Log ComSpec and add diagnostic to validate it

[karthiknadig]

We see this often where ComSpec env variable is some how set to invalid path. We should detect this and log. This happens way too often.

Essentially the ENOENT (-4058) error with -c " " error usually means that Node exec tried to get default shell on Windows using ComSpec and since ComSpec was wrong it was failing while trying to shell execute python commands.

Originally posted by @karthiknadig in #16684 (comment)

[karthiknadig]

This should also log if PATH does not have C:\Windows\System32. see related issue: #18138

/cc @karrtikr Just FYI.

[karrtikr]

For that user ComSpec was set #18138 (comment), I don't understand why node ran using cmd.exe instead of the value in the env variable. The fact whether C:\Windows\System32 is not PATH should not matter as long as it was set.

Documentation says cmd.exe is only used as a fallback: https://nodejs.org/api/child_process.html#default-windows-shell.

[karrtikr]

Perhaps it'll be sufficient to make sure it carries:

• A path that exists.
• It's a file and not a folder.

Note according to https://postgrespro.com/list/thread-id/1529515 it may also end with a semicolon, so some investigation may also be required to double check the possibilities of the values it can carry.

[karrtikr]

Few known causes for such an error:

• Try setting the "ComSpec" environment variable to a valid Command Prompt path in your system to fix it: https://aka.ms/AAk3djo.
• Please edit "Path" environment it to make sure it contains the "SystemRoot" subdirectories: https://aka.ms/AAk744c.

[karrtikr]

Verification steps:

ONLY ON WINDOWS

• Set ComSpec to an invalid value and load Python extension: