Print built derivations as json for build

Add --json option to nix build to allow machine readable output on
stdout with all built derivations

Fixes #1930

src/nix/build.cc

@@ -5,9 +5,11 @@
 #include "store-api.hh"
 #include "local-fs-store.hh"
 
+#include <nlohmann/json.hpp>
+
 using namespace nix;
 
-struct CmdBuild : InstallablesCommand, MixDryRun, MixProfile
+struct CmdBuild : InstallablesCommand, MixDryRun, MixJSON, MixProfile
 {
     Path outLink = "result";
     BuildMode buildMode = bmNormal;
@@ -86,6 +88,8 @@ struct CmdBuild : InstallablesCommand, MixDryRun, MixProfile
                     }, buildables[i]);
 
         updateProfile(buildables);
+
+        if (json) logger->cout("%s", buildablesToJSON(buildables, store).dump());
     }
 };
 

src/nix/installables.cc

@@ -16,8 +16,35 @@
 #include <regex>
 #include <queue>
 
+#include <nlohmann/json.hpp>
+
 namespace nix {
 
+nlohmann::json BuildableOpaque::toJSON(ref<Store> store) const {
+    nlohmann::json res;
+    res["path"] = store->printStorePath(path);
+    return res;
+}
+
+nlohmann::json BuildableFromDrv::toJSON(ref<Store> store) const {
+    nlohmann::json res;
+    res["drvPath"] = store->printStorePath(drvPath);
+    for (const auto& [output, path] : outputs) {
+        res["outputs"][output] = path ? store->printStorePath(*path) : "";
+    }
+    return res;
+}
+
+nlohmann::json buildablesToJSON(const Buildables & buildables, ref<Store> store) {
+    auto res = nlohmann::json::array();
+    for (const Buildable & buildable : buildables) {
+        std::visit([&res, store](const auto & buildable) {
+            res.push_back(buildable.toJSON(store));
+        }, buildable);
+    }
+    return res;
+}
+
 void completeFlakeInputPath(
     ref<EvalState> evalState,
     const FlakeRef & flakeRef,

src/nix/installables.hh

@@ -7,6 +7,8 @@
 
 #include <optional>
 
+#include <nlohmann/json_fwd.hpp>
+
 namespace nix {
 
 struct DrvInfo;
@@ -16,11 +18,13 @@ namespace eval_cache { class EvalCache; class AttrCursor; }
 
 struct BuildableOpaque {
     StorePath path;
+    nlohmann::json toJSON(ref<Store> store) const;
 };
 
 struct BuildableFromDrv {
     StorePath drvPath;
     std::map<std::string, std::optional<StorePath>> outputs;
+    nlohmann::json toJSON(ref<Store> store) const;
 };
 
 typedef std::variant<
@@ -29,6 +33,7 @@ typedef std::variant<
 > Buildable;
 
 typedef std::vector<Buildable> Buildables;
+nlohmann::json buildablesToJSON(const Buildables & buildables, ref<Store> store);
 
 struct App
 {