Make ParseSink a bit better

I wouldn't call it *good* yet, but this will do for now.

- `RetrieveRegularNARSink` renamed to `RegularFileSink` and moved
  accordingly because it actually has nothing to do with NARs in
  particular.

  - its `fd` field is also marked private

- `copyRecursive` introduced to dump a `SourceAccessor` into a
  `ParseSink`.

- `NullParseSink` made so `ParseSink` no longer has sketchy default
  methods.

This was done while updating #8918 to work with the new
`SourceAccessor`.

src/libstore/daemon.cc

@@ -454,13 +454,13 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
                        eagerly consume the entire stream it's given, past the
                        length of the Nar. */
                     TeeSource savedNARSource(from, saved);
-                    ParseSink sink; /* null sink; just parse the NAR */
+                    NullParseSink sink; /* just parse the NAR */
                     parseDump(sink, savedNARSource);
                 } else {
                     /* Incrementally parse the NAR file, stripping the
                        metadata, and streaming the sole file we expect into
                        `saved`. */
-                    RetrieveRegularNARSink savedRegular { saved };
+                    RegularFileSink savedRegular { saved };
                     parseDump(savedRegular, from);
                     if (!savedRegular.regular) throw Error("regular file expected");
                 }
@@ -899,7 +899,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
                 source = std::make_unique<TunnelSource>(from, to);
             else {
                 TeeSource tee { from, saved };
-                ParseSink ether;
+                NullParseSink ether;
                 parseDump(ether, tee);
                 source = std::make_unique<StringSource>(saved.s);
             }

src/libstore/export-import.cc

@@ -65,7 +65,7 @@ StorePaths Store::importPaths(Source & source, CheckSigsFlag checkSigs)
         /* Extract the NAR from the source. */
         StringSink saved;
         TeeSource tee { source, saved };
-        ParseSink ether;
+        NullParseSink ether;
         parseDump(ether, tee);
 
         uint32_t magic = readInt(source);

src/libstore/local-store.cc

@@ -1200,7 +1200,7 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
     bool narRead = false;
     Finally cleanup = [&]() {
         if (!narRead) {
-            ParseSink sink;
+            NullParseSink sink;
             parseDump(sink, source);
         }
     };

src/libstore/store-api.cc

@@ -410,7 +410,7 @@ ValidPathInfo Store::addToStoreSlow(std::string_view name, const Path & srcPath,
     /* Note that fileSink and unusualHashTee must be mutually exclusive, since
        they both write to caHashSink. Note that that requisite is currently true
        because the former is only used in the flat case. */
-    RetrieveRegularNARSink fileSink { caHashSink };
+    RegularFileSink fileSink { caHashSink };
     TeeSink unusualHashTee { narHashSink, caHashSink };
 
     auto & narSink = method == FileIngestionMethod::Recursive && hashAlgo != htSHA256
@@ -428,10 +428,10 @@ ValidPathInfo Store::addToStoreSlow(std::string_view name, const Path & srcPath,
        information to narSink. */
     TeeSource tapped { *fileSource, narSink };
 
-    ParseSink blank;
+    NullParseSink blank;
     auto & parseSink = method == FileIngestionMethod::Flat
-        ? fileSink
-        : blank;
+        ? (ParseSink &) fileSink
+        : (ParseSink &) blank;
 
     /* The information that flows from tapped (besides being replicated in
        narSink), is now put in parseSink. */