memtx: fix wrong index mask in rebound read tracker during rollback

During rollback read trackers are rebound to newer or older story, if
present: the index mask passed to `memtx_tx_track_read_story_slow`
is equal to the current index from which the story is unlinked and it
overwrites the tracker's index mask — pass the tracker's index mask
instead.

Closes tarantool#7828

NO_DOC=bugfix

changelogs/unreleased/gh-7828-rollback_rebind_read_tracker_wrong_idx_mask.md

@@ -0,0 +1,4 @@
+## bugfix/memtx
+
+* Fixed phantom read possible after reads from different indexes followed by a
+  rollback (gh-7828).

src/box/memtx_tx.c

@@ -1235,7 +1235,7 @@ memtx_tx_story_unlink_both_on_rollback(struct memtx_story *story, uint32_t idx)
 		if ((tracker->index_mask & index_mask) != 0) {
 			memtx_tx_track_read_story_slow(tracker->reader,
 						       rebind_story,
-						       index_mask);
+						       tracker->index_mask);
 			rlist_del(&tracker->in_reader_list);
 			rlist_del(&tracker->in_read_set);
 		}

test/box-luatest/gh_7828_rollback_rebind_read_tracker_wrong_idx_mask_test.lua

@@ -0,0 +1,50 @@
+local server = require('test.luatest_helpers.server')
+local t = require('luatest')
+
+local g = t.group()
+
+g.before_all(function(cg)
+    cg.server = server:new {
+        alias   = 'dflt',
+        box_cfg = {memtx_use_mvcc_engine = true}
+    }
+    cg.server:start()
+    cg.server:exec(function()
+        local s = box.schema.create_space('s')
+        s:create_index('pk')
+        s:create_index('sk', {parts = {{2}}})
+    end)
+end)
+
+g.after_all(function(cg)
+    cg.server:drop()
+end)
+
+--[[
+Checks that during rollback read tracker is rebound with correct index mask.
+]]
+g.test_rollback_rebind_read_tracker_correct_idx_mask = function(cg)
+    local stream1 = cg.server.net_box:new_stream()
+    local stream2 = cg.server.net_box:new_stream()
+    local stream3 = cg.server.net_box:new_stream()
+
+    stream1:begin()
+    stream2:begin()
+    stream3:begin()
+
+    stream1.space.s:insert{1, 0}
+    stream2.space.s:insert{1, 0, 0}
+
+    stream3.space.s.index[1]:get{0}
+    stream3.space.s:select{1}
+
+    stream1:rollback()
+
+    cg.server:exec(function()
+        box.space.s:insert{0, 0}
+    end)
+
+    t.assert_equals(stream3.space.s:select{}, {})
+    t.assert_error_msg_content_equals('Transaction has been aborted by conflict',
+                                      function() stream3.space.s:replace {0, 0} end)
+end