Might this need something similar to strutils.escape(...) if paths contain spaces or other non-alphanumeric characters?

no, I've found the problem. To add more spaces, the .git folder is deleted in Windows CI.

The .git folder being deleted in the Windows CI evidently indeed was the key issue here, but this still won't work when the sourcePath contains spaces or similar characters.

The current approach in 8773258 isn't sufficient either:

$ ls -l /tmp/staging/
total 4
drwxrwxr-x 3 user user 4096 May 28 18:23 'path with " in it'
$ git -C /tmp/staging/path\ with\ \"\ in\ it/ rev-parse --short=8 HEAD
8dc8fb68
$ cat a.nim
const sourcePath = "/tmp/staging/path with \" in it"
from std/os import dirExists
doAssert dirExists(sourcePath)
echo "directory exists named: " & sourcePath
const foo = staticExec("git -C \"" & sourcePath & "\" rev-parse --short=8 HEAD")
echo "commithash is " & foo
$ ./env nim c -r a
...
directory exists named: /tmp/staging/path with " in it
commithash is /bin/sh: 1: Syntax error: Unterminated quoted string

ha ha forget to escape

7e284dd seems to add extra quotation marks now? strutils.escape already adds them:

import std/strutils
echo strutils.escape("some string")
echo strutils.escape("some")
echo strutils.escape("embedded \" character")

results in

"some string"
"some"
"embedded \" character"

huh?, I thought strutils.escape only escape chars, turn out it also add prefix and suffix.

You introduced this last year: 4893097

What were the conditions then such that it made sense, and have they persisted?

iirc at that time windows runner have less than 14GB of disk space, although the documentation says all OS are given the same disk space. Don't know today.

As long as it the CI builds still work, however much disk space the CI runners have, not an issue, then. If reverting this change triggers disk exhaustion in Windows CI runners, can try other approaches.