From 965cc5da574c8ee2310d0cc0a7d75363f422b4c3 Mon Sep 17 00:00:00 2001
From: John Ericson <John.Ericson@Obsidian.Systems>
Date: Sat, 9 Dec 2017 14:27:24 -0500
Subject: [PATCH 1/2] top-level: Fix splicing in the presence of evaluation
 errors

 - No more `or {}`, this was misleading at best since those values
   wouldn't be used unless the attr they are defined from was present
   anyways.

 - `tryEval` for get outputs. This ensures that if some derivations fail, they
   won't take out the others. This benefited from the `or {}` before, but that
   was never good enough. `tryEval` is strictly better.
---
 pkgs/top-level/splice.nix | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/pkgs/top-level/splice.nix b/pkgs/top-level/splice.nix
index b13fa86a9958..d2c339b04df8 100644
--- a/pkgs/top-level/splice.nix
+++ b/pkgs/top-level/splice.nix
@@ -37,12 +37,15 @@ let
       inherit name;
       value = let
         defaultValue = mash.${name};
-        buildValue = buildPkgs.${name} or {};
-        runValue = runPkgs.${name} or {};
+        buildValue = buildPkgs.${name};
+        runValue = runPkgs.${name};
         augmentedValue = defaultValue
           // (lib.optionalAttrs (buildPkgs ? ${name}) { nativeDrv = buildValue; })
           // (lib.optionalAttrs (runPkgs ? ${name}) { crossDrv = runValue; });
         # Get the set of outputs of a derivation
+        tryGetOutputs = value0: let
+          eval = builtins.tryEval value0;
+        in getOutputs (if eval.success then eval.value else {});
         getOutputs = value: lib.genAttrs
           (value.outputs or (lib.optional (value ? out) "out"))
           (output: value.${output});
@@ -54,7 +57,7 @@ let
         # The derivation along with its outputs, which we recur
         # on to splice them together.
         else if lib.isDerivation defaultValue then augmentedValue
-          // splicer (getOutputs buildValue) (getOutputs runValue)
+          // splicer (tryGetOutputs buildValue) (getOutputs runValue)
         # Just recur on plain attrsets
         else if lib.isAttrs defaultValue then splicer buildValue runValue
         # Don't be fancy about non-derivations. But we could have used used

From 6c36d3c9e0b615c4e76fc3b3582e6f3ec1637af3 Mon Sep 17 00:00:00 2001
From: John Ericson <John.Ericson@Obsidian.Systems>
Date: Sun, 10 Dec 2017 16:38:48 -0500
Subject: [PATCH 2/2] top-level: Fix splicing, again

---
 pkgs/top-level/splice.nix | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/pkgs/top-level/splice.nix b/pkgs/top-level/splice.nix
index d2c339b04df8..ea81b110080d 100644
--- a/pkgs/top-level/splice.nix
+++ b/pkgs/top-level/splice.nix
@@ -37,26 +37,25 @@ let
       inherit name;
       value = let
         defaultValue = mash.${name};
-        buildValue = buildPkgs.${name};
-        runValue = runPkgs.${name};
+        # `or {}` is for the non-derivation attsert splicing case, where `{}` is the identity.
+        buildValue = buildPkgs.${name} or {};
+        runValue = runPkgs.${name} or {};
         augmentedValue = defaultValue
           // (lib.optionalAttrs (buildPkgs ? ${name}) { nativeDrv = buildValue; })
           // (lib.optionalAttrs (runPkgs ? ${name}) { crossDrv = runValue; });
-        # Get the set of outputs of a derivation
+        # Get the set of outputs of a derivation. If one derivation fails to
+        # evaluate we don't want to diverge the entire splice, so we fall back
+        # on {}
         tryGetOutputs = value0: let
-          eval = builtins.tryEval value0;
-        in getOutputs (if eval.success then eval.value else {});
+          inherit (builtins.tryEval value0) success value;
+        in getOutputs (lib.optionalAttrs success value);
         getOutputs = value: lib.genAttrs
           (value.outputs or (lib.optional (value ? out) "out"))
           (output: value.${output});
       in
-        # Certain *Cross derivations will fail assertions, but we need their
-        # nativeDrv. We are assuming anything that fails to evaluate is an
-        # attrset (including derivation) and thus can be unioned.
-        if !(builtins.tryEval defaultValue).success then augmentedValue
         # The derivation along with its outputs, which we recur
         # on to splice them together.
-        else if lib.isDerivation defaultValue then augmentedValue
+        if lib.isDerivation defaultValue then augmentedValue
           // splicer (tryGetOutputs buildValue) (getOutputs runValue)
         # Just recur on plain attrsets
         else if lib.isAttrs defaultValue then splicer buildValue runValue