Avoid crash while reloading non-existing file (#1308)

It can happen during a rebase operation that a file appears/disappears
multiple times in a very short timeframe. Specifically, since
`didChangeWatchedFiles` notifications are asynchronous, it can happen
that a file is deleted before a notification is processed by the
server. In such a case, the server should simply ignore the
notification, since a new one will arrive.

Also, there is no need to deeply index files during a rebase, so let's
convert to a shallow indexing.

We mark the source of the file as 'app', which has the only side
effect of indexing references.

apps/els_lsp/src/els_indexing.erl

@@ -10,6 +10,7 @@
     maybe_start/0,
     ensure_deeply_indexed/1,
     shallow_index/2,
+    shallow_index/3,
     deep_index/1,
     remove/1
 ]).

apps/els_lsp/src/els_text_synchronization.erl

@@ -99,9 +99,26 @@ handle_file_change(Uri, Type) when Type =:= ?FILE_CHANGE_TYPE_DELETED ->
 
 -spec reload_from_disk(uri()) -> ok.
 reload_from_disk(Uri) ->
-    {ok, Text} = file:read_file(els_uri:path(Uri)),
-    {ok, Document} = els_utils:lookup_document(Uri),
-    els_indexing:deep_index(Document#{text => Text}),
+    Path = els_uri:path(Uri),
+    case file:read_file(Path) of
+        {ok, Text} ->
+            els_indexing:shallow_index(Uri, Text, app);
+        {error, Error} ->
+            %% File is not accessible. This can happen, for example,
+            %% during a rebase operation, when the file "appears and
+            %% disappears" multiple times in a very short
+            %% timeframe. Just log the fact as a warning, but keep
+            %% going. It should be possible to buffer
+            %% 'didChangeWatchedFiles' requests, but it's not a big
+            %% deal.
+            ?LOG_WARNING(
+                "Error while reloading from disk. Ignoring. "
+                "[uri=~p] [error=~p]",
+                [
+                    Uri, Error
+                ]
+            )
+    end,
     ok.
 
 -spec background_index(els_dt_document:item()) -> {ok, pid()}.