docker_registry.app: Add a /_versions endpoint

For debugging it's good to know what package versions folks are
running.  This endpoint allows folks using floating dependencies (or
just different versions of docker-registry) to figure out the exact
version of all of their Python dependencies.

The only basic dependency that *doesn't* define a __version__ is
itsdangerous (as of v0.24), which doesn't provide a
programmatically-accessible version number.  I haven't checked all the
peripheral dependencies outside of what gets pulled in by a stock:

  $ pip install depends/docker-registry-core/
  $ pip install .

To avoid accidentally leaking implementation details, the /_versions
endpoint is disabled in the common config_sample.yml block, and only
enabled by default in the dev block.  In both cases, you can use the
DEBUG_VERSIONS environment variable to enable/disable the endpoint for
your particular instance.

Author: wking

README.md

@@ -149,6 +149,7 @@ When using the `config_sample.yml`, you can pass all options through as environm
 1. `loglevel`: string, level of debugging. Any of python's
    [logging](http://docs.python.org/2/library/logging.html) module levels:
    `debug`, `info`, `warn`, `error` or `critical`
+1. `debug_versions`: boolean, enable the `/_versions` endpoint for debugging.
 1. `storage_redirect`: Redirect resource requested if storage engine supports
    this, e.g. S3 will redirect signed URLs, this can be used to offload the
    server.

config/config_sample.yml

@@ -2,6 +2,8 @@
 common: &common
     # Default log level is info
     loglevel: _env:LOGLEVEL:info
+    # Enable the debugging /_versions endpoint
+    debug_versions: _env:DEBUG_VERSIONS:false
     # By default, the registry acts standalone (eg: doesn't query the index)
     standalone: _env:STANDALONE:true
     # The default endpoint to use (if NOT standalone) is index.docker.io
@@ -143,6 +145,7 @@ elliptics:
 dev: &dev
     <<: *local
     loglevel: _env:LOGLEVEL:debug
+    debug_versions: _env:DEBUG_VERSIONS:true
     search_backend: _env:SEARCH_BACKEND:sqlalchemy
 
 # This flavor is used by unit tests

docker_registry/app.py

@@ -3,6 +3,7 @@
 import logging
 import logging.handlers
 import os
+import sys
 
 try:
     import bugsnag
@@ -32,6 +33,30 @@ def ping():
     return toolkit.response(headers=headers)
 
 
+@app.route('/_versions')
+@app.route('/v1/_versions')
+def versions():
+    """Return a JSON object ({"package-name": "package-version", ...}).
+
+    This is an unofficial endpoint for debugging your docker-registry
+    install.  If you're running a publicly-accessible endpoint, it's
+    probably best to disable this endpoint to avoid leaking
+    implementation details.
+    """
+    versions = {}
+    if cfg.debug_versions:
+        for name, module in sys.modules.items():
+            if name.startswith('_'):
+                continue
+            try:
+                version = module.__version__
+            except AttributeError:
+                continue
+            versions[name] = version
+        versions['python'] = sys.version
+    return toolkit.response(versions)
+
+
 @app.route('/')
 def root():
     return toolkit.response('docker-registry server ({0}) (v{1})'