exportReferencesGraph: Export more complete info in JSON format

This writes info about every path in the closure in the same format as
‘nix path-info --json’. Thus it also includes NAR hashes and sizes.

Example:

  [
    {
      "path": "/nix/store/10h6li26i7g6z3mdpvra09yyf10mmzdr-hello-2.10",
      "narHash": "sha256:0ckdc4z20kkmpqdilx0wl6cricxv90lh85xpv2qljppcmz6vzcxl",
      "narSize": 197648,
      "references": [
        "/nix/store/10h6li26i7g6z3mdpvra09yyf10mmzdr-hello-2.10",
        "/nix/store/27binbdy296qvjycdgr1535v8872vz3z-glibc-2.24"
      ],
      "closureSize": 20939776
    },
    {
      "path": "/nix/store/27binbdy296qvjycdgr1535v8872vz3z-glibc-2.24",
      "narHash": "sha256:1nfn3m3p98y1c0kd0brp80dn9n5mycwgrk183j17rajya0h7gax3",
      "narSize": 20742128,
      "references": [
        "/nix/store/27binbdy296qvjycdgr1535v8872vz3z-glibc-2.24"
      ],
      "closureSize": 20742128
    }
  ]

Fixes #1134.

src/libstore/build.cc

@@ -10,6 +10,7 @@
 #include "builtins.hh"
 #include "finally.hh"
 #include "compression.hh"
+#include "json.hh"
 
 #include <algorithm>
 #include <iostream>
@@ -2273,9 +2274,18 @@ void DerivationGoal::doExportReferencesGraph()
             }
         }
 
-        /* Write closure info to `fileName'. */
+        /* Write closure info to <fileName>. */
         writeFile(tmpDir + "/" + fileName,
             worker.store.makeValidityRegistration(paths, false, false));
+
+        /* Write a more comprehensive JSON serialisation to
+           <fileName>.json. */
+        std::ostringstream str;
+        {
+            JSONPlaceholder jsonRoot(str, true);
+            worker.store.pathInfoToJSON(jsonRoot, paths, false, true);
+        }
+        writeFile(tmpDir + "/" + fileName + ".json", str.str());
     }
 }
 

src/libstore/store-api.cc

@@ -4,6 +4,7 @@
 #include "util.hh"
 #include "nar-info-disk-cache.hh"
 #include "thread-pool.hh"
+#include "json.hh"
 
 #include <future>
 
@@ -439,6 +440,64 @@ string Store::makeValidityRegistration(const PathSet & paths,
 }
 
 
+void Store::pathInfoToJSON(JSONPlaceholder & jsonOut, const PathSet & storePaths,
+    bool includeImpureInfo, bool showClosureSize)
+{
+    auto jsonList = jsonOut.list();
+
+    for (auto storePath : storePaths) {
+        auto info = queryPathInfo(storePath);
+        storePath = info->path;
+
+        auto jsonPath = jsonList.object();
+        jsonPath
+            .attr("path", storePath)
+            .attr("narHash", info->narHash.to_string())
+            .attr("narSize", info->narSize);
+
+        {
+            auto jsonRefs = jsonPath.list("references");
+            for (auto & ref : info->references)
+                jsonRefs.elem(ref);
+        }
+
+        if (info->ca != "")
+            jsonPath.attr("ca", info->ca);
+
+        if (showClosureSize)
+            jsonPath.attr("closureSize", getClosureSize(storePath));
+
+        if (!includeImpureInfo) continue;
+
+        if (info->deriver != "")
+            jsonPath.attr("deriver", info->deriver);
+
+        if (info->registrationTime)
+            jsonPath.attr("registrationTime", info->registrationTime);
+
+        if (info->ultimate)
+            jsonPath.attr("ultimate", info->ultimate);
+
+        if (!info->sigs.empty()) {
+            auto jsonSigs = jsonPath.list("signatures");
+            for (auto & sig : info->sigs)
+                jsonSigs.elem(sig);
+        }
+    }
+}
+
+
+unsigned long long Store::getClosureSize(const Path & storePath)
+{
+    unsigned long long totalSize = 0;
+    PathSet closure;
+    computeFSClosure(storePath, closure, false, false);
+    for (auto & p : closure)
+        totalSize += queryPathInfo(p)->narSize;
+    return totalSize;
+}
+
+
 const Store::Stats & Store::getStats()
 {
     {

src/libstore/store-api.hh

@@ -22,6 +22,7 @@ struct Derivation;
 class FSAccessor;
 class NarInfoDiskCache;
 class Store;
+class JSONPlaceholder;
 
