Get rid of addWantedOutputs

This is just a code cleanup; it should not be behavior change.

`addWantedOutputs` is removed by introducing `DerivationTrampolineGoal`.

`DerivationGoal` now only tracks a single output, and is back to
tracking a plain store path `drvPath`, not a deriving path one. Its
`addWantedOutputs` method is gone. These changes will allow subsequent
PRs to simplify it greatly.

Because the purpose of each goal is back to being immutable, we can also
once again make `Goal::buildResult` a public field, and get rid of the
`getBuildResult` method. This simplifies things also.

`DerivationTrampolineGoal` is, as the nane is supposed to indicate, a
cheap "trampoline" goal. It takes immutable sets of wanted outputs, and
just kicks of `DerivationGoal`s for them. Since now "actual work" is
done in these goals, it is not wasteful to have separate ones for
separate sets of outputs, even if those outputs (and the derivations
they are from) overlap.

This design is described in more detail in the doc comments on the goal
types, which I've now greatly expanded.

---

This separation of concerns will make it possible for future work on
issues like #11928, and to continue the path of having more goal types,
but each goal type does fewer things (issue #12628).

---

This commit in some sense reverts
f4f28cd, but that one kept around
`addWantedOutputs`. I am quite sure it was having two layers of goals
with `addWantedOutputs` that caused the issues --- restarting logic like
`addWantedOutputs` has is very tempermental! In this version of the
change, we have *zero* layers of `addWantedOutputs` --- no goal type
needs it, or otherwise has a mutable objective --- and so I think this
change is safe.

Co-authored-by: Sergei Zimmerman <[email protected]>

Author: Ericson2314

src/libstore/build/derivation-building-goal.cc

@@ -1,5 +1,5 @@
 #include "nix/store/build/derivation-building-goal.hh"
-#include "nix/store/build/derivation-goal.hh"
+#include "nix/store/build/derivation-trampoline-goal.hh"
 #ifndef _WIN32 // TODO enable build hook on Windows
 #  include "nix/store/build/hook-instance.hh"
 #  include "nix/store/build/derivation-builder.hh"
@@ -72,7 +72,7 @@ std::string DerivationBuildingGoal::key()
     /* Ensure that derivations get built in order of their name,
        i.e. a derivation named "aardvark" always comes before
        "baboon". And substitution goals always happen before
-       derivation goals (due to "b$"). */
+       derivation goals (due to "bd$"). */
     return "bd$" + std::string(drvPath.name()) + "$" + worker.store.printStorePath(drvPath);
 }
 
@@ -266,7 +266,7 @@ Goal::Co DerivationBuildingGoal::gaveUpOnSubstitution()
                     auto mEntry = get(inputGoals, drvPath);
                     if (!mEntry) return std::nullopt;
 
-                    auto buildResult = (*mEntry)->getBuildResult(DerivedPath::Built{drvPath, OutputsSpec::Names{outputName}});
+                    auto & buildResult = (*mEntry)->buildResult;
                     if (!buildResult.success()) return std::nullopt;
 
                     auto i = get(buildResult.builtOutputs, outputName);
@@ -296,30 +296,28 @@ Goal::Co DerivationBuildingGoal::gaveUpOnSubstitution()
                        worker.store.printStorePath(pathResolved),
                    });
 
-            // FIXME wanted outputs
-            auto resolvedDrvGoal = worker.makeDerivationGoal(
-                makeConstantStorePathRef(pathResolved), OutputsSpec::All{}, buildMode);
+            /* TODO https://github.com/NixOS/nix/issues/13247 we should
+               let the calling goal do this, so it has a change to pass
+               just the output(s) it cares about. */
+            auto resolvedDrvGoal = worker.makeDerivationTrampolineGoal(
+                pathResolved, OutputsSpec::All{}, drvResolved, buildMode);
             {
                 Goals waitees{resolvedDrvGoal};
                 co_await await(std::move(waitees));
             }
 
             trace("resolved derivation finished");
 
-            auto resolvedDrv = *resolvedDrvGoal->drv;
-            auto resolvedResult = resolvedDrvGoal->getBuildResult(DerivedPath::Built{
-                .drvPath = makeConstantStorePathRef(pathResolved),
-                .outputs = OutputsSpec::All{},
-            });
+            auto resolvedResult = resolvedDrvGoal->buildResult;
 
             SingleDrvOutputs builtOutputs;
 
             if (resolvedResult.success()) {
-                auto resolvedHashes = staticOutputHashes(worker.store, resolvedDrv);
+                auto resolvedHashes = staticOutputHashes(worker.store, drvResolved);
 
                 StorePathSet outputPaths;
 
-                for (auto & outputName : resolvedDrv.outputNames()) {
+                for (auto & outputName : drvResolved.outputNames()) {
                     auto initialOutput = get(initialOutputs, outputName);
                     auto resolvedHash = get(resolvedHashes, outputName);
                     if ((!initialOutput) || (!resolvedHash))
@@ -340,7 +338,7 @@ Goal::Co DerivationBuildingGoal::gaveUpOnSubstitution()
 
                       throw Error(
                           "derivation '%s' doesn't have expected output '%s' (derivation-goal.cc/realisation)",
-                          resolvedDrvGoal->drvReq->to_string(worker.store), outputName);
+                          worker.store.printStorePath(pathResolved), outputName);
                     }();
 
                     if (!drv->type().isImpure()) {