1 files changed, 124 insertions, 0 deletions
diff --git a/actionpack/lib/action_dispatch/middleware/debug_locks.rb b/actionpack/lib/action_dispatch/middleware/debug_locks.rb
new file mode 100644
index 0000000000..93c6c85a71
--- /dev/null
+++ b/actionpack/lib/action_dispatch/middleware/debug_locks.rb
@@ -0,0 +1,124 @@
+# frozen_string_literal: true
+
+module ActionDispatch
+  # This middleware can be used to diagnose deadlocks in the autoload interlock.
+  #
+  # To use it, insert it near the top of the middleware stack, using
+  # <tt>config/application.rb</tt>:
+  #
+  #     config.middleware.insert_before Rack::Sendfile, ActionDispatch::DebugLocks
+  #
+  # After restarting the application and re-triggering the deadlock condition,
+  # <tt>/rails/locks</tt> will show a summary of all threads currently known to
+  # the interlock, which lock level they are holding or awaiting, and their
+  # current backtrace.
+  #
+  # Generally a deadlock will be caused by the interlock conflicting with some
+  # other external lock or blocking I/O call. These cannot be automatically
+  # identified, but should be visible in the displayed backtraces.
+  #
+  # NOTE: The formatting and content of this middleware's output is intended for
+  # human consumption, and should be expected to change between releases.
+  #
+  # This middleware exposes operational details of the server, with no access
+  # control. It should only be enabled when in use, and removed thereafter.
+  class DebugLocks
+    def initialize(app, path = "/rails/locks")
+      @app = app
+      @path = path
+    end
+
+    def call(env)
+      req = ActionDispatch::Request.new env
+
+      if req.get?
+        path = req.path_info.chomp("/")
+        if path == @path
+          return render_details(req)
+        end
+      end
+
+      @app.call(env)
+    end
+
+    private
+      def render_details(req)
+        threads = ActiveSupport::Dependencies.interlock.raw_state do |raw_threads|
+          # The Interlock itself comes to a complete halt as long as this block
+          # is executing. That gives us a more consistent picture of everything,
+          # but creates a pretty strong Observer Effect.
+          #
+          # Most directly, that means we need to do as little as possible in
+          # this block. More widely, it means this middleware should remain a
+          # strictly diagnostic tool (to be used when something has gone wrong),
+          # and not for any sort of general monitoring.
+
+          raw_threads.each.with_index do |(thread, info), idx|
+            info[:index] = idx
+            info[:backtrace] = thread.backtrace
+          end
+
+          raw_threads
+        end
+
+        str = threads.map do |thread, info|
+          if info[:exclusive]
+            lock_state = +"Exclusive"
+          elsif info[:sharing] > 0
+            lock_state = +"Sharing"
+            lock_state << " x#{info[:sharing]}" if info[:sharing] > 1
+          else
+            lock_state = +"No lock"
+          end
+
+          if info[:waiting]
+            lock_state << " (yielded share)"
+          end
+
+          msg = +"Thread #{info[:index]} [0x#{thread.__id__.to_s(16)} #{thread.status || 'dead'}]  #{lock_state}\n"
+
+          if info[:sleeper]
+            msg << "  Waiting in #{info[:sleeper]}"
+            msg << " to #{info[:purpose].to_s.inspect}" unless info[:purpose].nil?
+            msg << "\n"
+
+            if info[:compatible]
+              compat = info[:compatible].map { |c| c == false ? "share" : c.to_s.inspect }
+              msg << "  may be pre-empted for: #{compat.join(', ')}\n"
+            end
+
+            blockers = threads.values.select { |binfo| blocked_by?(info, binfo, threads.values) }
+            msg << "  blocked by: #{blockers.map { |i| i[:index] }.join(', ')}\n" if blockers.any?
+          end
+
+          blockees = threads.values.select { |binfo| blocked_by?(binfo, info, threads.values) }
+          msg << "  blocking: #{blockees.map { |i| i[:index] }.join(', ')}\n" if blockees.any?
+
+          msg << "\n#{info[:backtrace].join("\n")}\n" if info[:backtrace]
+        end.join("\n\n---\n\n\n")
+
+        [200, { "Content-Type" => "text/plain", "Content-Length" => str.size }, [str]]
+      end
+
+      def blocked_by?(victim, blocker, all_threads)
+        return false if victim.equal?(blocker)
+
+        case victim[:sleeper]
+        when :start_sharing
+          blocker[:exclusive] ||
+            (!victim[:waiting] && blocker[:compatible] && !blocker[:compatible].include?(false))
+        when :start_exclusive
+          blocker[:sharing] > 0 ||
+            blocker[:exclusive] ||
+            (blocker[:compatible] && !blocker[:compatible].include?(victim[:purpose]))
+        when :yield_shares
+          blocker[:exclusive]
+        when :stop_exclusive
+          blocker[:exclusive] ||
+            victim[:compatible] &&
+            victim[:compatible].include?(blocker[:purpose]) &&
+            all_threads.all? { |other| !other[:compatible] || blocker.equal?(other) || other[:compatible].include?(blocker[:purpose]) }
+        end
+      end
+  end
+end