src/drv.cc

@@ -43,7 +43,7 @@ queryIsCached(nix::Store &store,
 
 /* The fields of a derivation that are printed in json form */
 Drv::Drv(std::string &attrPath, nix::EvalState &state, nix::DrvInfo &drvInfo,
-         MyArgs &args) {
+         MyArgs &args, std::optional<Constituents> constituents) : constituents(constituents) {
 
     auto localStore = state.ctx.store.dynamic_pointer_cast<nix::LocalFSStore>();
 
@@ -128,6 +128,11 @@ void to_json(nlohmann::json &json, const Drv &drv) {
         json["meta"] = drv.meta.value();
     }
 
+    if (auto constituents = drv.constituents) {
+        json["constituents"] = constituents->constituents;
+        json["namedConstituents"] = constituents->namedConstituents;
+    }
+
     if (drv.cacheStatus != Drv::CacheStatus::Unknown) {
         json["isCached"] = drv.cacheStatus == Drv::CacheStatus::Cached;
     }

src/drv.hh

@@ -17,6 +17,13 @@ class EvalState;
 struct DrvInfo;
 }  // namespace nix
 
+struct Constituents
+{
+    std::vector<std::string> constituents;
+    std::vector<std::string> namedConstituents;
+    Constituents(std::vector<std::string> constituents, std::vector<std::string> namedConstituents) : constituents(constituents), namedConstituents(namedConstituents) { };
+};
+
 /* The fields of a derivation that are printed in json form */
 struct Drv {
     std::string name;
@@ -27,7 +34,8 @@ struct Drv {
     std::map<std::string, std::optional<std::string>> outputs;
     std::map<std::string, std::set<std::string>> inputDrvs;
     std::optional<nlohmann::json> meta;
+    std::optional<Constituents> constituents;
 
-    Drv(std::string &attrPath, nix::EvalState &state, nix::DrvInfo &drvInfo, MyArgs &args);
+    Drv(std::string &attrPath, nix::EvalState &state, nix::DrvInfo &drvInfo, MyArgs &args, std::optional<Constituents> constituents);
 };
 void to_json(nlohmann::json &json, const Drv &drv);

src/eval-args.cc

@@ -62,6 +62,10 @@ MyArgs::MyArgs() : MixCommonArgs("nix-eval-jobs") {
              .description = "include derivation meta field in output",
              .handler = {&meta, true}});
 
+    addFlag({.longName = "constituents",
+             .description = "whether to evaluate constituents for Hydra's aggregate feature",
+             .handler = {&constituents, true}});
+
     addFlag({.longName = "check-cache-status",
              .description =
                  "Check if the derivations are present locally or in "

src/eval-args.hh

@@ -23,6 +23,7 @@ class MyArgs : virtual public nix::MixEvalArgs,
     bool impure = false;
     bool forceRecurse = false;
     bool checkCacheStatus = false;
+    bool constituents = false;
     size_t nrWorkers = 1;
     size_t maxMemorySize = 4096;
 

src/nix-eval-jobs.cc

@@ -1,5 +1,7 @@
 #include <lix/config.h> // IWYU pragma: keep
 
+#include <lix/libstore/derivations.hh>
+#include <lix/libstore/local-fs-store.hh>
 #include <lix/libexpr/eval-settings.hh>
 #include <lix/libmain/shared.hh>
 #include <lix/libutil/sync.hh>
@@ -148,6 +150,7 @@ private:
 struct State {
     std::set<json> todo = json::array({json::array()});
     std::set<json> active;
+    std::map<std::string, json> jobs;
     std::exception_ptr exc;
 };
 
@@ -308,8 +311,12 @@ void collector(Sync<State> &state_, std::condition_variable &wakeup) {
                 }
             } else {
                 auto state(state_.lock());
+                state->jobs.insert_or_assign(response["attr"], response);
+                auto named = response.find("namedConstituents");
+                if (named == response.end() || named->empty()) {
                     std::cout << respString << "\n" << std::flush;
                 }
+            }
 
             proc_ = std::move(proc);
             fromReader_ = std::move(fromReader);
@@ -383,5 +390,81 @@ int main(int argc, char **argv) {
 
