From cb414aad45539b97aeb4ad71e0d85545cea486c5 Mon Sep 17 00:00:00 2001
From: Jean-Paul Calderone <exarkun@twistedmatrix.com>
Date: Fri, 27 Sep 2019 13:42:54 -0400
Subject: [PATCH] comments
---
nixos/modules/overlays.nix | 27 +++++++++++++++++++++++----
1 file changed, 23 insertions(+), 4 deletions(-)
diff --git a/nixos/modules/overlays.nix b/nixos/modules/overlays.nix
index e510fbd7..779384ab 100644
--- a/nixos/modules/overlays.nix
+++ b/nixos/modules/overlays.nix
@@ -1,19 +1,38 @@
let
+ # Define a Python packageOverride that puts our version of Twisted into
+ # python27Packages.
pythonTwistedOverride = python-self: python-super: {
- twisted = python-super.callPackage ../pkgs/twisted.nix { inherit (python-super) twisted; };
+ # 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
+ # is, `python-self.callPackage` and `python-super.callPackage` will *both*
+ # try to pass `python-self.twisted`. So we take it upon ourselves to pass
+ # the "correct" Twisted (it is correct because we call its override method
+ # and that never converges if it is the fixed point Twisted).
+ twisted = python-self.callPackage ../pkgs/twisted.nix {
+ inherit (python-super) twisted;
+ };
};
in
self: super: {
+ # Use self.python27 to get the fixed point of all packages (that is, to
+ # respect all of the overrides). This is important since we want the
+ # overridden Twisted as a dependency of this env, not the original one.
+ #
+ # This might seem to violate the advice to use super for "library
+ # functionality" but python27.buildEnv should be considered a derivation
+ # instead because it implies a whole mess of derivations (all of the Python
+ # modules available).
privatestorage = self.python27.buildEnv.override
{ extraLibs =
[ self.python27Packages.tahoe-lafs
self.python27Packages.zkapauthorizer
];
- # Twisted's dropin.cache always collides between different
- # plugin-providing packages.
- # ignoreCollisions = true;
};
+ # Using super.python27 here causes us to define a python27 that overrides
+ # the value from the previously overlay, not from the fixed point. This is
+ # important because this override never converges.
python27 = super.python27.override (old: {
packageOverrides = super.lib.composeExtensions old.packageOverrides pythonTwistedOverride;
});
--
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