Reuse the same reader across ReadChunk calls so that prefetched block…

…s are not discared.

In the sequential non-parallel path (ReadRecord->ReadAheadFromBuffer->ReadChunk) ReadChunk reads the whole chunk into memory. Most readers will round up the read to the blocksize and thereby prefetch some data from the next chunk.

Currently ReadChunk recreates the reader each time and the previously prefetched data is lost and must be read again.

Instead use get_backing_reader() directly each time. We can do this since ReadChunk is only called in single threaded context (Initialize, ReadRecord (parallelism disabled)) so there cannot be any concurrent access to the underlying reader.

ReadTrace (before):
```
offset=64, size=1M <-- 1st chunk
offset=1M, size=1M
...
offset=8M, size=1M <-- block aligned read crosses chunk boundary
offset=8.3M, size=0.7M <-- redundant read for 2nd chunk
offset=9M, size=1M
...
```

ReadTrace (after):
```
offset=64, size=1M <-- 1st chunk
offset=1M, size=1M
...
offset=8M, size=1M <-- block aligned read crosses chunk boundary and is reused for the 2nd chunk
offset=9M, size=1M
...
```
PiperOrigin-RevId: 577312179

cpp/BUILD

@@ -168,6 +168,7 @@ cc_library(
         "@com_google_riegeli//riegeli/base:options_parser",
         "@com_google_riegeli//riegeli/base:status",
         "@com_google_riegeli//riegeli/bytes:reader",
+        "@com_google_riegeli//riegeli/bytes:wrapped_reader",
         "@com_google_riegeli//riegeli/chunk_encoding:chunk_decoder",
         "@com_google_riegeli//riegeli/records:chunk_reader",
         "@com_google_riegeli//riegeli/records:record_position",

cpp/array_record_reader.cc

@@ -46,6 +46,7 @@ limitations under the License.
 #include "riegeli/base/options_parser.h"
 #include "riegeli/base/status.h"
 #include "riegeli/bytes/reader.h"
+#include "riegeli/bytes/wrapped_reader.h"
 #include "riegeli/chunk_encoding/chunk_decoder.h"
 #include "riegeli/records/chunk_reader.h"
 #include "riegeli/records/record_position.h"
@@ -70,6 +71,7 @@ using riegeli::ChunkReader;
 using riegeli::OptionsParser;
 using riegeli::Reader;
 using riegeli::ValueParser;
+using riegeli::WrappedReader;
 
 template <class T>
 T CeilOfRatio(T x, T d) {
@@ -159,20 +161,15 @@ ArrayRecordReaderBase& ArrayRecordReaderBase::operator=(
   return *this;
 }
 
-// After the first access to the underlying `riegeli::Reader`, the lazily
-// evaluated variables for random access are all initialized. Therefore it's
-// safe to access the reader from multiple threads later on, even though the
-// methods wasn't const.
-ChunkDecoder ReadChunk(const ThreadCompatibleSharedPtr<Reader>& reader,
-                       size_t pos, size_t len) {
+ChunkDecoder ReadChunk(Reader* mutable_reader, size_t pos, size_t len) {
   ChunkDecoder decoder;
-  if (!reader->ok()) {
-    decoder.Fail(reader->status());
+  if (!mutable_reader->ok()) {
+    decoder.Fail(mutable_reader->status());
     return decoder;
   }
-  Reader* mutable_reader =
-      const_cast<Reader*>(reinterpret_cast<const Reader*>(reader.get()));
-  MaskedReader masked_reader(mutable_reader->NewReader(pos), len);
+  mutable_reader->Seek(pos);
+  MaskedReader masked_reader(std::make_unique<WrappedReader<>>(mutable_reader),
+                             len);
   if (!masked_reader.ok()) {
     decoder.Fail(masked_reader.status());
     return decoder;
@@ -227,7 +224,7 @@ void ArrayRecordReaderBase::Initialize() {
     }
     RiegeliPostscript postscript;
     auto postscript_decoder =
-        ReadChunk(reader, size - kRiegeliBlockSize, kRiegeliBlockSize);
+        ReadChunk(mutable_reader, size - kRiegeliBlockSize, kRiegeliBlockSize);
     if (!postscript_decoder.ReadRecord(postscript)) {
       Fail(Annotate(postscript_decoder.status(),
                     "Failed to read RiegeliPostscript"));
@@ -245,7 +242,7 @@ void ArrayRecordReaderBase::Initialize() {
     }
     state_->footer_offset = postscript.footer_offset();
     footer_decoder =
-        ReadChunk(reader, postscript.footer_offset(),
+        ReadChunk(mutable_reader, postscript.footer_offset(),
                   size - kRiegeliBlockSize - postscript.footer_offset());
 
     if (!footer_decoder.ReadRecord(footer_metadata)) {
@@ -670,11 +667,13 @@ bool ArrayRecordReaderBase::ReadAheadFromBuffer(uint64_t buffer_idx) {
     uint64_t chunk_end = std::min(state_->chunk_offsets.size(),
                                   (buffer_idx + 1) * state_->chunk_group_size);
     const auto reader = get_backing_reader();
+    Reader* mutable_reader =
+        const_cast<Reader*>(reinterpret_cast<const Reader*>(reader.get()));
     for (uint64_t chunk_idx = chunk_start; chunk_idx < chunk_end; ++chunk_idx) {
       uint64_t chunk_offset = state_->chunk_offsets[chunk_idx];
       uint64_t chunk_end_offset = state_->ChunkEndOffset(chunk_idx);
-      decoders.push_back(
-          ReadChunk(reader, chunk_offset, chunk_end_offset - chunk_offset));
+      decoders.push_back(ReadChunk(mutable_reader, chunk_offset,
+                                   chunk_end_offset - chunk_offset));
     }
     state_->buffer_idx = buffer_idx;
     state_->current_decoders = std::move(decoders);