Demonstrate how flexible read could look like

file/file_prefetch_buffer.h

@@ -526,6 +526,11 @@ class FilePrefetchBuffer {
     read_req.offset = offset;
     read_req.len = n;
     read_req.scratch = nullptr;
+    // In IOOptions we may want to pass in option to allow/disallow
+    // additional_readahead_size Another option is for FSReadRequest to just
+    // have a bool allow_read_to_block_boundary, so that we do not even need to
+    // expose the block size information to begin with..
+    read_req.additional_read_size = reader->GetBlockSize();
     IOStatus s = reader->MultiRead(opts, &read_req, 1, nullptr);
     if (!s.ok()) {
       return s;

file/random_access_file_reader.h

@@ -192,5 +192,7 @@ class RandomAccessFileReader {
                      AlignedBuf* aligned_buf);
 
   void ReadAsyncCallback(FSReadRequest& req, void* cb_arg);
+
+  size_t GetBlockSize() const { return file_->GetBlockSize(); }
 };
 }  // namespace ROCKSDB_NAMESPACE

include/rocksdb/file_system.h

@@ -812,6 +812,11 @@ struct FSReadRequest {
   // returns fewer bytes if end of file is hit (or `status` is not OK).
   size_t len;
 
+  // Allows more than "len" bytes to be returned
+  // Alternative: bool allow_more_data which allows returning up to the next
+  // block
+  size_t additional_read_size;
+
   // A buffer that MultiRead() can optionally place data in. It can
   // ignore this and allocate its own buffer.
   // The lifecycle of scratch will be until IO is completed.
@@ -950,6 +955,12 @@ class FSRandomAccessFile {
   // aligned buffer for Direct I/O
   virtual size_t GetRequiredBufferAlignment() const { return kDefaultPageSize; }
 
+  // Default of 0 for when block size concept does not apply
+  // This method is not necessary if our FSReadRequest contains an option to
+  // allow a variable amount of additional data to be sent along the response
+  // (unrelated to the block size)
+  virtual size_t GetBlockSize() const { return 0; }
+
   // Remove any kind of caching of data from the offset to offset+length
   // of this file. If the length is 0, then it refers to the end of file.
   // If the system is not caching the file contents, then this is a noop.