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Debugger is not Fiber-safe #490

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Debugger is not Fiber-safe#490

Description

@st0012
st0012
opened on Jan 12, 2022

Your environment

  • ruby -v: 3.1.0
  • rdbg -v: 1.4.0

Describe the bug

If a Fiber can trigger breakpoints and is resumed during a debugging session, it'll crash the debugger due to repeatedly entering subsession: already in subsession (RuntimeError).

To Reproduce

Script:

def foo
end

f = Fiber.new do
  foo
end

binding.b(pre: "b Object#foo ;; f.resume")

Command:

$ exe/rdbg -e "c" target.rb

Result:

❯ exe/rdbg -e "c" target.rb
[1, 9] in target.rb
=>   1| def foo
     2| end
     3|
     4| f = Fiber.new do
     5|   foo
     6| end
     7|
     8| binding.b(pre: "b Object#foo ;; f.resume")
     9|
=>#0    <main> at target.rb:1
(rdbg:commands) c
[3, 9] in target.rb
     3|
     4| f = Fiber.new do
     5|   foo
     6| end
     7|
=>   8| binding.b(pre: "b Object#foo ;; f.resume")
     9|
=>#0    <main> at target.rb:8
(rdbg:binding.break) b Object#foo
#0  BP - Method  Object#foo at target.rb:1
(rdbg:binding.break) f.resume
#<Thread:0x0000000110a725e0@DEBUGGER__::SESSION@server /Users/st0012/projects/debug/lib/debug/session.rb:147 run> terminated with exception (report_on_exception is true):
/Users/st0012/projects/debug/lib/debug/session.rb:1505:in `enter_subsession': already in subsession (RuntimeError)
        from /Users/st0012/projects/debug/lib/debug/session.rb:233:in `process_event'
        from /Users/st0012/projects/debug/lib/debug/session.rb:197:in `session_server_main'
        from /Users/st0012/projects/debug/lib/debug/session.rb:167:in `block in activate'
["DEBUGGER Exception: /Users/st0012/projects/debug/lib/debug/thread_client.rb:994",
 #<RuntimeError: already in subsession>,
 ["/Users/st0012/projects/debug/lib/debug/session.rb:1505:in `enter_subsession'", "/Users/st0012/projects/debug/lib/debug/session.rb:233:in `process_event'", "/Users/st0012/projects/debug/lib/debug/session.rb:197:in `session_server_main'", "/Users/st0012/projects/debug/lib/debug/session.rb:167:in `block in activate'"]]
["DEBUGGER Exception: /Users/st0012/projects/debug/lib/debug/thread_client.rb:994",
 #<NoMethodError: undefined method `width' for nil:NilClass

      @ui.width
         ^^^^^^>,
 ["/Users/st0012/projects/debug/lib/debug/session.rb:1588:in `width'",
  "/Users/st0012/projects/debug/lib/debug/thread_client.rb:805:in `wait_next_action_'",
  "/Users/st0012/projects/debug/lib/debug/thread_client.rb:684:in `wait_next_action'",
  "/Users/st0012/projects/debug/lib/debug/thread_client.rb:292:in `suspend'",
  "/Users/st0012/projects/debug/lib/debug/thread_client.rb:224:in `on_breakpoint'",
  "/Users/st0012/projects/debug/lib/debug/breakpoint.rb:65:in `suspend'",
  "/Users/st0012/projects/debug/lib/debug/breakpoint.rb:157:in `block in setup'",
  "/Users/st0012/projects/debug/lib/debug/session.rb:2195:in `debugger'",
  "target.rb:8:in `<main>'"]]
/Users/st0012/projects/debug/lib/debug/session.rb:1588:in `width': undefined method `width' for nil:NilClass (NoMethodError)

      @ui.width
         ^^^^^^
        from /Users/st0012/projects/debug/lib/debug/thread_client.rb:805:in `wait_next_action_'
        from /Users/st0012/projects/debug/lib/debug/thread_client.rb:684:in `wait_next_action'
        from /Users/st0012/projects/debug/lib/debug/thread_client.rb:292:in `suspend'
        from /Users/st0012/projects/debug/lib/debug/thread_client.rb:224:in `on_breakpoint'
        from /Users/st0012/projects/debug/lib/debug/breakpoint.rb:65:in `suspend'
        from /Users/st0012/projects/debug/lib/debug/breakpoint.rb:157:in `block in setup'
        from /Users/st0012/projects/debug/lib/debug/session.rb:2195:in `debugger'
        from target.rb:8:in `<main>'

Result with debug logs (from #469)

