gh-154160: Make sys.executable absolute when resolved from a relative PATH entry

[fedonman]

sys.executable is documented as "a string giving the absolute path of the executable binary for the Python interpreter". When the interpreter is started by a bare program name (no separator) and found through a relative directory in PATH, it was left as that relative path instead:

$ python3 -m venv venv
$ cd /tmp && PATH="venv/bin" python3 -c 'import sys; print(sys.executable)'
venv/bin/python3          # expected an absolute path

getpath absolutizes the executable in the sibling branch that handles a program name containing a separator (executable = abspath(program_name)), but the PATH-resolution loop assigned the joined path directly. This makes it call abspath() there too:

$ cd /tmp && PATH="venv/bin" python3 -c 'import sys; print(sys.executable)'
/tmp/venv/bin/python3

This is a regression from the getpath.c rewrite in 3.11 (bpo-45582 / #29041); 3.10 returned an absolute path.

The issue also notes a separate, cosmetic double-slash case (//usr/local/bin/python3 when invoked as usr/local/bin/python3 from /); that comes from a different branch and is not addressed here.

• Issue: sys.executable can be a relative path contrary to documented behavior [3.11 regression] #154160

[python-cla-bot]

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