Upgrade Tahoe-LAFS on the storage servers

Created by: exarkun

Tahoe-LAFS is being ported to Python 3. This is potentially very disruptive as it will involve changes across the entire codebase.

To bound the risk of the porting enterprise it is beneficial to have real-world testing performed as the porting effort progresses. Every master revision along the way should be correct.

nixos/modules/overlays.nix

 let
-  # Define a Python packageOverride that puts our version of Twisted into
-  # python27Packages.
-  pythonTwistedOverride = python-self: python-super: {
+  # Define a Python packageOverride that puts our version of some Python
+  # packages into python27Packages.
+  pythonPackageOverride = python-self: python-super: rec {
     # Get our Twisted derivation.  Pass in the old one so it can have pieces
     # overridden.  It needs to be passed in explicitly because callPackage is
     # specially crafted to always pull attributes from the fixed-point.  That
@@ -12,6 +12,15 @@ let
     twisted = python-self.callPackage ../pkgs/twisted.nix {
       inherit (python-super) twisted;
     };
+
+    # Put in our preferred version of tahoe-lafs as well.
+    tahoe-lafs = python-self.callPackage ../pkgs/tahoe-lafs.nix { };
+
+    # This is handy too...
+    zkapauthorizer = python-self.callPackage ../pkgs/zkapauthorizer.nix {
+      # And explicitly configure it with our preferred version of Tahoe-LAFS.
+      inherit tahoe-lafs;
+    };
   };
 in
 self: super: {
@@ -26,7 +35,9 @@ self: super: {
   # instead because it implies a whole mess of derivations (all of the Python
   # modules available).
   privatestorage = self.python27.buildEnv.override
-  { extraLibs =
+  { # ... for dropin.cache
+    ignoreCollisions = true;
+    extraLibs =
     [ self.python27Packages.tahoe-lafs
       self.python27Packages.zkapauthorizer
     ];
@@ -38,8 +49,8 @@ self: super: {
   python27 = super.python27.override (old: {
     packageOverrides =
       if old ? packageOverrides then
-        super.lib.composeExtensions old.packageOverrides pythonTwistedOverride
+        super.lib.composeExtensions old.packageOverrides pythonPackageOverride
       else
-        pythonTwistedOverride;
+        pythonPackageOverride;
   });
 }