Unfreeze threads for object-evaluating commands

There are several commands that evaluate objects and call methods on them,
such as:
- info
- ls (outline)
- trace object
- display

If the called method acquires a mutex shared with another thread (e.g.
when using Timeout), then it'd cause a deadlock as all threads are stopped.

For example, if there's an ActiveRecord Relation object stored as a local
variable, and the user runs the `info` command, then it'd call `inspect`
on it and trigger a database query, it could cause a deadlock as described
in #877.

This commit fixes the issue by unfreezing all threads before evaluating
those commands and freezing them again after receiving the result event.

lib/debug/session.rb

@@ -256,6 +256,11 @@ def request_tc(req)
       @tc << req
     end
 
+    def request_tc_with_freed_threads(req)
+      restart_all_threads
+      request_tc(req)
+    end
+
     def process_event evt
       # variable `@internal_info` is only used for test
       tc, output, ev, @internal_info, *ev_args = evt
@@ -314,7 +319,7 @@ def process_event evt
         if @displays.empty?
           wait_command_loop
         else
-          request_tc [:eval, :display, @displays]
+          request_eval :display, @displays
         end
       when :result
         raise "[BUG] not in subsession" if @subsession_stack.empty?
@@ -329,6 +334,7 @@ def process_event evt
             end
           end
 
+          stop_all_threads
         when :method_breakpoint, :watch_breakpoint
           bp = ev_args[1]
           if bp
@@ -342,6 +348,7 @@ def process_event evt
           obj_inspect = ev_args[2]
           opt = ev_args[3]
           add_tracer ObjectTracer.new(@ui, obj_id, obj_inspect, **opt)
+          stop_all_threads
         else
           stop_all_threads
         end
@@ -810,15 +817,15 @@ def register_default_command
 
         case sub
         when nil
-          request_tc [:show, :default, pat] # something useful
+          request_tc_with_freed_threads [:show, :default, pat] # something useful
         when :locals
-          request_tc [:show, :locals, pat]
+          request_tc_with_freed_threads [:show, :locals, pat]
         when :ivars
-          request_tc [:show, :ivars, pat, opt]
+          request_tc_with_freed_threads [:show, :ivars, pat, opt]
         when :consts
-          request_tc [:show, :consts, pat, opt]
+          request_tc_with_freed_threads [:show, :consts, pat, opt]
         when :globals
-          request_tc [:show, :globals, pat]
+          request_tc_with_freed_threads [:show, :globals, pat]
         when :threads
           thread_list
           :retry
@@ -838,7 +845,7 @@ def register_default_command
       #   * Show you available methods and instance variables of the given object.
       #   * If the object is a class/module, it also lists its constants.
       register_command 'outline', 'o', 'ls', unsafe: false do |arg|
-        request_tc [:show, :outline, arg]
+        request_tc_with_freed_threads [:show, :outline, arg]
       end
 
       # * `display`
@@ -848,9 +855,9 @@ def register_default_command
       register_command 'display', postmortem: false do |arg|
         if arg && !arg.empty?
           @displays << arg
-          request_tc [:eval, :try_display, @displays]
+          request_eval :try_display, @displays
         else
-          request_tc [:eval, :display, @displays]
+          request_eval :display, @displays
         end
       end
 
@@ -864,7 +871,7 @@ def register_default_command
           if @displays[n = $1.to_i]
             @displays.delete_at n
           end
-          request_tc [:eval, :display, @displays]
+          request_eval :display, @displays
         when nil
           if ask "clear all?", 'N'
             @displays.clear
@@ -983,7 +990,7 @@ def register_default_command
           :retry
 
         when /\Aobject\s+(.+)/
-          request_tc [:trace, :object, $1.strip, {pattern: pattern, into: into}]
+          request_tc_with_freed_threads [:trace, :object, $1.strip, {pattern: pattern, into: into}]
 
         when /\Aoff\s+(\d+)\z/
           if t = @tracers.values[$1.to_i]
@@ -1165,8 +1172,7 @@ def process_command line
     end
 
     def request_eval type, src
-      restart_all_threads
-      request_tc [:eval, type, src]
+      request_tc_with_freed_threads [:eval, type, src]
     end
 
     def step_command type, arg

test/console/info_test.rb

@@ -367,4 +367,36 @@ def test_ivar_abbrev
       end
     end
   end
+
+  class InfoThreadLockingTest < ConsoleTestCase
+    def program
+      <<~RUBY
+     1| th0 = Thread.new{sleep}
+     2| $m = Mutex.new
+     3| th1 = Thread.new do
+     4|   $m.lock
+     5|   sleep 1
+     6|   $m.unlock
+     7| end
+     8|
+     9| o = Object.new
+    10| def o.inspect
+    11|   $m.lock
+    12|   "foo"
+    13| end
+    14|
+    15| sleep 0.5
+    16| debugger
+      RUBY
+    end
+
+    def test_info_doesnt_cause_deadlock
+      debug_code(program) do
+        type 'c'
+        type 'info'
+        assert_line_text(/%self = main/)
+        type 'c'
+      end
+    end
+  end
 end

test/console/outline_test.rb

@@ -67,4 +67,35 @@ def test_outline_aliases
       end
     end
   end
+
+  class OutlineThreadLockingTest < ConsoleTestCase
+    def program
+      <<~RUBY
+     1| th0 = Thread.new{sleep}
+     2| $m = Mutex.new
+     3| th1 = Thread.new do
+     4|   $m.lock
+     5|   sleep 1
+     6|   $m.unlock
+     7| end
+     8|
+     9| def self.constants # overriding constants is only one of the ways to cause deadlock with outline
+    10|   $m.lock
+    11|   []
+    12| end
+    13|
+    14| sleep 0.5
+    15| debugger
+      RUBY
+    end
+
+    def test_outline_doesnt_cause_deadlock
+      debug_code(program) do
+        type 'c'
+        type 'ls'
+        assert_line_text(/locals: th0/)
+        type 'c'
+      end
+    end
+  end
 end

test/console/trace_test.rb

@@ -473,6 +473,36 @@ def test_block_doesnt_break_tracer
       end
     end
 
+    class ThreadLockingTest < ConsoleTestCase
+      def program
+        <<~RUBY
+      1| th0 = Thread.new{sleep}
+      2| $m = Mutex.new
+      3| th1 = Thread.new do
+      4|   $m.lock
+      5|   sleep 1
+      6|   $m.unlock
+      7| end
+      8|
+      9| def inspect
+      10|   m.lock
+      11|   ""
+      12| end
+      13|
+      14| sleep 0.5
+      15| debugger
+        RUBY
+      end
+
+      def test_object_tracer_doesnt_cause_deadlock
+        debug_code(program) do
+          type 'c'
+          type 'trace object self'
+          type 'c'
+        end
+      end
+    end
+
     class TraceCallReceiverTest < ConsoleTestCase
       def program
         <<~RUBY
@@ -544,7 +574,7 @@ def program
           6| p a
         RUBY
       end
-  
+
       def test_1656237686
         debug_code(program) do
           type 'trace line'