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Tuesday, May 27, 2014

Race conditions in Emacs' process filter functions

Race conditions are always fun. They are particularly surprising when they happen in a single-threaded program like Emacs. I just ran into this problem for the second time.

When Emacs reads data from a process, it executes a filter function associated with that process. The idea is that this filter function handles the data, and after it returns, Emacs continues with other work. Due to some peculiarities of Emacs, it is easily possible for filter functions to run while other filter functions are running, seemingly randomly.

Emacs runs filter functions whenever it reads data from a process, even if this happens while a filter function is already running. And there are many places where Emacs reads data from processes. Even if you knew all the functions that do so, you do not control or know about the code Emacs runs most of the time. For example, if you create a buffer in a particular mode, that mode’s hook is run, which could execute pretty much anything.

I first encountered this problem in Circe, my IRC client for Emacs. In a filter function, it can open a new buffer in a specific mode (a query). That mode can have flyspell active, Emacs spell checker. But flyspell starts a new process for spell checking. Starting a new process can cause Emacs to read from processes, running filter functions. This resulted in a difficult to debug problem that happened only when a user received two messages in very close succession.

You can read my comments on the problem in the Circe source code.

I just ran into the same problem again in Elpy, my Python development environment. To my surprise, Emacs’ process-send-string does not only send data, it also reads data from the process and handles it in the same call. In this particular case, the usual sequence of action is to run an initialization function, and then set a flag that initialization happened, so no need to run it again. But the initialization function sends data to a process. And the filter function for this process might call the original function. Which would notice that no initialization happened, send data, which would read data, which would notice that no initialization happened yet, so send data again … stack overflow.

Solution? A dynamic scope “lock” (not a real lock as it does not block, but close enough).

(defvar my-lock nil)
(when (not my-lock)
  (let ((my-lock t))

For a single-threaded application like Emacs, this looks completely useless. But it works.