diff --git a/src/libstore/daemon.cc b/src/libstore/daemon.cc
index e370e278c52..c3728e2b3c1 100644
--- a/src/libstore/daemon.cc
+++ b/src/libstore/daemon.cc
@@ -147,7 +147,7 @@ struct TunnelLogger : public Logger
     }
 };
 
-struct TunnelSink : Sink
+struct TunnelSink : public Sink
 {
     Sink & to;
     TunnelSink(Sink & to) : to(to) { }
@@ -175,7 +175,7 @@ struct TunnelSource : BufferedSource
 
 /* If the NAR archive contains a single file at top-level, then save
    the contents of the file to `s'.  Otherwise barf. */
-struct RetrieveRegularNARSink : ParseSink
+struct RetrieveRegularNARSink : public ParseSink
 {
     bool regular;
     string s;
@@ -721,9 +721,10 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
         if (GET_PROTOCOL_MINOR(clientVersion) >= 21)
             source = std::make_unique<TunnelSource>(from, to);
         else {
-            TeeSink tee(from);
-            parseDump(tee, tee.source);
-            saved = std::move(*tee.source.data);
+            TeeSource tee(from);
+            ParseSink sink;
+            parseDump(sink, tee);
+            saved = std::move(*tee.data);
             source = std::make_unique<StringSource>(saved);
         }
 
diff --git a/src/libstore/export-import.cc b/src/libstore/export-import.cc
index cb9da027dd6..38c21cb6c7b 100644
--- a/src/libstore/export-import.cc
+++ b/src/libstore/export-import.cc
@@ -7,7 +7,7 @@
 
 namespace nix {
 
-struct HashAndWriteSink : Sink
+struct HashAndWriteSink : public Sink
 {
     Sink & writeSink;
     HashSink hashSink;
@@ -77,8 +77,9 @@ StorePaths Store::importPaths(Source & source, std::shared_ptr<FSAccessor> acces
         if (n != 1) throw Error("input doesn't look like something created by 'nix-store --export'");
 
         /* Extract the NAR from the source. */
-        TeeSink tee(source);
-        parseDump(tee, tee.source);
+        TeeSource tee(source);
+        ParseSink sink;
+        parseDump(sink, tee);
 
         uint32_t magic = readInt(source);
         if (magic != exportMagic)
@@ -94,15 +95,15 @@ StorePaths Store::importPaths(Source & source, std::shared_ptr<FSAccessor> acces
         if (deriver != "")
             info.deriver = parseStorePath(deriver);
 
-        info.narHash = hashString(htSHA256, *tee.source.data);
-        info.narSize = tee.source.data->size();
+        info.narHash = hashString(htSHA256, *tee.data);
+        info.narSize = tee.data->size();
 
         // Ignore optional legacy signature.
         if (readInt(source) == 1)
             readString(source);
 
         // Can't use underlying source, which would have been exhausted
-        auto source = StringSource { *tee.source.data };
+        auto source = StringSource { *tee.data };
         addToStore(info, source, NoRepair, checkSigs, accessor);
 
         res.push_back(info.path);
diff --git a/src/libstore/filetransfer.cc b/src/libstore/filetransfer.cc
index c954ace7f77..43cebf01d7b 100644
--- a/src/libstore/filetransfer.cc
+++ b/src/libstore/filetransfer.cc
@@ -22,6 +22,7 @@
 #include <queue>
 #include <random>
 #include <thread>
+#include <variant>
 
 using namespace std::string_literals;
 
@@ -56,7 +57,6 @@ struct curlFileTransfer : public FileTransfer
         Callback<FileTransferResult> callback;
         CURL * req = 0;
         bool active = false; // whether the handle has been added to the multi object
-        std::string status;
 
         unsigned int attempt = 0;
 
@@ -72,6 +72,8 @@ struct curlFileTransfer : public FileTransfer
 
         curl_off_t writtenToSink = 0;
 
+        inline static const std::set<long> successfulStatuses;
+
         /* Get the HTTP status code, or 0 for other protocols. */
         long getHTTPStatus()
         {
@@ -98,7 +100,7 @@ struct curlFileTransfer : public FileTransfer
 
