nix path-info: Add

Forgot to commit this earlier...

src/nix/path-info.cc

+#include "command.hh"
+#include "progress-bar.hh"
+#include "shared.hh"
+#include "store-api.hh"
+
+#include <iomanip>
+
+using namespace nix;
+
+struct CmdPathInfo : StorePathsCommand
+{
+    bool showSize = false;
+    bool showClosureSize = false;
+
+    CmdPathInfo()
+    {
+        mkFlag('s', "size", "print size of the NAR dump of each path", &showSize);
+        mkFlag('S', "closure-size", "print sum size of the NAR dumps of the closure of each path", &showClosureSize);
+    }
+
+    std::string name() override
+    {
+        return "path-info";
+    }
+
+    std::string description() override
+    {
+        return "query information about store paths";
+    }
+
+    Examples examples() override
+    {
+        return {
+            Example{
+                "To show the closure sizes of every path in the current NixOS system closure, sorted by size:",
+                "nix path-info -rS /run/current-system | sort -nk2"
+            },
+            Example{
+                "To check the existence of a path in a binary cache:",
+                "nix path-info -r /nix/store/7qvk5c91...-geeqie-1.1 --store https://cache.nixos.org/"
+            },
+        };
+    }
+
+    void run(ref<Store> store, Paths storePaths) override
+    {
+        size_t pathLen = 0;
+        for (auto & storePath : storePaths)
+            pathLen = std::max(pathLen, storePath.size());
+
+        for (auto & storePath : storePaths) {
+            if (!store->isValidPath(storePath))
+                throw Error(format("path ‘%s’ is not valid") % storePath);
+
+            std::cout << storePath << std::string(pathLen - storePath.size(), ' ');
+
+            if (showSize) {
+                std::cout << '\t' << std::setw(11) << store->queryPathInfo(storePath)->narSize;
+            }
+
+            if (showClosureSize) {
+                size_t totalSize = 0;
+                PathSet closure;
+                store->computeFSClosure(storePath, closure, false, false);
+                for (auto & p : closure)
+                    totalSize += store->queryPathInfo(p)->narSize;
+                std::cout << '\t' << std::setw(11) << totalSize;
+            }
+
+            std::cout << std::endl;
+        }
+    }
+};
+
+static RegisterCommand r1(make_ref<CmdPathInfo>());