file_stream: proper seek support.

Our file_stream interface supports seek, but when we try to seek to arbitrary
locations that are smaller than an aio-boundary (say, for instance, f->seek(4)),
we will end up not being able to perform the read.

We need to guarantee the reads are aligned, and will then present to the caller
the buffer properly offset.

Signed-off-by: Glauber Costa <[email protected]>

core/fstream.cc

@@ -17,10 +17,18 @@ file_data_source_impl::get() {
     auto q = static_cast<char*>(p);
     temporary_buffer<char> buf(q, _buffer_size, make_free_deleter(p));
     auto old_pos = _pos;
-    _pos += _buffer_size;
+    // dma_read needs to be aligned. It doesn't have to be page-aligned,
+    // though, and we could get away with something much smaller. However, if
+    // we start reading in things outside page boundaries, we will end up with
+    // various pages around, some of them with overlapping ranges. Those would
+    // be very challenging to cache.
+    old_pos &= ~4095;
+    auto front = _pos - old_pos;
+    _pos += _buffer_size - front;
     return _file.dma_read(old_pos, q, _buffer_size).then(
-            [buf = std::move(buf)] (size_t size) mutable {
+            [buf = std::move(buf), front] (size_t size) mutable {
         buf.trim(size);
+        buf.trim_front(front);
         return make_ready_future<temporary_buffer<char>>(std::move(buf));
     });
 }