                     /* Only write data to the sink if this is a
                        successful response. */
-                    if (httpStatus == 0 || httpStatus == 200 || httpStatus == 201 || httpStatus == 206) {
+                    if (successfulStatuses.find(httpStatus) != successfulStatuses.end()) {
                         writtenToSink += len;
                         this->request.dataCallback((char *) data, len);
                     }
@@ -122,7 +124,7 @@ struct curlFileTransfer : public FileTransfer
             if (requestHeaders) curl_slist_free_all(requestHeaders);
             try {
                 if (!done)
-                    fail(FileTransferError(Interrupted, "download of '%s' was interrupted", request.uri));
+                    fail(FileTransferError(Interrupted, nullptr, "download of '%s' was interrupted", request.uri));
             } catch (...) {
                 ignoreException();
             }
@@ -142,7 +144,9 @@ struct curlFileTransfer : public FileTransfer
         }
 
         LambdaSink finalSink;
-        std::shared_ptr<CompressionSink> decompressionSink;
+        // std::shared_ptr< std::variant<Sink, CompressionSink> > decompressionSink;
+        // std::variant< std::shared_ptr<Sink>, std::shared_ptr<CompressionSink> > decompressionSink;
+        std::shared_ptr<Sink> decompressionSink;
 
         std::exception_ptr writeException;
 
@@ -152,8 +156,13 @@ struct curlFileTransfer : public FileTransfer
                 size_t realSize = size * nmemb;
                 result.bodySize += realSize;
 
-                if (!decompressionSink)
-                    decompressionSink = makeDecompressionSink(encoding, finalSink);
+                // if (!decompressionSink)
+                //     // decompressionSink = makeDecompressionSink(encoding, finalSink);
+                //     decompressionSink.reset(*makeDecompressionSink(encoding, finalSink));
+
+                // Let's see some types:
+                // makeDecompressionSink(...) :: ref<CompressionSink>
+                //
 
                 (*decompressionSink)((unsigned char *) contents, realSize);
 
@@ -174,6 +183,7 @@ struct curlFileTransfer : public FileTransfer
             size_t realSize = size * nmemb;
             std::string line((char *) contents, realSize);
             printMsg(lvlVomit, format("got header for '%s': %s") % request.uri % trim(line));
+            std::string status;
             if (line.compare(0, 5, "HTTP/") == 0) { // new response starts
                 result.etag = "";
                 auto ss = tokenizeString<vector<string>>(line, " ");
@@ -182,6 +192,13 @@ struct curlFileTransfer : public FileTransfer
                 result.bodySize = 0;
                 acceptRanges = false;
                 encoding = "";
+                if (successfulStatuses.find(std::stoi(status)) != successfulStatuses.end()) {
+                    // Here we want to construct just the sink
+                    decompressionSink.reset(static_cast<Sink>(makeDecompressionSink(encoding, finalSink).get_ptr()));
+                } else {
+                    // In this case we want to construct a TeeSink, to keep the error around
+                    decompressionSink = std::make_shared<TeeSink>{*decompressionSink};
+                }
             } else {
                 auto i = line.find(':');
                 if (i != string::npos) {
@@ -352,8 +369,7 @@ struct curlFileTransfer : public FileTransfer
             if (writeException)
                 failEx(writeException);
 
-            else if (code == CURLE_OK &&
-                (httpStatus == 200 || httpStatus == 201 || httpStatus == 204 || httpStatus == 206 || httpStatus == 304 || httpStatus == 0 /* other protocol */))
+            else if (code == CURLE_OK && successfulStatuses.find(httpStatus) != successfulStatuses.end())
             {
                 result.cached = httpStatus == 304;
                 act.progress(result.bodySize, result.bodySize);
@@ -410,14 +426,14 @@ struct curlFileTransfer : public FileTransfer
 
                 auto exc =
                     code == CURLE_ABORTED_BY_CALLBACK && _isInterrupted
-                    ? FileTransferError(Interrupted, fmt("%s of '%s' was interrupted", request.verb(), request.uri))
+                    ? FileTransferError(Interrupted, nullptr, fmt("%s of '%s' was interrupted", request.verb(), request.uri))
                     : httpStatus != 0
-                    ? FileTransferError(err,
+                    ? FileTransferError(err, nullptr,
                         fmt("unable to %s '%s': HTTP error %d",
                             request.verb(), request.uri, httpStatus)
                         + (code == CURLE_OK ? "" : fmt(" (curl error: %s)", curl_easy_strerror(code)))
                         )
-                    : FileTransferError(err,
+                    : FileTransferError(err, nullptr,
                         fmt("unable to %s '%s': %s (%d)",
                             request.verb(), request.uri, curl_easy_strerror(code), code));
 
