Scrub KUBERNETES_ environment from the postmaster (#1407)

The KUBERNETES_ environment variables are not required for PostgreSQL, yet having them exposed to the postmaster will also expose them to backends and to regular database users (using pl/perl for example).

patroni/__init__.py

@@ -9,6 +9,7 @@ from patroni.version import __version__
 logger = logging.getLogger(__name__)
 
 PATRONI_ENV_PREFIX = 'PATRONI_'
+KUBERNETES_ENV_PREFIX = 'KUBERNETES_'
 
 
 class Patroni(object):

patroni/postgresql/postmaster.py

@@ -7,7 +7,7 @@ import signal
 import subprocess
 import sys
 
-from patroni import PATRONI_ENV_PREFIX
+from patroni import PATRONI_ENV_PREFIX, KUBERNETES_ENV_PREFIX
 
 # avoid spawning the resource tracker process
 if sys.version_info >= (3, 8):  # pragma: no cover
@@ -176,7 +176,8 @@ class PostmasterProcess(psutil.Process):
         # In order to make everything portable we can't use fork&exec approach here, so  we will call
         # ourselves and pass list of arguments which must be used to start postgres.
         # On Windows, in order to run a side-by-side assembly the specified env must include a valid SYSTEMROOT.
-        env = {p: os.environ[p] for p in os.environ if not p.startswith(PATRONI_ENV_PREFIX)}
+        env = {p: os.environ[p] for p in os.environ if not p.startswith(
+            PATRONI_ENV_PREFIX) and not p.startswith(KUBERNETES_ENV_PREFIX)}
         try:
             proc = PostmasterProcess._from_pidfile(data_dir)
             if proc and not proc._is_postmaster_process():