nix dump-path: Add

This is primarily useful for extracting NARs from other stores (like
binary caches), which "nix-store --dump" cannot do.

src/nix/command.cc

@@ -129,4 +129,14 @@ void StorePathsCommand::run(ref<Store> store)
     run(store, storePaths);
 }
 
+void StorePathCommand::run(ref<Store> store)
+{
+    auto storePaths = buildInstallables(store, false);
+
+    if (storePaths.size() != 1)
+        throw UsageError("this command requires exactly one store path");
+
+    run(store, *storePaths.begin());
+}
+
 }

src/nix/command.hh

@@ -118,6 +118,16 @@ public:
     bool useDefaultInstallables() override { return !all; }
 };
 
+/* A command that operates on exactly one store path. */
+struct StorePathCommand : public InstallablesCommand
+{
+    using StoreCommand::run;
+
+    virtual void run(ref<Store> store, const Path & storePath) = 0;
+
+    void run(ref<Store> store) override;
+};
+
 typedef std::map<std::string, ref<Command>> Commands;
 
 /* An argument parser that supports multiple subcommands,

src/nix/dump-path.cc

@@ -0,0 +1,35 @@
+#include "command.hh"
+#include "store-api.hh"
+
+using namespace nix;
+
+struct CmdDumpPath : StorePathCommand
+{
+    std::string name() override
+    {
+        return "dump-path";
+    }
+
+    std::string description() override
+    {
+        return "dump a store path to stdout (in NAR format)";
+    }
+
+    Examples examples() override
+    {
+        return {
+            Example{
+                "To get a NAR from the binary cache https://cache.nixos.org/:",
+                "nix dump-path --store https://cache.nixos.org/ /nix/store/7crrmih8c52r8fbnqb933dxrsp44md93-glibc-2.25"
+            },
+        };
+    }
+
+    void run(ref<Store> store, const Path & storePath) override
+    {
+        FdSink sink(STDOUT_FILENO);
+        store->narFromPath(storePath, sink);
+    }
+};
+
+static RegisterCommand r1(make_ref<CmdDumpPath>());