@@ -675,7 +691,7 @@ struct curlFileTransfer : public FileTransfer
                 auto s3Res = s3Helper.getObject(bucketName, key);
                 FileTransferResult res;
                 if (!s3Res.data)
-                    throw FileTransferError(NotFound, fmt("S3 object '%s' does not exist", request.uri));
+                    throw FileTransferError(NotFound, nullptr, fmt("S3 object '%s' does not exist", request.uri));
                 res.data = s3Res.data;
                 callback(std::move(res));
 #else
diff --git a/src/libstore/filetransfer.hh b/src/libstore/filetransfer.hh
index 11dca2fe0f0..44f550d6186 100644
--- a/src/libstore/filetransfer.hh
+++ b/src/libstore/filetransfer.hh
@@ -103,9 +103,10 @@ class FileTransferError : public Error
 {
 public:
     FileTransfer::Error error;
+    std::optional<string> response;
     template<typename... Args>
-    FileTransferError(FileTransfer::Error error, const Args & ... args)
-        : Error(args...), error(error)
+    FileTransferError(FileTransfer::Error error, std::optional<string> response, const Args & ... args)
+        : Error(args...), error(error), response(response)
     { }
 };
 
diff --git a/src/libstore/references.cc b/src/libstore/references.cc
index a10d536a355..4bb556c0a97 100644
--- a/src/libstore/references.cc
+++ b/src/libstore/references.cc
@@ -46,7 +46,7 @@ static void search(const unsigned char * s, size_t len,
 }
 
 
-struct RefScanSink : Sink
+struct RefScanSink : public Sink
 {
     HashSink hashSink;
     StringSet hashes;
diff --git a/src/libutil/archive.cc b/src/libutil/archive.cc
index 6a848470520..92213adc969 100644
--- a/src/libutil/archive.cc
+++ b/src/libutil/archive.cc
@@ -295,7 +295,7 @@ void parseDump(ParseSink & sink, Source & source)
 }
 
 
-struct RestoreSink : ParseSink
+struct RestoreSink : public ParseSink
 {
     Path dstPath;
     AutoCloseFD fd;
diff --git a/src/libutil/archive.hh b/src/libutil/archive.hh
index 768fe25368d..32d98a6104d 100644
--- a/src/libutil/archive.hh
+++ b/src/libutil/archive.hh
@@ -63,13 +63,6 @@ struct ParseSink
     virtual void createSymlink(const Path & path, const string & target) { };
 };
 
-struct TeeSink : ParseSink
-{
-    TeeSource source;
-
-    TeeSink(Source & source) : source(source) { }
-};
-
 void parseDump(ParseSink & sink, Source & source);
 
 void restorePath(const Path & path, Source & source);
diff --git a/src/libutil/compression.cc b/src/libutil/compression.cc
index a117ddc72e1..86c8df8cb8d 100644
--- a/src/libutil/compression.cc
+++ b/src/libutil/compression.cc
@@ -18,7 +18,7 @@
 namespace nix {
 
 // Don't feed brotli too much at once.
-struct ChunkedCompressionSink : CompressionSink
+struct ChunkedCompressionSink : public CompressionSink
 {
     uint8_t outbuf[32 * 1024];
 
@@ -36,7 +36,7 @@ struct ChunkedCompressionSink : CompressionSink
     virtual void writeInternal(const unsigned char * data, size_t len) = 0;
 };
 
-struct NoneSink : CompressionSink
+struct NoneSink : public CompressionSink
 {
     Sink & nextSink;
     NoneSink(Sink & nextSink) : nextSink(nextSink) { }
@@ -44,7 +44,7 @@ struct NoneSink : CompressionSink
     void write(const unsigned char * data, size_t len) override { nextSink(data, len); }
 };
 
-struct GzipDecompressionSink : CompressionSink
+struct GzipDecompressionSink : public CompressionSink
 {
     Sink & nextSink;
     z_stream strm;
@@ -104,7 +104,7 @@ struct GzipDecompressionSink : CompressionSink
     }
 };
 
-struct XzDecompressionSink : CompressionSink
+struct XzDecompressionSink : public CompressionSink
 {
     Sink & nextSink;
     uint8_t outbuf[BUFSIZ];
@@ -156,7 +156,7 @@ struct XzDecompressionSink : CompressionSink
     }
 };
 
