os/bluestore: Fix BlueFS::truncate()

In `struct bluefs_fnode_t` there is a vector `extents` and
the vector `extents_index` that is a log2 seek cache.

Until modifications to truncate() we never removed extents from files.
Modified truncate() did not update extents_index.

For example 10 extents long files when truncated to 0 will have:
0 extents, 10 extents_index.
After writing some data to file:
1 extents, 11 extents_index.

Now, `bluefs_fnode_t::seek` will binary search extents_index,
lets say it located seek at item #3.
It will then jump up from #0 extent (that exists) to #3 extent which
does not exist at.
The worst part is that code is now broken, as #3 != extent.end().

There are 3 parts of the fix:
1) assert in `bluefs_fnode_t::seek` to protect against
   jumping outside extents
2) code in BlueFS::truncate to sync up `extents_index` with `extents`
3) dampening down assert in _replay to give a way out of cases
   where incorrect "offset 12345" (12345 is file size) instead of
   "offset 20000" (allocations occupied) was written to log.

Fixes: https://tracker.ceph.com/issues/69481

Signed-off-by: Adam Kupczyk <akupczyk@ibm.com>

src/os/bluestore/BlueFS.cc

@@ -1706,7 +1706,8 @@ int BlueFS::_replay(bool noop, bool to_stdout)
 		   << " fnode=" << fnode
 		   << " delta=" << delta
 		   << dendl;
-	      ceph_assert(delta.offset == fnode.allocated);
+	      // be leanient, if there is no extents just produce error message
+	      ceph_assert(delta.offset == fnode.allocated || delta.extents.empty());
 	    }
 	    if (cct->_conf->bluefs_log_replay_check_allocations) {
               int r = _check_allocations(fnode,
@@ -3830,6 +3831,7 @@ int BlueFS::truncate(FileWriter *h, uint64_t offset)/*_WF_L*/
       fnode.size = offset;
       fnode.allocated = new_allocated;
       fnode.reset_delta();
+      fnode.recalc_allocated();
       log.t.op_file_update(fnode);
       // sad, but is_dirty must be set to signal flushing of the log
       h->file->is_dirty = true;

src/os/bluestore/bluefs_types.cc

@@ -154,7 +154,9 @@ mempool::bluefs::vector<bluefs_extent_t>::iterator bluefs_fnode_t::seek(
     assert(it != extents_index.begin());
     --it;
     assert(offset >= *it);
-    p += it - extents_index.begin();
+    uint32_t skip = it - extents_index.begin();
+    ceph_assert(skip <= extents.size());
+    p += skip;
     offset -= *it;
   }
 

src/os/bluestore/bluefs_types.h

@@ -89,6 +89,7 @@ struct bluefs_fnode_t {
   void recalc_allocated() {
     allocated = 0;
     extents_index.reserve(extents.size());
+    extents_index.clear();
     for (auto& p : extents) {
       extents_index.emplace_back(allocated);
       allocated += p.length;