         if (state->exc)
             std::rethrow_exception(state->exc);
+
+        if (myArgs.constituents) {
+            auto store = myArgs.evalStoreUrl
+                 ? openStore(*myArgs.evalStoreUrl)
+                 : openStore();
+            for (auto & [attr, job_json] : state->jobs) {
+                auto namedConstituents = job_json.find("namedConstituents");
+                if (namedConstituents != job_json.end() && !namedConstituents->empty()) {
+                    bool broken = false;
+                    auto drvPathAggregate = store->parseStorePath((std::string) job_json["drvPath"]);
+                    auto drvAggregate = store->readDerivation(drvPathAggregate);
+                    if (!job_json.contains("constituents")) {
+                        job_json["constituents"] = json::array();
+                    }
+                    std::vector<std::string> errors;
+                    for (auto child : *namedConstituents) {
+                        auto childJob = state->jobs.find(child);
+                        if (childJob == state->jobs.end()) {
+                            broken = true;
+                            errors.push_back(fmt("%s: does not exist", child));
+                        } else if (childJob->second.find("error") != childJob->second.end()) {
+                            broken = true;
+                            errors.push_back(fmt("%s: %s", child, childJob->second["error"]));
+                        } else {
+                            auto drvPathChild = store->parseStorePath((std::string) childJob->second["drvPath"]);
+                            auto drvChild = store->readDerivation(drvPathChild);
+                            job_json["constituents"].push_back(store->printStorePath(drvPathChild));
+                            drvAggregate.inputDrvs.map[drvPathChild].value = {drvChild.outputs.begin()->first};
+                        }
+                    }
+
+                    if (broken) {
+                        json out;
+                        out["attr"] = job_json["attr"];
+                        out["error"] = concatStringsSep("\n", errors);
+                        out["constituents"] = json::array();
+                        std::cout << out.dump() << "\n" << std::flush;
+                    } else {
+                        std::string drvName(drvPathAggregate.name());
+                        assert(drvName.ends_with(nix::drvExtension));
+                        drvName.resize(drvName.size() - nix::drvExtension.size());
+
+                        auto hashModulo = nix::hashDerivationModulo(*store, drvAggregate, true);
+                        if (hashModulo.kind != nix::DrvHash::Kind::Regular) continue;
+
+                        auto h = hashModulo.hashes.find("out");
+                        if (h == hashModulo.hashes.end()) continue;
+                        auto outPath = store->makeOutputPath("out", h->second, drvName);
+                        drvAggregate.env["out"] = store->printStorePath(outPath);
+                        drvAggregate.outputs.insert_or_assign("out", nix::DerivationOutput::InputAddressed { .path = outPath });
+                        auto newDrvPath = store->printStorePath(nix::writeDerivation(*store, drvAggregate));
+
+                        if (myArgs.gcRootsDir != "") {
+                            nix::Path root =
+                                myArgs.gcRootsDir + "/" +
+                                std::string(nix::baseNameOf(newDrvPath));
+                            if (!nix::pathExists(root)) {
+                                auto localStore =
+                                    store
+                                        .dynamic_pointer_cast<nix::LocalFSStore>();
+                                auto storePath =
+                                    localStore->parseStorePath(newDrvPath);
+                                localStore->addPermRoot(storePath, root);
+                            }
+                        }
+
+                        debug("rewrote aggregate derivation %s -> %s", store->printStorePath(drvPathAggregate), newDrvPath);
+
+                        job_json["drvPath"] = newDrvPath;
+                        job_json["outputs"]["out"] = store->printStorePath(outPath);
+                        job_json.erase("namedConstituents");
+                        std::cout << job_json.dump() << "\n" << std::flush;
+                    }
+                }
+            }
+        }
     });
 }

src/worker.cc

@@ -127,7 +127,40 @@ void worker(nix::ref<nix::eval_cache::CachingEvaluator> evaluator, nix::Bindings
 
