Fixed the incorrect operation when setuptools plugins output something into stdout.

docs/changelog/2335.bugfix.rst

@@ -0,0 +1 @@
+Fix the incorrect operation when ``setuptools`` plugins output something into ``stdout``.

src/virtualenv/discovery/cached_py_info.py

@@ -8,8 +8,10 @@
 
 import logging
 import os
+import random
 import sys
 from collections import OrderedDict
+from string import ascii_lowercase, ascii_uppercase, digits
 
 from virtualenv.app_data import AppDataDisabled
 from virtualenv.discovery.py_info import PythonInfo
@@ -85,10 +87,27 @@ def _get_via_file_cache(cls, app_data, path, exe, env):
     return py_info
 
 
+COOKIE_LENGTH = 32  # type: int
+
+
+def gen_cookie():
+    return "".join(random.choice("".join((ascii_lowercase, ascii_uppercase, digits))) for _ in range(COOKIE_LENGTH))
+
+
 def _run_subprocess(cls, exe, app_data, env):
     py_info_script = Path(os.path.abspath(__file__)).parent / "py_info.py"
+    # Cookies allow to split the serialized stdout output generated by the script collecting the info from the output
+    # generated by something else. The right way to deal with it is to create an anonymous pipe and pass its descriptor
+    # to the child and output to it. But AFAIK all of them are either not cross-platform or too big to implement and are
+    # not in the stdlib. So the easiest and the shortest way I could mind is just using the cookies.
+    # We generate pseudorandom cookies because it easy to implement and avoids breakage from outputting modules source
+    # code, i.e. by debug output libraries. We reverse the cookies to avoid breakages resulting from variable values
+    # appearing in debug output.
+
+    start_cookie = gen_cookie()
+    end_cookie = gen_cookie()
     with app_data.ensure_extracted(py_info_script) as py_info_script:
-        cmd = [exe, str(py_info_script)]
+        cmd = [exe, str(py_info_script), start_cookie, end_cookie]
         # prevent sys.prefix from leaking into the child process - see https://bugs.python.org/issue22490
         env = env.copy()
         env.pop("__PYVENV_LAUNCHER__", None)
@@ -108,6 +127,26 @@ def _run_subprocess(cls, exe, app_data, env):
             out, err, code = "", os_error.strerror, os_error.errno
     result, failure = None, None
     if code == 0:
+        out_starts = out.find(start_cookie[::-1])
+
+        if out_starts > -1:
+            pre_cookie = out[:out_starts]
+
+            if pre_cookie:
+                sys.stdout.write(pre_cookie)
+
+            out = out[out_starts + COOKIE_LENGTH :]
+
+        out_ends = out.find(end_cookie[::-1])
+
+        if out_ends > -1:
+            post_cookie = out[out_ends + COOKIE_LENGTH :]
+
+            if post_cookie:
+                sys.stdout.write(post_cookie)
+
+            out = out[:out_ends]
+
         result = cls._from_json(out)
         result.executable = exe  # keep original executable as this may contain initialization code
     else:

src/virtualenv/discovery/py_info.py

@@ -524,4 +524,21 @@ def _possible_base(self):
 if __name__ == "__main__":
     # dump a JSON representation of the current python
     # noinspection PyProtectedMember
-    print(PythonInfo()._to_json())
+    argv = sys.argv[1:]
+
+    if len(argv) >= 1:
+        start_cookie = argv[0]
+        argv = argv[1:]
+    else:
+        start_cookie = ""
+
+    if len(argv) >= 1:
+        end_cookie = argv[0]
+        argv = argv[1:]
+    else:
+        end_cookie = ""
+
+    sys.argv = sys.argv[:1] + argv
+
+    info = PythonInfo()._to_json()
+    sys.stdout.write("".join((start_cookie[::-1], info, end_cookie[::-1])))