-struct BzipDecompressionSink : ChunkedCompressionSink
+struct BzipDecompressionSink : public ChunkedCompressionSink
 {
     Sink & nextSink;
     bz_stream strm;
@@ -209,7 +209,7 @@ struct BzipDecompressionSink : ChunkedCompressionSink
     }
 };
 
-struct BrotliDecompressionSink : ChunkedCompressionSink
+struct BrotliDecompressionSink : public ChunkedCompressionSink
 {
     Sink & nextSink;
     BrotliDecoderState * state;
@@ -285,7 +285,7 @@ ref<CompressionSink> makeDecompressionSink(const std::string & method, Sink & ne
         throw UnknownCompressionMethod("unknown compression method '%s'", method);
 }
 
-struct XzCompressionSink : CompressionSink
+struct XzCompressionSink : public CompressionSink
 {
     Sink & nextSink;
     uint8_t outbuf[BUFSIZ];
@@ -364,7 +364,7 @@ struct XzCompressionSink : CompressionSink
     }
 };
 
-struct BzipCompressionSink : ChunkedCompressionSink
+struct BzipCompressionSink : public ChunkedCompressionSink
 {
     Sink & nextSink;
     bz_stream strm;
@@ -417,7 +417,7 @@ struct BzipCompressionSink : ChunkedCompressionSink
     }
 };
 
-struct BrotliCompressionSink : ChunkedCompressionSink
+struct BrotliCompressionSink : public ChunkedCompressionSink
 {
     Sink & nextSink;
     uint8_t outbuf[BUFSIZ];
diff --git a/src/libutil/compression.hh b/src/libutil/compression.hh
index dd666a4e19f..9c68c1dbd30 100644
--- a/src/libutil/compression.hh
+++ b/src/libutil/compression.hh
@@ -8,7 +8,7 @@
 
 namespace nix {
 
-struct CompressionSink : BufferedSink
+struct CompressionSink : public BufferedSink
 {
     virtual void finish() = 0;
 };
diff --git a/src/libutil/hash.hh b/src/libutil/hash.hh
index 180fb76334d..42f7335dbe2 100644
--- a/src/libutil/hash.hh
+++ b/src/libutil/hash.hh
@@ -125,7 +125,7 @@ string printHashType(HashType ht);
 
 union Ctx;
 
-struct AbstractHashSink : virtual Sink
+struct AbstractHashSink : public virtual Sink
 {
     virtual HashResult finish() = 0;
 };
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index a04118512fc..bfc18a8dba4 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -24,7 +24,7 @@ struct Sink
 
 
 /* A buffered abstract sink. */
-struct BufferedSink : virtual Sink
+struct BufferedSink : public virtual Sink
 {
     size_t bufSize, bufPos;
     std::unique_ptr<unsigned char[]> buffer;
@@ -86,7 +86,7 @@ protected:
 
 
 /* A sink that writes data to a file descriptor. */
-struct FdSink : BufferedSink
+struct FdSink : public BufferedSink
 {
     int fd;
     bool warn = false;
@@ -144,7 +144,7 @@ private:
 
 
 /* A sink that writes data to a string. */
-struct StringSink : Sink
+struct StringSink : public Sink
 {
     ref<std::string> s;
     StringSink() : s(make_ref<std::string>()) { };
@@ -181,6 +181,25 @@ struct TeeSource : Source
     }
 };
 
+struct TeeSink : public Sink
+{
+    Sink & orig;
+    ref<std::string> data;
+    TeeSink(TeeSink &&) = default;
+    TeeSink(Sink & orig)
+        : orig(orig), data(make_ref<std::string>()) { }
+    void operator () (const unsigned char * data, size_t len) {
+        this->data->append((const char *) data, len);
+        Sink::operator()(data, len);
+    }
+    void operator () (const std::string & s)
+    {
+        *data += s;
+        Sink::operator()(s);
+    }
+};
+
+
 /* A reader that consumes the original Source until 'size'. */
 struct SizedSource : Source
 {
@@ -213,7 +232,7 @@ struct SizedSource : Source
 };
 
 /* Convert a function into a sink. */
-struct LambdaSink : Sink
+struct LambdaSink : public Sink
 {
     typedef std::function<void(const unsigned char *, size_t)> lambda_t;