Make image conversion use a proper python interpreter for prlimit

The image conversion plugin does a processutils exec(), which needs
to spawn python for prlimit support. Under uwsgi, sys.executable
points to uwsgi itself, which won't work in this case. This introduces
a [wsgi]/python_interpreter config option (because I don't think
there is any way to get this from uwsgi itself) which we use for
the exec. By default, it's sys.executable, which is what is used
right now so nobody should notice a change unless they need it.

Note: Making this depend on the devstack change to remove the wsgi
import restriction so we can get a test on it.

Partial-Bug: #1888713
Change-Id: I7cb2e135d6ea2cb21de55060df3f7bf40b3e64b6

.zuul.yaml

@@ -242,6 +242,13 @@
           $GLANCE_API_CONF:
             DEFAULT:
               enabled_import_methods: "[\"copy-image\", \"glance-direct\"]"
+            wsgi:
+              python_interpreter: /usr/bin/python3
+          $GLANCE_IMAGE_IMPORT_CONF:
+            image_import_opts:
+              image_import_plugins: "['image_conversion']"
+            image_conversion:
+              output_format: raw
 
 - project:
     templates:

glance/async_/flows/plugins/image_conversion.py

@@ -70,6 +70,7 @@ class _ConvertImage(task.Task):
         self.image_repo = image_repo
         self.image_id = image_id
         self.dest_path = ""
+        self.python = CONF.wsgi.python_interpreter
         super(_ConvertImage, self).__init__(
             name='%s-Convert_Image-%s' % (task_type, task_id))
 
@@ -88,6 +89,7 @@ class _ConvertImage(task.Task):
                                            "--output=json",
                                            src_path,
                                            prlimit=utils.QEMU_IMG_PROC_LIMITS,
+                                           python_exec=self.python,
                                            log_errors=putils.LOG_ALL_ERRORS,)
         except OSError as exc:
             with excutils.save_and_reraise_exception():

glance/common/config.py

@@ -19,6 +19,7 @@ Routines for configuring Glance
 
 import logging
 import os
+import sys
 
 from oslo_config import cfg
 from oslo_middleware import cors
@@ -578,6 +579,14 @@ too large, you *may* have increased memory footprint per worker and/or you
 may overwhelm other system resources such as disk or outbound network
 bandwidth. If this is too small, image import requests will have to wait
 until a thread becomes available to begin processing.""")),
+    cfg.StrOpt('python_interpreter',
+               default=sys.executable,
+               help=_("""
+Path to the python interpreter to use when spawning external
+processes. By default this is sys.executable, which should be the
+same interpreter running Glance itself. However, in some situations
+(i.e. uwsgi) this may not actually point to a python interpreter
+itself.""")),
 ]