❯ RUBY_DEBUG_LOG_LEVEL=DEBUG exe/rdbg -e "c" target.rb
DEBUGGER (INFO): Session start (pid: 69294)
DEBUGGER (DEBUG): #<DBG:TC 1:waiting@<internal:/Users/st0012/.rbenv/versions/3.1.0/lib/ruby/3.1.0/rubygems/core_ext/kernel_require.rb>:85:in `require'> changed mode (waiting -> running)
DEBUGGER (INFO): Thread #1 is created.
DEBUGGER (DEBUG): #<DBG:TC 2:waiting@/Users/st0012/projects/debug/lib/debug/session.rb:152:in `block in activate'> changed mode (waiting -> running)
DEBUGGER (INFO): Thread #2 is created.
DEBUGGER (DEBUG): #<DBG:TC 1:running@/Users/st0012/projects/debug/lib/debug/session.rb:117:in `block in initialize'> changed mode (running -> waiting)
DEBUGGER (DEBUG): #<DBG:TC 1:waiting@/Users/st0012/projects/debug/lib/debug/session.rb:117:in `block in initialize'> sent Event { type: :load, args: ["<RubyVM::InstructionSequence:<main>@target.rb:0>", "nil"] } to Session
DEBUGGER (INFO): Load /Users/st0012/projects/debug/target.rb
DEBUGGER (DEBUG): #<DBG:TC 1:waiting@/Users/st0012/projects/debug/lib/debug/session.rb:117:in `block in initialize'> received Cmd { type: :continue, args: [] } from Session
DEBUGGER (DEBUG): #<DBG:TC 1:waiting@/Users/st0012/projects/debug/lib/debug/session.rb:117:in `block in initialize'> changed mode (waiting -> running)
DEBUGGER (DEBUG): #<DBG:TC 1:running@target.rb:1:in `<main>'> was suspended for :line
DEBUGGER (DEBUG): #<DBG:TC 1:running@target.rb:1:in `<main>'> changed mode (running -> waiting)
DEBUGGER (DEBUG): #<DBG:TC 1:waiting@target.rb:1:in `<main>'> sent Event { type: :suspend, args: [":breakpoint", "[\"/Users/st0012/projects/debug/target.rb\", 1]"] } to Session
DEBUGGER (INFO): enter_subsession
[1, 9] in target.rb
=>   1| def foo
     2| end
     3|
     4| f = Fiber.new do
     5|   foo
     6| end
     7|
     8| binding.b(pre: "b Object#foo ;; f.resume")
     9|
=>#0    <main> at target.rb:1
(rdbg:commands) c
DEBUGGER (INFO): leave_subsession
DEBUGGER (DEBUG): #<DBG:TC 1:waiting@target.rb:1:in `<main>'> received Cmd { type: :continue, args: [] } from Session
DEBUGGER (DEBUG): #<DBG:TC 1:waiting@target.rb:1:in `<main>'> changed mode (waiting -> running)
DEBUGGER (DEBUG): #<DBG:TC 1:running@target.rb:8:in `<main>'> was suspended for :line
DEBUGGER (DEBUG): #<DBG:TC 1:running@target.rb:8:in `<main>'> changed mode (running -> waiting)
DEBUGGER (DEBUG): #<DBG:TC 1:waiting@target.rb:8:in `<main>'> sent Event { type: :suspend, args: [":breakpoint", "[\"/Users/st0012/projects/debug/lib/debug/session.rb\", 2195]"] } to Session
DEBUGGER (INFO): enter_subsession
[3, 9] in target.rb
     3|
     4| f = Fiber.new do
     5|   foo
     6| end
     7|
=>   8| binding.b(pre: "b Object#foo ;; f.resume")
     9|
=>#0    <main> at target.rb:8
(rdbg:binding.break) b Object#foo
DEBUGGER (DEBUG): #<DBG:TC 1:waiting@target.rb:8:in `<main>'> received Cmd { type: :breakpoint, args: [":method", "\"Object\"", "\"#\"", "\"foo\"", "nil", "nil", "nil"] } from Session
DEBUGGER (DEBUG): #<DBG:TC 1:waiting@target.rb:8:in `<main>'> changed mode (waiting -> running)
DEBUGGER (DEBUG): #<DBG:TC 1:running@target.rb:8:in `<main>'> sent Event { type: :result, args: [":method_breakpoint", "#<DEBUGGER__::MethodBreakpoint:0x000000010da4a908 @sig_klass_name=\"Object\", @sig_op=\"#\", @sig_method_name=\"foo\", @klass_eval_binding=nil, @override_method=false, @klass=Object, @method=#<UnboundMethod: Object#foo()
target.rb:1>, @cond=nil, @cond_class=nil, @command=nil, @path=nil, @key=\"Object#foo\", @deleted=false, @tp=#<TracePoint:line /Users/st0012/projects/debug/lib/debug/session.rb:2195 in `debugger'>>"] } to Session
DEBUGGER (DEBUG): #<DBG:TC 1:running@target.rb:8:in `<main>'> changed mode (running -> waiting)
#0  BP - Method  Object#foo at target.rb:1
(rdbg:binding.break) f.resume
DEBUGGER (DEBUG): #<DBG:TC 1:waiting@target.rb:8:in `<main>'> received Cmd { type: :eval, args: [":pp", "\"f.resume\""] } from Session
DEBUGGER (DEBUG): #<DBG:TC 1:waiting@target.rb:8:in `<main>'> changed mode (waiting -> running)
DEBUGGER (DEBUG): #<DBG:TC 1:running@target.rb:5:in `block in <main>'> was suspended for :call
DEBUGGER (DEBUG): #<DBG:TC 1:running@target.rb:5:in `block in <main>'> changed mode (running -> waiting)
DEBUGGER (DEBUG): #<DBG:TC 1:waiting@target.rb:5:in `block in <main>'> sent Event { type: :suspend, args: [":breakpoint", "\"Object#foo\""] } to Session
#<Thread:0x000000010d768938@DEBUGGER__::SESSION@server /Users/st0012/projects/debug/lib/debug/session.rb:147 run> terminated with exception (report_on_exception is true):
/Users/st0012/projects/debug/lib/debug/session.rb:1505:in `enter_subsession': already in subsession (RuntimeError)
        from /Users/st0012/projects/debug/lib/debug/session.rb:233:in `process_event'
        from /Users/st0012/projects/debug/lib/debug/session.rb:197:in `session_server_main'
        from /Users/st0012/projects/debug/lib/debug/session.rb:167:in `block in activate'
DEBUGGER (DEBUG): #<DBG:TC 1:waiting@target.rb:5:in `block in <main>'> changed mode (waiting -> running)
["DEBUGGER Exception: /Users/st0012/projects/debug/lib/debug/thread_client.rb:994",
 #<RuntimeError: already in subsession>,
 ["/Users/st0012/projects/debug/lib/debug/session.rb:1505:in `enter_subsession'", "/Users/st0012/projects/debug/lib/debug/session.rb:233:in `process_event'", "/Users/st0012/projects/debug/lib/debug/session.rb:197:in `session_server_main'", "/Users/st0012/projects/debug/lib/debug/session.rb:167:in `block in activate'"]]
["DEBUGGER Exception: /Users/st0012/projects/debug/lib/debug/thread_client.rb:994",
 #<NoMethodError: undefined method `width' for nil:NilClass

