[ruby-core:51128] [ruby-trunk - Bug #7624][Open] How to handle exceptions raised in event hook?

Issue #7624 has been reported by ko1 (Koichi Sasada).

----------------------------------------
Bug #7624: How to handle exceptions raised in event hook?
https://bugs.ruby-lang.org/issues/7624

Author: ko1 (Koichi Sasada)
Status: Open
Priority: Normal
Assignee: ko1 (Koichi Sasada)
Category: core
Target version: 2.0.0
ruby -v: ruby 2.0.0dev (2012-12-21 trunk 38515) [i386-mswin32_100]


The following program raises an exception in `return' event hook.

###
class FOO < RuntimeError; end
class BAR < RuntimeError; end
def m1; m2; end
def m2; m3; end
def m3; raise FOO; end
set_trace_func(lambda{|*args|
  if args[0] == 'return'
    p args
    raise BAR # raise in trace_func
  end})
begin
  m1
rescue => e
  p e
ensure
  p :ensure
end
###

And this program cause strange behavior in each versions.

---
# 2.0 trunk
ruby 2.0.0dev (2012-12-21 trunk 38515) [i386-mswin32_100]
["return", "test.rb", 5, :m3, #<Binding:0x9781c8>, Object]
test.rb:9:in `block in <main>': BAR (BAR)
	from test.rb:5:in `m3'
	from test.rb:4:in `m2'
	from test.rb:3:in `m1'
	from test.rb:12:in `<main>'

# 1.9.3p332 (2012-11-15 revision 37660)
ruby 1.9.3p332 (2012-11-15 revision 37660) [i386-mswin32_100]
["return", "test.rb", 5, :m3, #<Binding:0x2682980>, Object]
["return", "test.rb", 5, :m3, #<Binding:0x268278c>, Object]
... # infinite loop

# 1.8.7
ruby 1.8.7 (2012-06-29 patchlevel 370) [i386-cygwin]
["return", "test.rb", 5, :m3, #<Binding:0x80057adc>, Object]
["return", "test.rb", 5, :m2, #<Binding:0x80057a64>, Object]
["return", "test.rb", 5, :m1, #<Binding:0x80057884>, Object]
test.rb:13: [BUG] unexpected local variable assignment
ruby 1.8.7 (2012-06-29 patchlevel 370) [i386-cygwin]

      1 [sig] ruby 7388 open_stackdumpfile: Dumping stack trace to ruby.exe.stackdump

# [BUG] !! and line number is not correct.

---
# JRuby
$ bin/jruby --debug -v test.rb
jruby 1.7.2.dev (1.9.3p327) 2012-12-25 dc3af39 on OpenJDK Client VM 1.6.0_18-b18 [linux-i386]
["return", "test.rb", 5, :m3, #<Binding:0x1f7896f>, Object]
["return", "test.rb", 4, :m2, #<Binding:0x1abbec4>, Object]
["return", "test.rb", 3, :m1, #<Binding:0x2c03ff>, Object]
#<BAR: BAR>
:ensure

---
# Ancient Ruby
# 1.6.5
$ versions/install-tags_v1_6_5_/bin/ruby  'test.rb' 2>&1

["return", "test.rb", 3, :m3, #<Binding:0xf755c7d0>, Object]
["return", "test.rb", 12, :m2, #<Binding:0xf755c5c8>, Object]
["return", "test.rb", 5, :m1, #<Binding:0xf755c424>, Object]
#<BAR: BAR>
:ensure

# 1.8.6
$ versions/install-tags_v1_8_6_420_/bin/ruby  'test.rb' 2>&1

["return", "test.rb", 5, :m3, #<Binding:0x7f9bc3032530>, Object]
["return", "test.rb", 4, :m2, #<Binding:0x7f9bc3032440>, Object]
["return", "test.rb", 3, :m1, #<Binding:0x7f9bc3032080>, Object]
test.rb:13: [BUG] unexpected local variable assignment
ruby 1.8.6 (2010-09-02) [x86_64-linux]

---

1.8 and before, exceptions from trace_fook are raised as normal exceptions.
JRuby (with --debug) emulates this behavior correctly.

Also CRuby 1.9 tried to handle exceptions as normal exceptions.
However, it becomes infinite loop.


### (1) exception are occurred at m3
 m1
  m2
   m3
    raise

### (2) pop stack frames
 m1
  m2
   m3

### (3) try m3 return hook
 m1
  m2
   m3
    m3_return_hook

### (4) exception at m3_return_hook
 m1
  m2
   m3
    m3_return_hook
     raise

### (5) pop stack frames and go to (3)
 m1
  m2
   m3

MRI 1.8 invokes hooks *after* pop m3.
However, MRI 1.9 invoke hooks *before* pop m3.
This is why infinite loop was made.

----

To solve this issue, current 2.0 pop all stack frames before return hook (when handling exception).

Hoever, `ensure' etc was skipped.  This is also issues.
[Bug 7622], [Bug 7592]

This corner case is confusing.

----

There are several solution about it.

(1) Emulate 1.8's behavior.
Which JRuby does :)
No compatibility issue.

(2) Do not propagate any errors from trace block

Like thread, prohibit all of errors and escape (such as throw).

(2') Output errors if any errors are occurred

(3) Exit thread or program which current trunk done.

----

I think only few people use set_trace_func()
because send bug report on it.

(3) seems bad because ensure clause is ignored.
This means current trunk is buggy.

(1) is best way because no incompatibility
and no discussion is needed.

(2) (and (2')) has incompatible behavior from 1.9 and 1.8.
But this approach seems good because trace hooks should not affect
main program.


I think (1) and (2) is valuable to discuss.

----

I'm sorry to propose such issue includes *compatibility*.
I want to make it clear current status and choose correct way.




-- 
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