perf(libstore/derivation-builder): pre-compute outputGraph for linear complexity

Build the inverse of `scratchOuputs` before running topoSort, avoiding
quadratic complexity when determining which outputs reference each
other. This fixes the FIXME comment about building the inverted map up
front.

Inspired by Lix commit 10c04ce84 / Change Id
Ibdd46e7b2e895bfeeebc173046d1297b41998181, but ended up being completely
different code.

Co-Authored-By: Maximilian Bosch <[email protected]>
Co-Authored-By: Bernardo Meurer Costa <[email protected]>

Author: 3 people

src/libstore/unix/build/derivation-builder.cc

@@ -1398,6 +1398,11 @@ SingleDrvOutputs DerivationBuilderImpl::registerOutputs()
         StorePathSet refs;
     };
 
+    /* inverse map of scratchOutputs for efficient lookup */
+    std::map<StorePath, std::string> scratchOutputsInverse;
+    for (auto & [outputName, path] : scratchOutputs)
+        scratchOutputsInverse.insert_or_assign(path, outputName);
+
     std::map<std::string, std::variant<AlreadyRegistered, PerhapsNeedToRegister>> outputReferencesIfUnregistered;
     std::map<std::string, struct stat> outputStats;
     for (auto & [outputName, _] : drv.outputs) {
@@ -1486,11 +1491,9 @@ SingleDrvOutputs DerivationBuilderImpl::registerOutputs()
                                                [&](const AlreadyRegistered &) { return StringSet{}; },
                                                [&](const PerhapsNeedToRegister & refs) {
                                                    StringSet referencedOutputs;
-                                                   /* FIXME build inverted map up front so no quadratic waste here */
                                                    for (auto & r : refs.refs)
-                                                       for (auto & [o, p] : scratchOutputs)
-                                                           if (r == p)
-                                                               referencedOutputs.insert(o);
+                                                       if (auto * o = get(scratchOutputsInverse, r))
+                                                           referencedOutputs.insert(*o);
                                                    return referencedOutputs;
                                                },
                                            },