check whether symlink exists

[hallyn]

What type of PR is this?

Which issue does this PR fix:

What does this PR do / Why do we need it:

If an issue # is not available please add repro steps and logs showing the issue:

Testing done on this change:

Automation added to e2e:

Will this break upgrades or downgrades?

Does this PR introduce any user-facing change?:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #534 (9350b59) into main (67d1ffb) will decrease coverage by 0.14%.
Report is 2 commits behind head on main.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #534      +/-   ##
==========================================
- Coverage   13.14%   13.01%   -0.14%     
==========================================
  Files          40       40              
  Lines        5943     6003      +60     
==========================================
  Hits          781      781              
- Misses       5034     5094      +60     
  Partials      128      128              

Files	Coverage Δ
pkg/overlay/pack.go	0.00% <0.00%> (ø)

... and 4 files with indirect coverage changes

📣 Codecov offers a browser extension for seamless coverage viewing on GitHub. Try it in Chrome or Firefox today!

[rchincha]

lgtm but still need to understand whether our test cases are incomplete since we didn't need this check until now.

[hallyn]

install-base:
  build_only: true
  from:
    type: docker
    url: "docker://zothub.io/machine/bootkit/rootfs:v0.0.17.231018-squashfs"
    
install-rootfs-pkg:
  from:
    type: built
    tag: install-base
  build_only: true
  run: |
    #!/bin/sh -ex
    writefile() {
      mkdir -p "${1%/*}"
      echo "write $1" 1>&2
      cat >"$1"
    }

    writefile /etc/systemd/network/20-wire-enp0s-dhcp.network <<"END"
    [Match]
    Name=enp0s*
    [Network]
    DHCP=yes
    END

demo-zot:
  from:
    type: built
    tag: install-rootfs-pkg
  import:
    - x
  run: |
    #!/bin/sh -ex
    cp /stacker/imports/x /usr/bin/x

If I first stacker build --layer-type=squashfs, then --layer-type=tar, then this case broke for me in two ways. The first time, it would give me:

+ cp /stacker/imports/x /usr/bin/x
panic: runtime error: index out of range [-1]

goroutine 1 [running]:
stackerbuild.io/stacker/pkg/overlay.generateLayer({{0x0, 0x0}, {0xc000046270, 0x2b}, {0xc000046570, 0x26}, {0xc0000465d0, 0x28}, 0x0, {0x1bb3f7e, ...}, ...}, ...)
        /stacker-tree/pkg/overlay/pack.go:400 +0x157b
stackerbuild.io/stacker/pkg/overlay.repackOverlay({{0x0, 0x0}, {0xc000046270, 0x2b}, {0xc000046570, 0x26}, {0xc0000465d0, 0x28}, 0x0, {0x1bb3f7e, ...}, ...}, ...)
        /stacker-tree/pkg/overlay/pack.go:602 +0x707
stackerbuild.io/stacker/pkg/overlay.(*overlay).Repack(0xc000288ea0, {0xc0003a5bf0, 0x8}, {0xc000012648, 0x1, 0x1}, 0x4c?)
        /stacker-tree/pkg/overlay/pack.go:292 +0xe5
stackerbuild.io/stacker/pkg/stacker.(*Builder).build(0xc000052b50, {0x3490440, 0xc000288ea0}, {0xc000047ec0, 0x2f})
        /stacker-tree/pkg/stacker/build.go:512 +0x1176
stackerbuild.io/stacker/pkg/stacker.(*Builder).BuildMultiple(0xc000052b50, {0xc000441670, 0x1, 0x1})
        /stacker-tree/pkg/stacker/build.go:610 +0x5ea
main.doBuild(0x4024340?)

The second woudl give me:

+ cp /stacker/imports/x /usr/bin/x
error: failed to create squashfs symlink: symlink /home/serge/delme/stackerfail/orig/roots/sha256_1b0a2913c9959502eb1f534067eec3f5ced3f656f6ea3170cfbe2484e7e484e5 /home/serge/delme/stackerfail/orig/roots/sha256_c46ab27817af8af8deb0bb60493266913dcef9fc76a3e4d5d1a42a3ced6b0196: file exists
error: exit status 1

With this fixed version, it builds the tar layers properly without crashing.

[rchincha]

So this fails then?
#540