crimson/osd: clear data_included if read fails

* extract read_object_for_push_op() out of build_push_op().
  build_push_op() is composed of multiple steps. would be better
  if we can split and extract them into separated method for better
  readability
* clear push_op->data_included() if fiemap fails. before this change,
  a local copy of `copy_subset` is cleared if fiemap fails, but
  push_op->data_included is not updated in this case. we should
  clear the latter if no data is returned in push_op->data. so,
  in this change, push_op->data_included is cleared if fiemap
  fails. the same applies to other cases where no data is returned in
  push_op
* add comments to outline the steps in read_object_for_push_op().

Signed-off-by: Kefu Chai <kchai@redhat.com>

src/crimson/osd/replicated_recovery_backend.cc

@@ -452,48 +452,16 @@ seastar::future<ObjectRecoveryProgress> ReplicatedRecoveryBackend::build_push_op
 	});
       }
       return seastar::make_ready_future<>();
-    }).then([this, &recovery_info, &progress, &available, &new_progress, pop] {
+    }).then([this, &recovery_info, &progress, &available, pop] {
       logger().debug("build_push_op: available: {}, copy_subset: {}",
 		     available, recovery_info.copy_subset);
-      if (available > 0) {
-	if (!recovery_info.copy_subset.empty()) {
-	  return seastar::do_with(interval_set<uint64_t>(recovery_info.copy_subset),
-	    [this, &recovery_info, &progress, &available, pop, &new_progress]
-	    (auto& copy_subset) {
-	    return backend->fiemap(coll, ghobject_t(recovery_info.soid),
-			  0, copy_subset.range_end()).then(
-	      [&copy_subset](auto m) {
-	      interval_set<uint64_t> fiemap_included(std::move(m));
-	      copy_subset.intersection_of(fiemap_included);
-	      return seastar::make_ready_future<>();
-	    }).then([&recovery_info, &progress,
-	      &copy_subset, &available, pop, &new_progress] {
-	      pop->data_included.span_of(copy_subset, progress.data_recovered_to,
-					 available);
-	      if (pop->data_included.empty()) // zero filled section, skip to end!
-		new_progress.data_recovered_to =
-		  recovery_info.copy_subset.range_end();
-	      else
-		new_progress.data_recovered_to = pop->data_included.range_end();
-	      return seastar::make_ready_future<>();
-	    }).handle_exception([&copy_subset](auto e) {
-	      copy_subset.clear();
-	      return seastar::make_ready_future<>();
-	    });
-	  });
-	} else {
-	  return seastar::now();
-	}
-      } else {
-	pop->data_included.clear();
-	return seastar::make_ready_future<>();
-      }
-    }).then([this, &oi, pop] {
-      return store->readv(coll, ghobject_t{oi.soid}, pop->data_included, 0);
-    }).safe_then([&recovery_info, &progress,
-      &new_progress, stat, pop, &v]
-      (auto bl) {
-      pop->data.claim_append(bl);
+      return read_object_for_push_op(recovery_info.soid,
+				     recovery_info.copy_subset,
+				     progress.data_recovered_to,
+				     available, pop);
+    }).then([this, &recovery_info, &v, &progress, &new_progress, stat, pop]
+            (uint64_t recovered_to) {
+      new_progress.data_recovered_to = recovered_to;
       if (new_progress.is_complete(recovery_info)) {
 	new_progress.data_complete = true;
 	if (stat)
@@ -516,14 +484,58 @@ seastar::future<ObjectRecoveryProgress> ReplicatedRecoveryBackend::build_push_op
 		     pop->version, pop->data.length());
       return seastar::make_ready_future<ObjectRecoveryProgress>
 		(std::move(new_progress));
-    }, PGBackend::read_errorator::all_same_way([](auto e) {
-	logger().debug("build_push_op: read exception");
-	return seastar::make_exception_future<ObjectRecoveryProgress>(e);
-      })
-    );
+    });
   });
 }
 
+seastar::future<uint64_t>
+ReplicatedRecoveryBackend::read_object_for_push_op(
+    const hobject_t& oid,
+    const interval_set<uint64_t>& copy_subset,
+    uint64_t offset,
+    uint64_t max_len,
+    PushOp* push_op)
+{
+  if (max_len == 0 || copy_subset.empty()) {
+    push_op->data_included.clear();
+    return seastar::make_ready_future<uint64_t>(offset);
+  }
+  // 1. get the extents in the interested range
+  return backend->fiemap(coll, ghobject_t{oid},
+                         0, copy_subset.range_end()).then_wrapped(
+    [=](auto&& fiemap_included) mutable {
+    interval_set<uint64_t> extents;
+    try {
+      extents.intersection_of(copy_subset, fiemap_included.get0());
+    } catch (std::exception &) {
+      // if fiemap() fails, we will read nothing, as the intersection of
+      // copy_subset and an empty interval_set would be empty anyway
+      extents.clear();
+    }
+    // 2. we can read up to "max_len" bytes from "offset", so truncate the
+    //    extents down to this quota. no need to return the number of consumed
+    //    bytes, as this is the last consumer of this quota
+    push_op->data_included.span_of(extents, offset, max_len);
+    // 3. read the truncated extents
+    // TODO: check if the returned extents are pruned
+    return store->readv(coll, ghobject_t{oid}, push_op->data_included, 0);
+  }).safe_then([push_op, range_end=copy_subset.range_end()](auto &&bl) {
+    push_op->data.claim_append(std::move(bl));
+    uint64_t recovered_to = 0;
+    if (push_op->data_included.empty()) {
+      // zero filled section, skip to end!
+      recovered_to = range_end;
+    } else {
+      // note down the progress, we will start from there next time
+      recovered_to = push_op->data_included.range_end();
+    }
+    return seastar::make_ready_future<uint64_t>(recovered_to);
+  }, PGBackend::read_errorator::all_same_way([](auto e) {
+    logger().debug("build_push_op: read exception");
+    return seastar::make_exception_future<uint64_t>(e);
+  }));
+}
+
 std::list<std::map<pg_shard_t, pg_missing_t>::const_iterator>
 ReplicatedRecoveryBackend::get_shards_to_push(const hobject_t& soid)
 {

src/crimson/osd/replicated_recovery_backend.h

@@ -108,4 +108,18 @@ protected:
   seastar::future<> on_stop() final {
     return seastar::now();
   }
+private:
+  /// read the remaining extents of object to be recovered and fill push_op
+  /// with them
+  ///
+  /// @param oid object being recovered
+  /// @param copy_subset extents we want
+  /// @param offset the offset in object from where we should read
+  /// @return the new offset
+  seastar::future<uint64_t> read_object_for_push_op(
+    const hobject_t& oid,
+    const interval_set<uint64_t>& copy_subset,
+    uint64_t offset,
+    uint64_t max_len,
+    PushOp* push_op);
 };