Resolve environment variable values

Fix joke2k#421: add support for embedded variables.

environ/environ.py

@@ -17,8 +17,10 @@
 import os
 import re
 import sys
+import threading
 import urllib.parse as urlparselib
 import warnings
+from os.path import expandvars
 from urllib.parse import (
     parse_qs,
     ParseResult,
@@ -187,7 +189,10 @@ class Env:
     }
     CLOUDSQL = 'cloudsql'
 
+    VAR = re.compile(r'(?<!\\)\$\{?(?P<name>[A-Z_][0-9A-Z_]*)}?', re.IGNORECASE)
+
     def __init__(self, **scheme):
+        self._local = threading.local()
         self.smart_cast = True
         self.escape_proxy = False
         self.prefix = ""
@@ -358,9 +363,12 @@ def path(self, var, default=NOTSET, **kwargs):
         """
         return Path(self.get_value(var, default=default), **kwargs)
 
-    def get_value(self, var, cast=None, default=NOTSET, parse_default=False):
+    def get_value(self, var, cast=None, default=NOTSET, parse_default=False, add_prefix=True):
         """Return value for given environment variable.
 
+        - Substitute environment variable values.
+        - Detect infinite recursion in values (self-reference).
+
         :param str var:
             Name of variable.
         :param collections.abc.Callable or None cast:
@@ -369,15 +377,30 @@ def get_value(self, var, cast=None, default=NOTSET, parse_default=False):
              If var not present in environ, return this instead.
         :param bool parse_default:
             Force to parse default.
+        :param bool add_prefix:
+            Whether to add prefix to variable name.
         :returns: Value from environment or default (if set).
         :rtype: typing.IO[typing.Any]
         """
 
+        var_name = "{}{}".format(self.prefix, var) if add_prefix else var
+        if not hasattr(self._local, 'vars'):
+            self._local.vars = set()
+        if var_name in self._local.vars:
+            error_msg = "Environment variable '{}' recursively references itself (eventually)".format(var_name)
+            raise ImproperlyConfigured(error_msg)
+
+        self._local.vars.add(var_name)
+        try:
+            return self._get_value(var_name, cast=cast, default=default, parse_default=parse_default)
+        finally:
+            self._local.vars.remove(var_name)
+
+    def _get_value(self, var_name, cast=None, default=NOTSET, parse_default=False):
         logger.debug("get '{}' casted as '{}' with default '{}'".format(
-            var, cast, default
+            var_name, cast, default
         ))
 
-        var_name = "{}{}".format(self.prefix, var)
         if var_name in self.scheme:
             var_info = self.scheme[var_name]
 
@@ -403,26 +426,28 @@ def get_value(self, var, cast=None, default=NOTSET, parse_default=False):
             value = self.ENVIRON[var_name]
         except KeyError as exc:
             if default is self.NOTSET:
-                error_msg = "Set the {} environment variable".format(var)
+                error_msg = "Set the {} environment variable".format(var_name)
                 raise ImproperlyConfigured(error_msg) from exc
 
             value = default
 
+        # Substitute environment variables
+        if isinstance(value, (str, bytes)) and var_name != 'DJANGO_SECRET_KEY':
+            def repl(m):
+                return self.get_value(m['name'], cast=cast, default=default,
+                                      parse_default=parse_default, add_prefix=False)
+            value = self.VAR.sub(repl, value)
+            value = expandvars(value)
+
         # Resolve any proxied values
         prefix = b'$' if isinstance(value, bytes) else '$'
         escape = rb'\$' if isinstance(value, bytes) else r'\$'
-        if hasattr(value, 'startswith') and value.startswith(prefix):
-            value = value.lstrip(prefix)
-            value = self.get_value(value, cast=cast, default=default)
-
         if self.escape_proxy and hasattr(value, 'replace'):
             value = value.replace(escape, prefix)
 
         # Smart casting
-        if self.smart_cast:
-            if cast is None and default is not None and \
-                    not isinstance(default, NoValue):
-                cast = type(default)
+        if self.smart_cast and cast is None and default is not None and not isinstance(default, NoValue):
+            cast = type(default)
 
         value = None if default is None and value == '' else value