Add Store::dumpPath() method

This allows applying nix-store --verify-path to binary cache stores:

  NIX_REMOTE=https://cache.nixos.org nix-store --verify-path /nix/store/s5c7...

src/libstore/binary-cache-store.cc

@@ -156,10 +156,8 @@ bool BinaryCacheStore::isValidPath(const Path & storePath)
     return fileExists(narInfoFileFor(storePath));
 }
 
-void BinaryCacheStore::exportPath(const Path & storePath, bool sign, Sink & sink)
+void BinaryCacheStore::dumpPath(const Path & storePath, Sink & sink)
 {
-    assert(!sign);
-
     auto res = readNarInfo(storePath);
 
     auto nar = getFile(res.url);
@@ -183,6 +181,15 @@ void BinaryCacheStore::exportPath(const Path & storePath, bool sign, Sink & sink
     assert(nar.size() % 8 == 0);
 
     sink((unsigned char *) nar.c_str(), nar.size());
+}
+
+void BinaryCacheStore::exportPath(const Path & storePath, bool sign, Sink & sink)
+{
+    assert(!sign);
+
+    auto res = readNarInfo(storePath);
+
+    dumpPath(storePath, sink);
 
     // FIXME: check integrity of NAR.
 
@@ -264,7 +271,7 @@ Path BinaryCacheStore::addToStore(const string & name, const Path & srcPath,
     StringSink sink;
     Hash h;
     if (recursive) {
-        dumpPath(srcPath, sink, filter);
+        nix::dumpPath(srcPath, sink, filter);
         h = hashString(hashAlgo, *sink.s);
     } else {
         auto s = readFile(srcPath);

src/libstore/binary-cache-store.hh

@@ -124,6 +124,8 @@ public:
     Path addTextToStore(const string & name, const string & s,
         const PathSet & references, bool repair = false) override;
 
+    void dumpPath(const Path & path, Sink & sink) override;
+
     void exportPath(const Path & path, bool sign, Sink & sink) override;
 
     Paths importPaths(bool requireSignature, Source & source,

src/libstore/local-fs-store.cc

@@ -1,3 +1,4 @@
+#include "archive.hh"
 #include "fs-accessor.hh"
 #include "store-api.hh"
 
@@ -68,4 +69,9 @@ ref<FSAccessor> LocalFSStore::getFSAccessor()
     return make_ref<LocalStoreAccessor>(ref<Store>(shared_from_this()));
 }
 
+void LocalFSStore::dumpPath(const Path & path, Sink & sink)
+{
+    nix::dumpPath(path, sink);
+}
+
 }

src/libstore/local-store.cc

@@ -1418,7 +1418,7 @@ Path LocalStore::addToStore(const string & name, const Path & _srcPath,
        small files. */
     StringSink sink;
     if (recursive)
-        dumpPath(srcPath, sink, filter);
+        nix::dumpPath(srcPath, sink, filter);
     else
         sink.s = make_ref<std::string>(readFile(srcPath));
 

src/libstore/remote-store.cc

@@ -366,7 +366,7 @@ Path RemoteStore::addToStore(const string & name, const Path & _srcPath,
     try {
         conn->to.written = 0;
         conn->to.warn = true;
-        dumpPath(srcPath, conn->to, filter);
+        nix::dumpPath(srcPath, conn->to, filter);
         conn->to.warn = false;
         conn->processStderr();
     } catch (SysError & e) {

src/libstore/store-api.hh

@@ -215,6 +215,9 @@ public:
     virtual Path addTextToStore(const string & name, const string & s,
         const PathSet & references, bool repair = false) = 0;
 
+    /* Write a NAR dump of a store path. */
+    virtual void dumpPath(const Path & path, Sink & sink) = 0;
+
     /* Export a store path, that is, create a NAR dump of the store
        path and append its references and its deriver.  Optionally, a
        cryptographic signature (created by OpenSSL) of the preceding
@@ -354,6 +357,8 @@ public:
 
 class LocalFSStore : public Store
 {
+public:
+    void dumpPath(const Path & path, Sink & sink) override;
     ref<FSAccessor> getFSAccessor() override;
 };
 

src/nix-store/nix-store.cc

@@ -783,7 +783,9 @@ static void opVerifyPath(Strings opFlags, Strings opArgs)
         Path path = followLinksToStorePath(i);
         printMsg(lvlTalkative, format("checking path ‘%1%’...") % path);
         ValidPathInfo info = store->queryPathInfo(path);
-        HashResult current = hashPath(info.narHash.type, path);
+        HashSink sink(info.narHash.type);
+        store->dumpPath(path, sink);
+        auto current = sink.finish();
         if (current.first != info.narHash) {
             printMsg(lvlError,
                 format("path ‘%1%’ was modified! expected hash ‘%2%’, got ‘%3%’")