Merge "Fix commitlog chunks overwriting each other" from Paweł

"
This series fixes a problem in the commitlog cycle() function that
confused in-memory and on-disk size of chunks it wrote to disk. The
former was used to decide how much data needs to be actually written,
and the latter was used to compute the offset of the next chunk. If two
chunk writes happened concurrently one the one positioned earlier in
the file could corrupt the header of the next one.

Fixes #4231.

Tests: unit(dev), dtest(commitlog_test.py:TestCommitLog.test_commitlog_replay_on_startup,test_commitlog_replay_with_alter_table)
"

* tag 'fix-commitlog-cycle/v1' of https://github.com/pdziepak/scylla:
  commitlog: write the correct buffer size
  utils/fragmented_temporary_buffer_view: add remove suffix

(cherry picked from commit d95dec22d9)

db/commitlog/commitlog.cc

@@ -689,6 +689,8 @@ public:
         // but all previous write/flush pairs.
         return _pending_ops.run_with_ordered_post_op(rp, [this, size, off, buf = std::move(buf)]() mutable { ///////////////////////////////////////////////////
             auto view = fragmented_temporary_buffer::view(buf);
+            view.remove_suffix(buf.size_bytes() - size);
+            assert(size == view.size_bytes());
             return do_with(off, view, [&] (uint64_t& off, fragmented_temporary_buffer::view& view) {
                 if (view.empty()) {
                     return make_ready_future<>();