sstables: random_access_reader: let file_random_access_reader set the input stream

Allow file_random_access_reader constructor to set the
input stream to prepare for futurizing seek() by adding
a protected set() method.

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

sstables/random_access_reader.cc

@@ -42,7 +42,7 @@ void random_access_reader::seek(uint64_t pos) {
             return fut.then([in = std::move(in)] {});
         });
     }
-    _in = std::make_unique < input_stream < char >> (open_at(pos));
+    set(open_at(pos));
 }
 
 future<> random_access_reader::close() {
@@ -53,7 +53,7 @@ future<> random_access_reader::close() {
 
 file_random_access_reader::file_random_access_reader(file f, uint64_t file_size, size_t buffer_size, unsigned read_ahead)
     : _file(std::move(f)), _file_size(file_size), _buffer_size(buffer_size), _read_ahead(read_ahead) {
-    seek(0);
+    set(open_at(0));
 }
 
 input_stream<char> file_random_access_reader::open_at(uint64_t pos) {

sstables/random_access_reader.hh

@@ -36,6 +36,11 @@ class random_access_reader {
 protected:
     virtual input_stream<char> open_at(uint64_t pos) = 0;
 
+    void set(input_stream<char> in) {
+        assert(!_in);
+        _in = std::make_unique<input_stream<char>>(std::move(in));
+    }
+
 public:
     future <temporary_buffer<char>> read_exactly(size_t n);