      @ui.width
         ^^^^^^>,
 ["/Users/st0012/projects/debug/lib/debug/session.rb:1588:in `width'",
  "/Users/st0012/projects/debug/lib/debug/thread_client.rb:805:in `wait_next_action_'",
  "/Users/st0012/projects/debug/lib/debug/thread_client.rb:684:in `wait_next_action'",
  "/Users/st0012/projects/debug/lib/debug/thread_client.rb:292:in `suspend'",
  "/Users/st0012/projects/debug/lib/debug/thread_client.rb:224:in `on_breakpoint'",
  "/Users/st0012/projects/debug/lib/debug/breakpoint.rb:65:in `suspend'",
  "/Users/st0012/projects/debug/lib/debug/breakpoint.rb:157:in `block in setup'",
  "/Users/st0012/projects/debug/lib/debug/session.rb:2195:in `debugger'",
  "target.rb:8:in `<main>'"]]
/Users/st0012/projects/debug/lib/debug/session.rb:1588:in `width': undefined method `width' for nil:NilClass (NoMethodError)

      @ui.width
         ^^^^^^
        from /Users/st0012/projects/debug/lib/debug/thread_client.rb:805:in `wait_next_action_'
        from /Users/st0012/projects/debug/lib/debug/thread_client.rb:684:in `wait_next_action'
        from /Users/st0012/projects/debug/lib/debug/thread_client.rb:292:in `suspend'
        from /Users/st0012/projects/debug/lib/debug/thread_client.rb:224:in `on_breakpoint'
        from /Users/st0012/projects/debug/lib/debug/breakpoint.rb:65:in `suspend'
        from /Users/st0012/projects/debug/lib/debug/breakpoint.rb:157:in `block in setup'
        from /Users/st0012/projects/debug/lib/debug/session.rb:2195:in `debugger'
        from target.rb:8:in `<main>'

Expected behavior

Ideally, it should enter another debugging session (nested sessions). But it requires a large structural change. So before that happens, I hope the debugger would tell users that it needs to skip the inner session instead of crashing.

Activity

  1. st0012 commented on Mar 24, 2022

    @st0012
    st0012
    on Mar 24, 2022
    MemberAuthor

    This is now solved with the support of nested subsessions.

  2. st0012
    closed this as completedon Mar 24, 2022
  3. kingdonb
    added a commit that references this issue on Jun 22, 2022

    breakpoints do not work with fibers

    a85809a
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