Make `KeyedBuildResult`, `BuildResult` like before, and fix bug another way

In https://github.com/NixOS/nix/pull/6311#discussion_r834863823, I
realized since derivation goals' wanted outputs can "grow" due to
overlapping dependencies (See `DerivationGoal::addWantedOutputs`, called
by `Worker::makeDerivationGoalCommon`), the previous bug fix had an
unfortunate side effect of causing more pointless rebuilds.

In paticular, we have this situation:

1. Goal made from `DerivedPath::Built { foo, {a} }`.

2. Goal gives on on substituting, starts building.

3. Goal made from `DerivedPath::Built { foo, {b} }`, in fact is just
   modified original goal.

4. Though the goal had gotten as far as building, so all outputs were
   going to be produced, `addWantedOutputs` no longer knows that and so
   the goal is flagged to be restarted.

This might sound far-fetched with input-addressed drvs, where we usually
basically have all our goals "planned out" before we start doing
anything, but with CA derivation goals and especially RFC 92, where *drv
resolution* means goals are created after some building is completed, it
is more likely to happen.

So the first thing to do was restore the clearing of `wantedOutputs` we
used to do, and then filter the outputs in `buildPathsWithResults` to
only get the ones we care about.

But fix also has its own side effect in that the `DerivedPath` in the
`BuildResult` in `DerivationGoal` cannot be trusted; it is merely the
*first* `DerivedPath` for which this goal was originally created.

To remedy this, I made `BuildResult` be like it was before, and instead
made `KeyedBuildResult` be a subclass wit the path. Only
`buildPathsWithResults` returns `KeyedBuildResult`s, everything else
just becomes like it was before, where the "key" is unambiguous from
context.

I think separating the "primary key" field(s) from the other fields is
good practical in general anyways. (I would like to do the same thing
for `ValidPathInfo`.) Among other things, it allows constructions like
`std::map<Key, ThingWithKey>` where doesn't contain duplicate keys and
just precludes the possibility of those duplicate keys being out of
sync.

We might leverage the above someday to overload `buildPathsWithResults`
to take a *set* of return a *map* per the above.

-----

Unfortunately, we need to avoid C++20 strictness on designated
initializers.

(BTW
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2287r1.html
this offers some new syntax for this use-case. Hopefully this will be
adopted and we can eventually use it.)

No having that yet, maybe it would be better to not make
`KeyedBuildResult` a subclass to just avoid this.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

1 files changed, 15 insertions, 1 deletions

diff --git a/src/libstore/build/goal.hh b/src/libstore/build/goal.hh
index f4bf6f38b..c0e12a2ed 100644
--- a/src/libstore/build/goal.hh
+++ b/src/libstore/build/goal.hh
@@ -81,11 +81,26 @@ struct Goal : public std::enable_shared_from_this<Goal>
      */
     ExitCode exitCode = ecBusy;
 
+protected:
     /**
      * Build result.
      */
     BuildResult buildResult;
 
+public:
+
+    /**
+     * Project a `BuildResult` with just the information that pertains
+     * to the given request.
+     *
+     * In general, goals may be aliased between multiple requests, and
+     * the stored `BuildResult` has information for the union of all
+     * requests. We don't want to leak what the other request are for
+     * sake of both privacy and determinism, and this "safe accessor"
+     * ensures we don't.
+     */
+    BuildResult getBuildResult(const DerivedPath &);
+
     /**
      * Exception containing an error message, if any.
      */
@@ -93,7 +108,6 @@ struct Goal : public std::enable_shared_from_this<Goal>
 
     Goal(Worker & worker, DerivedPath path)
         : worker(worker)
-        , buildResult { .path = std::move(path) }
     { }
 
     virtual ~Goal()