Add arm/v6 cross-compiled build

[dagood]

• For Add linux-armv6l builders (armhf) #519

Includes related changes:

• Update bootstrap Go to 1.18 for strings.Cut.
• Add missing err checks in pack code.
• Diff reduction: include directory entries in the tar.gz/zip, use / rather than \ for zips even though they're for Windows.

To check the results, I cherry-picked to 1.18 and compared the file lists on the platforms we're currently building:

#> diff <(tar -tf go1.18.linux-amd64.tar.gz | sort) <(tar -tf eng/artifacts/bin/go.dev.linux-amd64.tar.gz | sort)
29d28
< go/codereview.cfg

#> diff <(tar -tf go1.18.linux-armv6l.tar.gz | sort) <(tar -tf eng/artifacts/bin/go.dev.linux-armv6l.tar.gz | sort)
29d28
< go/codereview.cfg
802d800
< go/pkg/linux_arm/os/signal/
804,805d801
< go/pkg/linux_arm/os/signal/internal/
< go/pkg/linux_arm/os/signal/internal/pty.a
821d816
< go/pkg/linux_arm/runtime/cgo.a

# (Copied from a non-cross build on a Windows machine.)
#> diff <(unzip -Z1 go1.18.windows-amd64.zip | sort) <(unzip -Z1 eng/artifacts/bin/go.dev.windows-amd64.zip | sort)
29d28
< go/codereview.cfg
598d596
< go/pkg/tool/windows_amd64/api.exe
11366,11367c11364,11365
< go/test/fixedbugs/issue27836.dir/Äfoo.go
< go/test/fixedbugs/issue27836.dir/Ämain.go
---
> go/test/fixedbugs/issue27836.dir/├Дfoo.go
> go/test/fixedbugs/issue27836.dir/├Дmain.go

• Missing go/codereview.cfg is normal.
• go/pkg/linux_arm/os/signal/internal/pty.a is interesting. According to the source, it's only used to run tests.
• go/pkg/linux_arm/runtime/cgo.a is probably missing because of the cross-compile: something like not being able to build cgo without running a C compiler that targets arm/v6. This might not be an issue, because when someone needs cgo their copy of Go might automatically build cgo if this .a prebuilt file is missing.
  • Building a simple cgo app works fine without this file. When I run go tool dist test, this file is built.
• go/pkg/tool/windows_amd64/api.exe is an existing diff. I think it's actually an upstream bug--filed x/build/cmd/release: Windows zip/installer includes unnecessary pkg/tool/windows_amd64/api.exe golang/go#52335
• go/test/fixedbugs/issue27836.dir/├Дfoo.go is an existing diff, not new to this change. However, it seems to only be a problem with unzip -Z1. When I extract the file on Windows, it ends up being Ä, not ├Д.

---

As a very basic check that the cross-compiled Go works, I installed Debian Bullseye armhf (arm/v7) onto my RPi 4 (arm64 chip), and on there, the armv6l build from CI works to:

• Build a basic app that uses cgo
• Run go tool dist test.

Our partners are going to be using armhf, so this is probably good enough for now.

[jaredpar]

One thing to be careful about is how block execution and variable scoping works in powershell. I've gotten burned by this in the past because I made bad assumptions about how name lookup works here. IIRC the variables are not actually captured, aka it's not a lambda, but instead the variable lookup occurs in the context where the block is evaluated.

Been many years since I got burned here so maybe I'm having bad memory but I would look into this a bit.

[dagood]

That's right, no capturing.

In this particular case, capture vs. non-capture doesn't have any impact because Invoke-CrossGoBlock doesn't set any variables that are being used in the block, the block is defined right here where it's being passed to the function, and the block doesn't escape. But definitely something to be aware of while modifying these scripts.

I could inline Invoke-CrossGoBlock to avoid the weirdness, but we rely on blocks for Invoke-WithRetry too, so it wouldn't really remove the need to know about this quirk.

I'm glad we're able to get out of this script and into the _core Go module relatively quickly. 😄

[dagood]

I ran an internal build on the latest commit: https://dev.azure.com/dnceng/internal/_build/results?buildId=1718680&view=results, and I tried ingesting it into go-images, but there are a couple things to fix:

The build asset json lists the build as "env": { "GOARCH": "armv6l", "GOOS": "linux" }, but it should be "GOARCH": "arm", "GOARM": "6". This requires a fix to go-infra and updating the version in eng/_util, so I'll fix that in this PR.

The dockerupdate command doesn't update the manifest.json file quite right. This fix is just on the go-infra side.

[dagood]

Build asset JSON from new dev internal build looks good: https://dev.azure.com/dnceng/internal/_build/results?buildId=1727301&view=results. Generates expected armhf results in go-images with latest dockerupdate.