libstore: fix reporting output cycles on drvs with references to other drvs

Closes #1064

The culprit here is that `genGraphString` is only invoked with the
store-paths associated with the outputs of the derivation, so when
filling `dependents`, the `graph_data.find(p)` call would return the end
of the iterator when doing this for references to other store-paths.

As a result, the code wrote information behind the graph data-structure
causing a corruption. For me, this resulted in a SIGSEGV most of the
time and in a few cases in an uncaught `map::at`-exception as reported
by Niklas.

This patch changes two aspects of the original implementation:

* When filling `dependents` in the node-set, use `map.at()` instead of
  `map.find()->second`. The latter doesn't make any sense and was the
  cause of corrupting memory. The `at` would've made it far easier to
  spot this in the first place.

* Filter out store-paths that don't belong to a different output of the
  derivation when creating `outputGraph`. This variable is used on two
  places, `genGraphString` and for topological sorting.

  The latter already filters out store-paths from a different drv, so
  this is happening now when creating the variable in the first place
  such that `genGraphString` never ends up with corrupt data in the
  first place. This is the actual bugfix.

Implemented a regression-test for this case to be sure.

Change-Id: Ie02144d89c32b0a776cb1ece0601d0229315ebc3
(cherry picked from commit 0a5f474a25)

tests/functional/check-outputs.nix

@@ -15,6 +15,18 @@ rec {
     '';
   };
 
+  cycle-with-deps = mkDerivation {
+    name = "cycle-with-deps";
+    inherit dep;
+    outputs = [ "foo" "bar" ];
+    builder = builtins.toFile "builder.sh" ''
+      mkdir -p $foo/bin $bar/lib
+      ln -sf $dep $bar/lib
+      echo $foo > $bar/txt
+      echo $bar > $foo/txt
+    '';
+  };
+
   as_dependency = mkDerivation {
     name = "depends-on-cycle";
     inherit cycle;

tests/functional/output-cycles.sh

@@ -16,3 +16,14 @@ error="$(! nix-build check-outputs.nix -A as_dependency 2>&1)"
 
 grepQuiet "cycle detected in build of '.*' in the references of output 'bar' from output 'foo'" <<<"$error"
 grepQuiet "error: 1 dependencies of derivation" <<<"$error"
+
+error="$(! nix-build check-outputs.nix -A cycle-with-deps 2>&1)"
+grepQuiet "cycle detected in build of '.*' in the references of output 'bar' from output 'foo'" <<<"$error"
+
+if [[ "$(uname -s)" = Linux ]]; then
+    echo "$error"
+    <<<"$error" grepQuiet "/store/.*-cycle-with-deps-bar"
+    <<<"$error" grepQuiet "└───txt: ….*cycle-with-deps-foo.*"
+    <<<"$error" grepQuiet "    →.*/store/.*-cycle-with-deps-foo"
+    <<<"$error" grepQuiet "    └───txt:.*-cycle-with-deps-bar.*"
+fi