Fix #480 -- Allow plugins to specify a minimum pretix version

doc/development/api/plugins.rst

@@ -49,15 +49,19 @@ description        string               A more verbose description of what your
 visible            boolean (optional)   ``True`` by default, can hide a plugin so it cannot be normally activated.
 restricted         boolean (optional)   ``False`` by default, restricts a plugin such that it can only be enabled
                                         for an event by system administrators / superusers.
+compatibility      string               Specifier for compatible pretix versions.
 ================== ==================== ===========================================================
 
 A working example would be::
 
-    from django.apps import AppConfig
+    try:
+        from pretix.base.plugins import PluginConfig
+    except ImportError:
+        raise RuntimeError("Please use pretix 2.7 or above to run this plugin!")
     from django.utils.translation import ugettext_lazy as _
 
 
-    class PaypalApp(AppConfig):
+    class PaypalApp(PluginConfig):
         name = 'pretix_paypal'
         verbose_name = _("PayPal")
 
@@ -68,6 +72,7 @@ A working example would be::
             visible = True
             restricted = False
             description = _("This plugin allows you to receive payments via PayPal")
+            compatibility = "pretix>=2.7.0"
 
 
     default_app_config = 'pretix_paypal.PaypalApp'

src/pretix/base/plugins.py

@@ -1,8 +1,10 @@
+import sys
 from enum import Enum
 from typing import List
 
-from django.apps import apps
+from django.apps import AppConfig, apps
 from django.conf import settings
+from django.core.exceptions import ImproperlyConfigured
 
 
 class PluginType(Enum):
@@ -39,3 +41,22 @@ def get_all_plugins(event=None) -> List[type]:
         plugins,
         key=lambda m: (0 if m.module.startswith('pretix.') else 1, str(m.name).lower().replace('pretix ', ''))
     )
+
+
+class PluginConfig(AppConfig):
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        if not hasattr(self, 'PretixPluginMeta'):
+            raise ImproperlyConfigured("A pretix plugin config should have a PretixPluginMeta inner class.")
+
+        if hasattr(self.PretixPluginMeta, 'compatibility'):
+            import pkg_resources
+            try:
+                pkg_resources.require(self.PretixPluginMeta.compatibility)
+            except pkg_resources.VersionConflict as e:
+                print("Incompatible plugins found!")
+                print("Plugin {} requires you to have {}, but you installed {}.".format(
+                    self.name, e.req, e.dist
+                ))
+                sys.exit(1)