Fix issue with sparse reading a segment that is being modified.

When a sparse read detects a modified segment and re-initialises
it is possible that the segment will again be modified before the
second read attempt. It is not necessary in this case to check
if the segment has been modified as the read plan would have been
generated before the segment was re-initialised so this commit
introduces a new ra_log_segment:read_sparse_no_checks function
that skips the modified check to avoid a crash.

src/ra_log_reader.erl

@@ -235,9 +235,18 @@ exec_read_plan(Dir, Plan, Open0, TransformFun, Options, Acc0)
                   {ok, _, Acc} ->
                       {Acc, Open2};
                   {error, modified} ->
+                      %% if the segment has been modified since it was opened
+                      %% it is not safe to attempt the read as the read plan
+                      %% may refer to indexes that weren't in the segment at
+                      %% that time. In this case we evict all segments and
+                      %% re-open what we need.
                       {_, Open3} = ra_flru:evict(BaseName, Open2),
                       {SegNew, Open} = get_segment_ext(Dir, Open3, BaseName, Options),
-                      {ok, _, Acc} = ra_log_segment:read_sparse(SegNew, Idxs, Fun, Acc1),
+                      %% at this point we can read without checking for modification
+                      %% as the read plan would have been created before we
+                      %% read the index from the segment
+                      {ok, _, Acc} = ra_log_segment:read_sparse_no_checks(
+                                       SegNew, Idxs, Fun, Acc1),
                       {Acc, Open}
               end
       end, {Acc0, Open0}, Plan).

src/ra_log_segment.erl

@@ -14,6 +14,7 @@
          fold/6,
          is_modified/1,
          read_sparse/4,
+         read_sparse_no_checks/4,
          term_query/2,
          close/1,
          range/1,
@@ -299,7 +300,8 @@ is_modified(#state{cfg = #cfg{fd = Fd},
             false;
         false ->
             %% get info and compare to data_offset
-            {ok, #file_info{size = Size}} =  prim_file:read_handle_info(Fd),
+            {ok, #file_info{size = Size}} =
+                prim_file:read_handle_info(Fd, [posix]),
             Size > DataOffset
     end.
 
@@ -308,17 +310,25 @@ is_modified(#state{cfg = #cfg{fd = Fd},
                   Acc) ->
     {ok, NumRead :: non_neg_integer(), Acc} | {error, modified}
       when Acc :: term().
-read_sparse(#state{index = Index,
-                   cfg = #cfg{fd = Fd}} = State,
-            Indexes, AccFun, Acc) ->
+read_sparse(#state{} = State, Indexes, AccFun, Acc) ->
     case is_modified(State) of
         true ->
             {error, modified};
         false ->
-            Cache0 = prepare_cache(Fd, Indexes, Index),
-            read_sparse0(Fd, Indexes, Index, Cache0, Acc, AccFun, 0)
+            read_sparse_no_checks(State, Indexes, AccFun, Acc)
     end.
 
+-spec read_sparse_no_checks(state(), [ra_index()],
+                            fun((ra:index(), ra_term(), binary(), Acc) -> Acc),
+                                Acc) ->
+    {ok, NumRead :: non_neg_integer(), Acc}
+      when Acc :: term().
+read_sparse_no_checks(#state{index = Index,
+                             cfg = #cfg{fd = Fd}},
+                      Indexes, AccFun, Acc) ->
+    Cache0 = prepare_cache(Fd, Indexes, Index),
+    read_sparse0(Fd, Indexes, Index, Cache0, Acc, AccFun, 0).
+
 read_sparse0(_Fd, [], _Index, _Cache, Acc, _AccFun, Num) ->
     {ok, Num, Acc};
 read_sparse0(Fd, [NextIdx | Rem] = Indexes, Index, Cache0, Acc, AccFun, Num)

test/ra_log_2_SUITE.erl

@@ -489,11 +489,13 @@ read_plan_modified(Config) ->
     Plan = ra_log:partial_read([1], Log2, fun (_, _, Cmd) -> Cmd end),
     {#{1 := _}, Flru} = ra_log_read_plan:execute(Plan, undefined),
 
-    Log = deliver_all_log_events(write_and_roll(2, 3, 1, Log2, 50), 100),
-    Plan2 = ra_log:partial_read([1,2], Log, fun (_, _, Cmd) -> Cmd end),
+    Log3 = deliver_all_log_events(write_and_roll(2, 3, 1, Log2, 50), 100),
+    Plan2 = ra_log:partial_read([1,2], Log3, fun (_, _, Cmd) -> Cmd end),
     %% assert we can read the newly appended item with the cached
     %% segment
-    {#{1 := _, 2 := _}, _} = ra_log_read_plan:execute(Plan2, Flru),
+    {#{1 := _, 2 := _}, Flru2} = ra_log_read_plan:execute(Plan2, Flru),
+    Log = deliver_all_log_events(write_and_roll(3, 4, 1, Log3, 50), 100),
+    {#{1 := _, 2 := _}, _} = ra_log_read_plan:execute(Plan2, Flru2),
     ra_log:close(Log),
     ok.
 

test/ra_log_segment_SUITE.erl

@@ -220,6 +220,8 @@ full_file(Config) ->
     {ok, Seg} = ra_log_segment:append(Seg1, 2, 2, Data),
     {error, full} = ra_log_segment:append(Seg, 3, 2, Data),
     ?assertNot(ra_log_segment:is_modified(Seg)),
+    {ok, R} = ra_log_segment:open(Fn, #{mode => read}),
+    ?assertNot(ra_log_segment:is_modified(R)),
     {1,2} = ra_log_segment:range(Seg),
     ok = ra_log_segment:close(Seg),
     ok.