support postmortem mode (config)

postmortem mode enables to see an environment (backtrace, local
variables and so on) when it exit by some exception.

```
DEBUGGER: Session start (pid: 29868)
[1, 8] in target.rb
      1|
=>    2| def foo a
      3|   raise 'foo'
      4| end
      5|
      6| foo(10)
      7|
      8| __END__
=>#0    <main> at target.rb:2
(rdbg:commands) config postmortem=true
postmortem = true              # CONTROL: Enable postmortem debug (default: false)
(rdbg:commands) c
Enter postmortem mode with #<RuntimeError: foo>
  target.rb:3:in `foo'
  target.rb:6:in `<main>'
[1, 8] in target.rb
      1|
      2| def foo a
=>    3|   raise 'foo'
      4| end
      5|
      6| foo(10)
      7|
      8| __END__
=>#0    Object#foo(a=10) at target.rb:3
  #1    <main> at target.rb:6
(rdbg:postmortem) i
%self = main
a = 10
```

Author: ko1

README.md

@@ -363,6 +363,7 @@ config set no_color true
   * `RUBY_DEBUG_SKIP_PATH` (`skip_path`): Skip showing/entering frames for given paths (default: [])
   * `RUBY_DEBUG_SKIP_NOSRC` (`skip_nosrc`): Skip on no source code lines (default: false)
   * `RUBY_DEBUG_KEEP_ALLOC_SITE` (`keep_alloc_site`): Keep allocation site and p, pp shows it (default: false)
+  * `RUBY_DEBUG_POSTMORTEM` (`postmortem`): Enable postmortem debug (default: false)
 
 * BOOT
   * `RUBY_DEBUG_NONSTOP` (`nonstop`): Nonstop mode

lib/debug/config.rb

@@ -15,6 +15,7 @@ module DEBUGGER__
     skip_path:      ['RUBY_DEBUG_SKIP_PATH',      "CONTROL: Skip showing/entering frames for given paths (default: [])", :path],
     skip_nosrc:     ['RUBY_DEBUG_SKIP_NOSRC',     "CONTROL: Skip on no source code lines (default: false)",              :bool],
     keep_alloc_site:['RUBY_DEBUG_KEEP_ALLOC_SITE',"CONTROL: Keep allocation site and p, pp shows it (default: false)",   :bool],
+    postmortem:     ['RUBY_DEBUG_POSTMORTEM',     "CONTROL: Enable postmortem debug (default: false)",                   :bool],
 
     # boot setting
     nonstop:        ['RUBY_DEBUG_NONSTOP',     "BOOT: Nonstop mode",                                                :bool],
@@ -99,6 +100,10 @@ def update conf
       else
         # ignore
       end
+
+      if !old_conf[:postmortem] != !conf[:postmortem]
+        SESSION.postmortem = conf[:postmortem]
+      end
     end
 
     private def config

lib/debug/console.rb

@@ -44,7 +44,7 @@ def get_command line
         end
       end
 
-      def readline prompt = "(rdbg) "
+      def readline prompt
         readline_setup prompt
         Reline.readmultiline(prompt, true){ true }
       end
@@ -71,14 +71,14 @@ def readline_setup
         }
       end
 
-      def readline
+      def readline prompt
         readline_setup
-        Readline.readline("(rdbg) ", true)
+        Readline.readline(prompt, true)
       end
 
     rescue LoadError
-      def readline
-        print "(rdbg) "
+      def readline prompt
+        print prompt
         gets
       end
     end

lib/debug/local.rb

@@ -59,9 +59,9 @@ def puts str = nil
       end
     end
 
-    def readline
+    def readline prompt = '(rdbg)'
       setup_interrupt do
-        (@console.readline || 'quit').strip
+        (@console.readline(prompt) || 'quit').strip
       end
     end
 

lib/debug/server.rb

@@ -195,7 +195,7 @@ def puts str = nil
       end
     end
 
-    def readline
+    def readline prompt
       (sock do |s|
         s.puts "input"
         sleep 0.01 until @q_msg

lib/debug/session.rb

@@ -55,6 +55,7 @@ def first_line
 
 module DEBUGGER__
   PresetCommand = Struct.new(:commands, :source, :auto_continue)
+  class PostmortemError < RuntimeError; end
 
   class Session
     def initialize ui
@@ -74,6 +75,8 @@ def initialize ui
       @tc = nil
       @tc_id = 0
       @preset_command = nil
+      @postmortem_hook = nil
+      @postmortem = false
 
       @frame_map = {} # {id => [threadId, frame_depth]} for DAP
       @var_map   = {1 => [:globals], } # {id => ...} for DAP
@@ -245,6 +248,14 @@ def wait_command_loop tc
       @tc = nil
     end
 
