Make nix nar cat work on pipes too

This was lost after 2.32 while making the accessor lazy. We can restore the support
for it pretty easily. Also this is significant optimization for nix nar cat.
E.g. with a NAR of a linux repo this speeds up by ~3x:

Benchmark 1: nix nar cat /tmp/linux.nar README
  Time (mean ± σ):     737.2 ms ±   5.6 ms    [User: 298.1 ms, System: 435.7 ms]
  Range (min … max):   728.6 ms … 746.9 ms    10 runs

Benchmark 2: build/src/nix/nix nar cat /tmp/linux.nar README
  Time (mean ± σ):     253.5 ms ±   2.9 ms    [User: 56.4 ms, System: 196.3 ms]
  Range (min … max):   248.1 ms … 258.7 ms    12 runs

src/libutil/include/nix/util/fs-sink.hh

@@ -12,7 +12,7 @@ namespace nix {
  *
  * See `FileSystemObjectSink::createRegularFile`.
  */
-struct CreateRegularFileSink : Sink
+struct CreateRegularFileSink : virtual Sink
 {
     /**
      * If set to true, the sink will not be called with the contents

src/nix/cat.cc

@@ -1,5 +1,6 @@
 #include "nix/cmd/command.hh"
 #include "nix/store/store-api.hh"
+#include "nix/util/archive.hh"
 #include "nix/util/nar-accessor.hh"
 #include "nix/util/serialise.hh"
 #include "nix/util/source-accessor.hh"
@@ -79,7 +80,40 @@ struct CmdCatNar : StoreCommand, MixCat
         if (!fd)
             throw SysError("opening NAR file '%s'", narPath);
         auto source = FdSource{fd.get()};
-        cat(makeLazyNarAccessor(source, seekableGetNarBytes(fd.get())), CanonPath{path});
+
+        struct CatRegularFileSink : NullFileSystemObjectSink
+        {
+            CanonPath neededPath = CanonPath::root;
+            bool found = false;
+
+            void createRegularFile(const CanonPath & path, std::function<void(CreateRegularFileSink &)> crf) override
+            {
+                struct : CreateRegularFileSink, FdSink
+                {
+                    void isExecutable() override {}
+                } crfSink;
+
+                crfSink.fd = INVALID_DESCRIPTOR;
+
+                if (path == neededPath) {
+                    logger->stop();
+                    crfSink.skipContents = false;
+                    crfSink.fd = STDOUT_FILENO;
+                    found = true;
+                } else {
+                    crfSink.skipContents = true;
+                }
+
+                crf(crfSink);
+            }
+        } sink;
+
+        sink.neededPath = CanonPath(path);
+        /* NOTE: We still parse the whole file to validate that it's a correct NAR. */
+        parseDump(sink, source);
+
+        if (!sink.found)
+            throw Error("NAR does not contain regular file '%1%'", path);
     }
 };
 

tests/functional/nar-access.sh

@@ -23,6 +23,8 @@ diff -u data.cat-nar "$storePath/foo/data"
 # Check that file contents of baz match.
 nix nar cat "$narFile" /foo/baz > baz.cat-nar
 diff -u baz.cat-nar "$storePath/foo/baz"
+nix nar cat /dev/stdin /foo/baz < "$narFile" > baz.cat-nar-pipe
+expect 1 nix nar cat "$narFile" /foo/baz/doesntexist 2>&1 | grep "NAR does not contain regular file '/foo/baz/doesntexist'"
 
 nix store cat "$storePath/foo/baz" > baz.cat-nar
 diff -u baz.cat-nar "$storePath/foo/baz"