bf->strip($flag, "-ffat-lto-objects -flto=auto");
+ # strip twice to workaround #1028044
+ $bf->strip($flag, "-ffat-lto-objects -flto=auto");
             # https://golang.org/issues/54313
             # https://github.com/golang/go/commit/365ca694
             $bf->strip($flag, "-fstack-protector-strong");
```

[Shengjing Zhu (zhsj)]

I'm inclined to do nothing in dh-golang.

This bug is caused by:

+ Ubuntu added lto flags twice, which I filled another bug at LP #2002201 and I have a MP for it.
+ dpkg $bf-strip is not robust, which I filled at https://bugs.debian.org/1028044, and is already fixed in trunk.

Either one is fixed, this bug no longer exists.

[Shengjing Zhu (zhsj)]

LP #2002201 is fixed. So let's make this invalid.