{"action":"created","comment":{"url":"https://api.github.com/repos/netty/netty/pulls/comments/27239834","id":27239834,"diff_hunk":"@@ -39,22 +39,30 @@\n import java.util.ArrayList;\n import java.util.Collection;\n import java.util.Collections;\n+import java.util.Comparator;\n import java.util.HashMap;\n import java.util.HashSet;\n-import java.util.LinkedHashSet;\n import java.util.List;\n import java.util.Map;\n import java.util.Set;\n+import java.util.concurrent.ConcurrentSkipListSet;\n \n /**\n  * Simple implementation of {@link Http2Connection}.\n  */\n public class DefaultHttp2Connection implements Http2Connection {\n \n+    private static final Comparator<Http2Stream> STREAMID_COMPARATOR = new Comparator<Http2Stream>() {\n+        @Override\n+        public int compare(Http2Stream o1, Http2Stream o2) {\n+            return o1.id() - o2.id();","path":"codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2Connection.java","position":21,"original_position":21,"commit_id":"269c97cacdc61ee5057bb38e9bbae6ff2f46dc48","original_commit_id":"269c97cacdc61ee5057bb38e9bbae6ff2f46dc48","user":{"login":"Scottmitch","id":7562868,"avatar_url":"https://avatars.githubusercontent.com/u/7562868?v=3","gravatar_id":"","url":"https://api.github.com/users/Scottmitch","html_url":"https://github.com/Scottmitch","followers_url":"https://api.github.com/users/Scottmitch/followers","following_url":"https://api.github.com/users/Scottmitch/following{/other_user}","gists_url":"https://api.github.com/users/Scottmitch/gists{/gist_id}","starred_url":"https://api.github.com/users/Scottmitch/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/Scottmitch/subscriptions","organizations_url":"https://api.github.com/users/Scottmitch/orgs","repos_url":"https://api.github.com/users/Scottmitch/repos","events_url":"https://api.github.com/users/Scottmitch/events{/privacy}","received_events_url":"https://api.github.com/users/Scottmitch/received_events","type":"User","site_admin":false},"body":"@buchgr - I don't think we need to order this by priority.  The expensive tree traversal stuff you glanced over is to be able to derive the relative priority order (because there is no total priority order).  The priority stuff is calculated while traversing the tree and this is just used after the fact to ensure all the calculated results are used.","created_at":"2015-03-26T18:09:07Z","updated_at":"2015-03-26T18:09:07Z","html_url":"https://github.com/netty/netty/pull/3537#discussion_r27239834","pull_request_url":"https://api.github.com/repos/netty/netty/pulls/3537","_links":{"self":{"href":"https://api.github.com/repos/netty/netty/pulls/comments/27239834"},"html":{"href":"https://github.com/netty/netty/pull/3537#discussion_r27239834"},"pull_request":{"href":"https://api.github.com/repos/netty/netty/pulls/3537"}}},"pull_request":{"url":"https://api.github.com/repos/netty/netty/pulls/3537","id":31981068,"html_url":"https://github.com/netty/netty/pull/3537","diff_url":"https://github.com/netty/netty/pull/3537.diff","patch_url":"https://github.com/netty/netty/pull/3537.patch","issue_url":"https://api.github.com/repos/netty/netty/issues/3537","number":3537,"state":"open","locked":false,"title":"Avoid ConcurrentModificationException when iterating over Http2Connectio...","user":{"login":"buchgr","id":1388179,"avatar_url":"https://avatars.githubusercontent.com/u/1388179?v=3","gravatar_id":"","url":"https://api.github.com/users/buchgr","html_url":"https://github.com/buchgr","followers_url":"https://api.github.com/users/buchgr/followers","following_url":"https://api.github.com/users/buchgr/following{/other_user}","gists_url":"https://api.github.com/users/buchgr/gists{/gist_id}","starred_url":"https://api.github.com/users/buchgr/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/buchgr/subscriptions","organizations_url":"https://api.github.com/users/buchgr/orgs","repos_url":"https://api.github.com/users/buchgr/repos","events_url":"https://api.github.com/users/buchgr/events{/privacy}","received_events_url":"https://api.github.com/users/buchgr/received_events","type":"User","site_admin":false},"body":"...n.activeStreams().\r\n\r\nMotivation:\r\n\r\nWe have seen `ConcurrentModificationExceptions` being thrown in `DefaultHttp2RemoteFlowController.writePendingBytes()`\r\nwhile iterating over all active streams and trying to flush their frames buffered in the flow controller.\r\n\r\nThe issue can be reproduced 100% of the time, when we receive a `WINDOW_UPDATE` for the connection and we\r\nsubsequently write a frame with the `END_STREAM` flag set of a stream that is already state `HALF_CLOSED_REMOTE`,\r\nthen the frame gets closed and removed from the set of active streams - thus a `ConcurrentModificationException`\r\nis raised.\r\n\r\nModifications:\r\n\r\n- Replace the `LinkedHashSet` with a `ConcurrentSkipListSet`, which allows for being modified while being iterated over.\r\n- Require `Http2Connection.activeStreams()` to return a collection that allows for concurrent modifications.\r\n\r\nResult:\r\n\r\nThe HTTP2 codec no longer crashes in the before mentioned 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