 
 /* Size of the hash part of store paths, in base-32 characters. */
@@ -469,6 +470,19 @@ public:
     string makeValidityRegistration(const PathSet & paths,
         bool showDerivers, bool showHash);
 
+    /* Write a JSON representation of store path metadata, such as the
+       hash and the references. If ‘includeImpureInfo’ is true,
+       variable elements such as the registration time are
+       included. If ‘showClosureSize’ is true, the closure size of
+       each path is included. */
+    void pathInfoToJSON(JSONPlaceholder & jsonOut, const PathSet & storePaths,
+        bool includeImpureInfo, bool showClosureSize);
+
+    /* Return the size of the closure of the specified path, that is,
+       the sum of the size of the NAR serialisation of each path in
+       the closure. */
+    unsigned long long getClosureSize(const Path & storePath);
+
     /* Optimise the disk space usage of the Nix store by hard-linking files
        with the same contents. */
     virtual void optimiseStore() = 0;

src/nix/path-info.cc

@@ -65,55 +65,12 @@ struct CmdPathInfo : StorePathsCommand
         for (auto & storePath : storePaths)
             pathLen = std::max(pathLen, storePath.size());
 
-        auto getClosureSize = [&](const Path & storePath) -> unsigned long long {
-            unsigned long long totalSize = 0;
-            PathSet closure;
-            store->computeFSClosure(storePath, closure, false, false);
-            for (auto & p : closure)
-                totalSize += store->queryPathInfo(p)->narSize;
-            return totalSize;
-        };
-
         if (json) {
-            JSONList jsonRoot(std::cout, true);
-
-            for (auto storePath : storePaths) {
-                auto info = store->queryPathInfo(storePath);
-                storePath = info->path;
-
-                auto jsonPath = jsonRoot.object();
-                jsonPath
-                    .attr("path", storePath)
-                    .attr("narHash", info->narHash.to_string())
-                    .attr("narSize", info->narSize);
-
-                if (showClosureSize)
-                    jsonPath.attr("closureSize", getClosureSize(storePath));
-
-                if (info->deriver != "")
-                    jsonPath.attr("deriver", info->deriver);
-
-                {
-                    auto jsonRefs = jsonPath.list("references");
-                    for (auto & ref : info->references)
-                        jsonRefs.elem(ref);
-                }
-
-                if (info->registrationTime)
-                    jsonPath.attr("registrationTime", info->registrationTime);
-
-                if (info->ultimate)
-                    jsonPath.attr("ultimate", info->ultimate);
-
-                if (info->ca != "")
-                    jsonPath.attr("ca", info->ca);
-
-                if (!info->sigs.empty()) {
-                    auto jsonSigs = jsonPath.list("signatures");
-                    for (auto & sig : info->sigs)
-                        jsonSigs.elem(sig);
-                }
-            }
+            JSONPlaceholder jsonRoot(std::cout, true);
+            store->pathInfoToJSON(jsonRoot,
+                // FIXME: preserve order?
+                PathSet(storePaths.begin(), storePaths.end()),
+                true, showClosureSize);
         }
 
         else {
@@ -128,7 +85,7 @@ struct CmdPathInfo : StorePathsCommand
                     std::cout << '\t' << std::setw(11) << info->narSize;
 
                 if (showClosureSize)
-                    std::cout << '\t' << std::setw(11) << getClosureSize(storePath);
+                    std::cout << '\t' << std::setw(11) << store->getClosureSize(storePath);
 
                 if (showSigs) {
                     std::cout << '\t';