the "How do you want to open this file?" dialog has resurfaced on windows 10 arm64 instances. with a little debugging (selecting notepad as the file association), i see that the file in question contains the following text:

#!/bin/sh
# Copyright (C) 2002, Earnie Boyd
#   mailto:earnie@users.sf.net
# This file is part of Minimal SYStem.
#   http://www.mingw.org/msys.shtml
# File: cmd  "$COMSPEC" "$@"

i'm guessing that something in the mozilla-build install (the installer contains msys) is triggering this.

i will do some more debugging and try to find a way to squash this.

after a little more debugging, i determined that:

• the file-path being opened during the mozilla-build/msys install is C:\mozilla-build\msys\bin\cmd which contains:

  #!/bin/sh
  # Copyright (C) 2002, Earnie Boyd
  #   mailto:earnie@users.sf.net
  # This file is part of Minimal SYStem.
  #   http://www.mingw.org/msys.shtml
  # File: cmd
  "$COMSPEC" "$@"
  

• crucially, the system PATH environment variable incorrectly contained an entry for C:\mozilla-build\msys\bin before the entry for C:\Windows\System32

• the default value setting of the system COMSPEC environment variable is CMD

these settings create an undesired or unexpected condition when the command cmd is run without a .exe suffix since the incorrect PATH setting causes cmd to be invoked at C:\mozilla-build\msys\bin\cmd instead of C:\Windows\System32\cmd.exe.

the invocation of cmd by some/any process is then not seen as an executable by the operating system since windows does not treat a file as an executable if it does not have an extension that matches one contained in the PATHEXT environment variable. instead, windows treats invocations of non-executables as a request to open a file with a name matching the invocation.

hence on our infra, whenever something invokes cmd on a system where the PATH contains a C:\mozilla-build\msys\bin entry before its C:\Windows\System32 entry, windows assumes we want to open the text file at C:\mozilla-build\msys\bin\cmd instead of invoking C:\Windows\System32\cmd.exe which is normally what the caller expects to happen.

the fix is to simply correct the PATH environment variable so that the C:\Windows\System32 entry precedes the C:\mozilla-build\msys\bin entry.

in occ, we use a dsc component to set the PATH environment variable. until now, this was achieved by prepending any entries in the manifest component to whatever was already in the system PATH. eg:

    {
      "ComponentName": "env_PATH",
      "ComponentType": "EnvironmentVariableUniquePrepend",
      "Name": "PATH",
      "Values": [
        ...,
        "C:\\mozilla-build\\msys\\bin",
        ...
      ],
      "Target": "Machine"
    },

would result in a PATH var containing: C:\mozilla-build\msys\bin;whatever-was-already-in-PATH

swapping EnvironmentVariableUniquePrepend for EnvironmentVariableUniqueAppend would have the desired effect on any new system and result in a PATH var containing: whatever-was-already-in-PATH;C:\mozilla-build\msys\bin.

however, i wanted a fix that would also correct any running systems that already have C:\mozilla-build\msys\bin in PATH in the wrong order (preceding C:\Windows\System32). so i modified the implementations of EnvironmentVariableUniquePrepend and EnvironmentVariableUniqueAppend so that they first remove any entries from PATH that also exist in the manifest component, before either appending or prepending all the manifest components to the PATH, resulting in the desired order of PATH components.

the fix patch is very simple and is implemented in the aarch64 branch here: https://github.com/mozilla-releng/OpenCloudConfig/commit/27e555c. it will be merged to master in due course