x/build: no error for missing file in windows workspace

[findleyr]

CL 430915 failed mysteriously with a seemingly missing file:
https://storage.googleapis.com/go-build-log/706d84fc/windows-386-2008_7e064017.log

The reason, we discovered after testing on a local windows machine, is that AUX (including aux.go) is a reserved name on windows:
https://learn.microsoft.com/en-us/windows/win32/fileio/naming-a-file#naming-conventions

Git produces "error: invalid path 'gopls/internal/lsp/generator/aux.go'". I don't think the windows TryBots use git directly, but it is surprising to me that the trybot run did not fail eagerly if it was unable to write a file to the workspace.

[cherrymui]

cc @golang/release

[dmitshur]

Notes from a quick look.

It is correctly reporting an error during the "PutTar" operation (which is used to place the code on the builder) on the windows-amd64-2016 TryBot:

  builder: windows-amd64-2016
      rev: 706d84fca2b36fdf670a0d921e6a8a3b481eaa05
 buildlet: http://10.128.0.48 GCE VM: buildlet-windows-amd64-2016-rnc0afa66
  started: 2022-09-20 00:02:45.720474088 +0000 UTC m=+288958.319726496
    ended: 2022-09-20 00:03:36.922840305 +0000 UTC m=+289009.522092725
  success: false

Events:
  2022-09-20T00:02:45Z get_buildlet 
  2022-09-20T00:03:24Z finish_get_buildlet after 39s
  2022-09-20T00:03:24Z using_buildlet 10.128.0.48:80
  2022-09-20T00:03:24Z write_snapshot_tar 
  2022-09-20T00:03:35Z finish_write_snapshot_tar after 10.7s
  2022-09-20T00:03:35Z make_and_test 
  2022-09-20T00:03:35Z fetching_subrepo tools
  2022-09-20T00:03:35Z get_source tools@81cf9a22b1c30e1effc2c4811a760300c7413b71
  2022-09-20T00:03:35Z finish_get_source after 0s; tools@81cf9a22b1c30e1effc2c4811a760300c7413b71
  2022-09-20T00:03:36Z finish_make_and_test after 992.4ms; err=runTests: 500 Internal Server Error; body: open C:\workdir\gopath\src\golang.org\x\tools\gopls\internal\lsp\protocol\generator\aux.go: Access is denied.


Build log:
windows-amd64-2016 at 706d84fca2b36fdf670a0d921e6a8a3b481eaa05 building tools at 81cf9a22b1c30e1effc2c4811a760300c7413b71



Error: runTests: 500 Internal Server Error; body: open C:\workdir\gopath\src\golang.org\x\tools\gopls\internal\lsp\protocol\generator\aux.go: Access is denied.

Though coordinator considers it an internal error and just keeps retrying (seemingly indefinitely due to #36226) rather than posting that as a failure message. That makes it harder to see (I got it from the TryBot status page, in between the retries).

It's a mystery to me what's different about the windows-386-2008 TryBot to cause it not to happen there too.