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authorJosé Valim <[email protected]>2014-05-01 17:44:18 +0200
committerLukas Larsson <[email protected]>2014-10-16 17:56:17 +0200
commit2d3a39b5729a295d4e0ac416ff0280e3edca44c6 (patch)
tree4d3af1051e0b8993bf1e629a628705dc94e0300a /erts/lib_src
parent48083e54b502afb2768066394074d29423162dc8 (diff)
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Fix io:columns/0 timeout when invoked via user
This patch fixes an issue where io:columns/0 times out when invoked from any application callback (or any supervisor/supervised module since the group leader is inherited). To reproduce the issue, one must simply call io:columns() from any application callback. You will notice the process will block for 2 seconds which then times out and returns {:error, :enotsup}. Note this bug only happens inside the erlang shell (using -noshell or escripts do not trigger the bug). To fix the bug, it is important to understand how io requests flow from application callback processes. Here are the steps followed: 1. Since io:columns/1 is timing out, the first step is to find out who is the group leader for the application callback process. Using process_info/1, we can see the parent process is the application_master and handles io_requests by delegating them to the group_leader. 2. By inspecting the application_master process, we can find the group_leader the message is sent is the registered process named user. The process is running the group module which does handle io:columns/1 requests delegating them to user_drv process. 3. The user_drv process does handle tty_geometry requests, except that a clause above ends up short-circuiting all tty_geometry requests from the user process. This patch moves the user clause below the specific driver messages. OTP-12241
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