set_env: a quoted value ending with a literal backslash loses its quote protection #4387

Describe the bug

A quoted set_env value loses its quote protection as soon as it ends with a literal backslash and contains a semicolon: tox then reads the remainder of the value as a marker and aborts.

To Reproduce

tox 4.67.11, tox.ini:

[testenv]
package = skip
set_env =
    WINPATH="C:\tools;C:\bin\\"
commands = python -c "import os; print(os.environ['WINPATH'])"

ValueError: invalid marker for set_env key 'WINPATH': Expected a marker variable or quoted string
    C:\bin\\"
    ^
escape a literal ';' in the value as '\;'

Expected

The value stays "C:\tools;C:\bin\\": the quotes are balanced, the ; sits inside them, so there is no marker.

Root cause

SetEnv._split_value_marker decides whether a quote or a semicolon is escaped by inspecting the single preceding character:

if respect_quotes and char in {'"', "'"} and (index == 0 or value[index - 1] != "\\"):

For "C:\tools;C:\bin\\" the closing quote is preceded by \\, which is a literal backslash rather than an escaped quote. The first pass therefore never closes the quote and ends with in_quotes = True, so the method retries with respect_quotes = False, splits on the first ; inside the quotes and turns the rest into a marker.

Counting the run of backslashes (odd = escaped, even = literal) fixes this, and makes \\; separate correctly as well.

Workaround

Escape the semicolons instead of quoting: WINPATH=C:\tools\;C:\bin\\.