+    def prompt
+      if @postmortem
+        '(rdbg:postmortem) '
+      else
+        '(rdbg) '
+      end
+    end
+
     def wait_command
       if @preset_command
         if @preset_command.commands.empty?
@@ -262,7 +273,7 @@ def wait_command
         end
       else
         @ui.puts "INTERNAL_INFO: #{JSON.generate(@internal_info)}" if ENV['RUBY_DEBUG_TEST_MODE']
-        line = @ui.readline
+        line = @ui.readline prompt
       end
 
       case line
@@ -298,18 +309,21 @@ def process_command line
       #   * Step in. Resume the program until next breakable point.
       when 's', 'step'
         cancel_auto_continue
+        check_postmortem
         @tc << [:step, :in]
 
       # * `n[ext]`
       #   * Step over. Resume the program until next line.
       when 'n', 'next'
         cancel_auto_continue
+        check_postmortem
         @tc << [:step, :next]
 
       # * `fin[ish]`
       #   * Finish this frame. Resume the program until the current frame is finished.
       when 'fin', 'finish'
         cancel_auto_continue
+        check_postmortem
         @tc << [:step, :finish]
 
       # * `c[ontinue]`
@@ -370,6 +384,8 @@ def process_command line
       #   * break if: `<expr>` is true at any lines.
       #   * Note that this feature is super slow.
       when 'b', 'break'
+        check_postmortem
+
         if arg == nil
           show_bps
           return :retry
@@ -386,6 +402,7 @@ def process_command line
 
       # skip
       when 'bv'
+        check_postmortem
         require 'json'
 
         h = Hash.new{|h, k| h[k] = []}
@@ -412,6 +429,8 @@ def process_command line
       # * `catch <Error>`
       #   * Set breakpoint on raising `<Error>`.
       when 'catch'
+        check_postmortem
+
         if arg
           bp = add_catch_breakpoint arg
           show_bps bp if bp
@@ -424,6 +443,8 @@ def process_command line
       #   * Stop the execution when the result of current scope's `@ivar` is changed.
       #   * Note that this feature is super slow.
       when 'wat', 'watch'
+        check_postmortem
+
         if arg && arg.match?(/\A@\w+/)
           @tc << [:breakpoint, :watch, arg]
         else
@@ -436,6 +457,8 @@ def process_command line
       # * `del[ete] <bpnum>`
       #   * delete specified breakpoint.
       when 'del', 'delete'
+        check_postmortem
+
         bp =
         case arg
         when nil
@@ -784,6 +807,9 @@ def process_command line
       return :retry
     rescue SystemExit
       raise
+    rescue PostmortemError => e
+      @ui.puts e.message
+      return :retry
     rescue Exception => e
       @ui.puts "[REPL ERROR] #{e.inspect}"
       @ui.puts e.backtrace.map{|e| '  ' + e}
@@ -1250,6 +1276,56 @@ def check_forked
         raise 'DEBUGGER: stop at forked process is not supported yet.'
       end
     end
+
+    def check_postmortem
+      if @postmortem
+        raise PostmortemError, "Can not use this command on postmortem mode."
+      end
+    end
+
+    def enter_postmortem_session frames
+      @postmortem = true
+      ThreadClient.current.on_suspend :postmortem, postmortem_frames: frames
+    ensure
+      @postmortem = false
+    end
+
+    def postmortem=(is_enable)
+      if is_enable
+        unless @postmortem_hook
+          @postmortem_hook = TracePoint.new(:raise){|tp|
+            exc = tp.raised_exception
+            frames = DEBUGGER__.capture_frames(__dir__)
+            exc.instance_variable_set(:@postmortem_frames, frames)
+          }
+          at_exit{
+            @postmortem_hook.disable
+            if CONFIG[:postmortem] && (exc = $!) != nil
+              begin
+                @ui.puts "Enter postmortem mode with #{exc.inspect}"
+                @ui.puts exc.backtrace.map{|e| '  ' + e}
+                @ui.puts "\n"
+
+                enter_postmortem_session exc.instance_variable_get(:@postmortem_frames)
+              rescue SystemExit
+                exit!
+              rescue Exception => e
+                @ui = STDERR unless @ui
+                @ui.puts "Error while postmortem console: #{e.inspect}"
+              end
+            end
+          }
+        end
+
+        if !@postmortem_hook.enabled?
+          @postmortem_hook.enable
+        end
+      else
+        if @postmortem_hook && @postmortem_hook.enabled?
+          @postmortem_hook.disable
+        end
+      end
+    end
   end
 
   class UI_Base

lib/debug/thread_client.rb

@@ -170,9 +170,15 @@ def on_trace trace_id, msg
       wait_next_action
     end
 
-    def on_suspend event, tp = nil, bp: nil, sig: nil
+    def on_suspend event, tp = nil, bp: nil, sig: nil, postmortem_frames: nil
       @current_frame_index = 0
-      @target_frames = DEBUGGER__.capture_frames __dir__
+
+      if postmortem_frames
+        @target_frames = postmortem_frames
+        @postmortem = true
+      else
+        @target_frames = DEBUGGER__.capture_frames(__dir__)
+      end
 
       cf = @target_frames.first
       if cf