             if (v->type() == nix::nAttrs) {
                 if (auto drvInfo = nix::getDerivation(*state, *v, false)) {
-                    auto drv = Drv(attrPathS, *state, *drvInfo, args);
+                    std::optional<Constituents> maybeConstituents;
+                    if (args.constituents) {
+                        std::vector<std::string> constituents;
+                        std::vector<std::string> namedConstituents;
+                        auto a = v->attrs->get(evaluator->symbols.create("_hydraAggregate"));
+                        if (a && state->forceBool(*a->value, a->pos, "while evaluating the `_hydraAggregate` attribute")) {
+                            auto a = v->attrs->get(evaluator->symbols.create("constituents"));
+                            if (!a)
+                                state->ctx.errors.make<nix::EvalError>("derivation must have a ‘constituents’ attribute").debugThrow();
+
+                            nix::NixStringContext context;
+                            state->coerceToString(a->pos, *a->value, context, "while evaluating the `constituents` attribute", true, false);
+                            for (auto & c : context)
+                                std::visit(nix::overloaded {
+                                    [&](const nix::NixStringContextElem::Built & b) {
+                                        constituents.push_back(b.drvPath->to_string(*evaluator->store));
+                                    },
+                                    [&](const nix::NixStringContextElem::Opaque & o) {
+                                    },
+                                    [&](const nix::NixStringContextElem::DrvDeep & d) {
+                                    },
+                                }, c.raw);
+
+                            state->forceList(*a->value, a->pos, "while evaluating the `constituents` attribute");
+                            for (unsigned int n = 0; n < a->value->listSize(); ++n) {
+                                auto v = a->value->listElems()[n];
+                                state->forceValue(*v, nix::noPos);
+                                if (v->type() == nix::nString)
+                                    namedConstituents.push_back(std::string(v->str()));
+                            }
+                        }
+                        maybeConstituents = Constituents(constituents, namedConstituents);
+                    }
+                    auto drv = Drv(attrPathS, *state, *drvInfo, args, maybeConstituents);
                     reply.update(drv);
 
                     /* Register the derivation as a GC root.  !!! This

tests/assets/flake.nix

@@ -1,7 +1,7 @@
 {
   inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
 
-  outputs = { nixpkgs, ... }:
+  outputs = { self, nixpkgs, ... }:
     let
       pkgs = nixpkgs.legacyPackages.x86_64-linux;
     in
@@ -24,6 +24,30 @@
               builder = ":";
             };
         };
+        success = {
+          aggregate = pkgs.runCommand "aggregate" {
+            _hydraAggregate = true;
+            constituents = [
+              self.hydraJobs.builtJob
+              "anotherone"
+            ];
+          } ''
+            touch $out
+          '';
+          anotherone = pkgs.writeText "constituent" "text";
+        };
+        failures = {
+          aggregate = pkgs.runCommand "aggregate" {
+            _hydraAggregate = true;
+            constituents = [
+              "doesntexist"
+              "doesnteval"
+            ];
+          } ''
+            touch $out
+          '';
+          doesnteval = pkgs.writeText "constituent" (toString {});
+        };
       };
     };
 }

tests/test_eval.py

@@ -95,6 +95,70 @@ def test_eval_error() -> None:
         assert "this is an evaluation error" in attrs["error"]
 
 
+def test_constituents() -> None:
+    with TemporaryDirectory() as tempdir:
+        cmd = [
+            str(BIN),
+            "--gc-roots-dir",
+            tempdir,
+            "--meta",
+            "--workers",
+            "1",
+            "--flake",
+            ".#legacyPackages.x86_64-linux.success",
+            "--constituents",
+        ]
+        res = subprocess.run(
+            cmd,
+            cwd=TEST_ROOT.joinpath("assets"),
+            text=True,
+            stdout=subprocess.PIPE,
+        )
+        print(res.stdout)
+        results = [json.loads(r) for r in res.stdout.split("\n") if r]
+        assert len(results) == 2
+        child = results[0]
+        assert child["attr"] == "anotherone"
+        aggregate = results[1]
+        assert aggregate["attr"] == "aggregate"
+        assert "namedConstituents" not in aggregate
+        assert aggregate["constituents"][0].endswith("-job1.drv")
+        assert aggregate["constituents"][1] == child["drvPath"]
+        assert "error" not in aggregate
+
+
+def test_constituents_error() -> None:
+    with TemporaryDirectory() as tempdir:
+        cmd = [
+            str(BIN),
+            "--gc-roots-dir",
+            tempdir,
+            "--meta",
+            "--workers",
+            "1",
+            "--flake",
+            ".#legacyPackages.x86_64-linux.failures",
+            "--constituents",
+        ]
+        res = subprocess.run(
+            cmd,
+            cwd=TEST_ROOT.joinpath("assets"),
+            text=True,
+            stdout=subprocess.PIPE,
+        )
+        print(res.stdout)
+        results = [json.loads(r) for r in res.stdout.split("\n") if r]
+        assert len(results) == 2
+        child = results[0]
+        assert child["attr"] == "doesnteval"
+        assert "error" in child
+        aggregate = results[1]
+        assert aggregate["attr"] == "aggregate"
+        assert "namedConstituents" not in aggregate
+        assert aggregate["error"].startswith('"doesntexist": does not exist\n"doesnteval": "error: derivation ')
+        assert aggregate["constituents"] == []
+
+
 @pytest.mark.infiniterecursion
 def test_recursion_error() -> None:
     with TemporaryDirectory() as tempdir: