Add Store::buildPathsWithResults()

This function is like buildPaths(), except that it returns a vector of
BuildResults containing the exact statuses and output paths of each
derivation / substitution. This is convenient for functions like
Installable::build(), because they then don't need to do another
series of calls to get the outputs of CA derivations. It's also a
precondition to impure derivations, where we *can't* query the output
of those derivations since they're not stored in the Nix database.

Note that PathSubstitutionGoal can now also return a BuildStatus.

src/libcmd/installables.cc

@@ -12,6 +12,7 @@
 #include "eval-cache.hh"
 #include "url.hh"
 #include "registry.hh"
+#include "build-result.hh"
 
 #include <regex>
 #include <queue>
@@ -769,8 +770,7 @@ BuiltPaths getBuiltPaths(ref<Store> evalStore, ref<Store> store, const DerivedPa
                             throw Error(
                                 "the derivation '%s' doesn't have an output named '%s'",
                                 store->printStorePath(bfd.drvPath), output);
-                        if (settings.isExperimentalFeatureEnabled(
-                                Xp::CaDerivations)) {
+                        if (settings.isExperimentalFeatureEnabled(Xp::CaDerivations)) {
                             auto outputId =
                                 DrvOutput{outputHashes.at(output), output};
                             auto realisation =
@@ -816,12 +816,31 @@ BuiltPaths Installable::build(
         pathsToBuild.insert(pathsToBuild.end(), b.begin(), b.end());
     }
 
-    if (mode == Realise::Nothing || mode == Realise::Derivation)
+    switch (mode) {
+    case Realise::Nothing:
+    case Realise::Derivation:
         printMissing(store, pathsToBuild, lvlError);
-    else if (mode == Realise::Outputs)
-        store->buildPaths(pathsToBuild, bMode, evalStore);
-
-    return getBuiltPaths(evalStore, store, pathsToBuild);
+        return getBuiltPaths(evalStore, store, pathsToBuild);
+    case Realise::Outputs: {
+        BuiltPaths res;
+        for (auto & buildResult : store->buildPathsWithResults(pathsToBuild, bMode, evalStore)) {
+            if (!buildResult.success())
+                buildResult.rethrow();
+            if (buildResult.drvPath) {
+                std::map<std::string, StorePath> outputs;
+                for (auto & path : buildResult.builtOutputs)
+                    outputs.emplace(path.first.outputName, path.second.outPath);
+                res.push_back(BuiltPath::Built{*buildResult.drvPath, outputs});
+            } else if (buildResult.outPath) {
+                res.push_back(BuiltPath::Opaque{*buildResult.outPath});
+            } else
+                abort();
+        }
+        return res;
+    }
+    default:
+        assert(false);
+    }
 }
 
 BuiltPaths Installable::toBuiltPaths(

src/libstore/build-result.hh

@@ -28,6 +28,7 @@ struct BuildResult
         LogLimitExceeded,
         NotDeterministic,
         ResolvesToAlreadyValid,
+        NoSubstituters,
     } status = MiscFailure;
     std::string errorMsg;
 
@@ -63,15 +64,26 @@ struct BuildResult
        non-determinism.) */
     bool isNonDeterministic = false;
 
+    /* For derivations, the derivation path and the wanted outputs. */
+    std::optional<StorePath> drvPath;
     DrvOutputs builtOutputs;
 
+    /* For substitutions, the substituted path. */
+    std::optional<StorePath> outPath;
+
     /* The start/stop times of the build (or one of the rounds, if it
        was repeated). */
     time_t startTime = 0, stopTime = 0;
 
-    bool success() {
+    bool success()
+    {
         return status == Built || status == Substituted || status == AlreadyValid || status == ResolvesToAlreadyValid;
     }
+
+    void rethrow()
+    {
+        throw Error("%s", errorMsg);
+    }
 };
 
 }