|Title:||Retry system calls failing with EINTR|
|Last-Modified:||2014-10-29 11:42:14 +0100 (Wed, 29 Oct 2014)|
|Author:||Charles-François Natali <cf.natali at gmail.com>, Victor Stinner <victor.stinner at gmail.com>|
|BDFL-Delegate:||Antoine Pitrou < email@example.com >|
- Backward Compatibility
Retry system calls failing with the EINTR error and recompute timeout if needed.
On POSIX systems, signals are common. Your program must be prepared to handle them. Examples of signals:
- The most common signal is SIGINT , signal sent when CTRL+c is pressed. By default, Python raises a KeyboardInterrupt exception when this signal is received.
- When running subprocesses, the SIGCHLD signal is sent when a child process exits.
- Resizing the terminal sends the SIGWINCH signal to the applications running in the terminal.
- Putting the application in background (ex: press CTRL-z and then type the bg command) sends the SIGCONT signal.
Writing a signal handler is difficult, only "async-signal safe" functions can be called. For example, printf() and malloc() are not async-signal safe. When a signal is sent to a process calling a system call, the system call can fail with the EINTR error to give the program an opportunity to handle the signal without the restriction on signal safe functions. Depending on the platform, on the system call and the SA_RESTART flag, the system call may or may not fail with EINTR .
If the signal handler was set with the SA_RESTART flag set, the kernel retries some the system call instead of failing with EINTR . For example, read() is retried, whereas select() is not retried. The Python function signal.signal() clears the SA_RESTART flag when setting the signal handler: all system calls should fail with EINTR in Python.
The problem is that handling EINTR should be done for all system calls. The problem is similar to handling errors in the C language which does not have exceptions: you must check all function returns to check for error, and usually duplicate the code checking for errors. Python does not have this issue, it uses exceptions to notify errors.
In Python 3.4, the code to handle the InterruptedError exception ( EINTR error) is duplicated on case by case. Only a few Python modules handle this exception, and fixes usually took several years to cover a whole module. Example of code retrying file.read() on InterruptedError :
while True: try: data = file.read(size) break except InterruptedError: continue
List of Python modules of the standard library which handle InterruptedError :
- io , _pyio
Other programming languages like Perl, Java and Go already retry system calls failing with EINTR .
In most cases, you don't want to be interrupted by signals and you don't expect to get InterruptedError exceptions. For example, do you really want to write such complex code for an "Hello World" example?
while True: try: print("Hello World") break except InterruptedError: continue
InterruptedError can happen in unexpected places. For example, os.close() and FileIO.close() can raises InterruptedError : see the article close() and EINTR .
The Python issues related to EINTR section below gives examples of bugs caused by "EINTR".
The expectation is that Python hides the InterruptedError : retry system calls failing with the EINTR error.
Sometimes, you expect some signals and you want to handle them as soon as possible. For example, you may want to quit immediatly a program using the CTRL+c keyboard shortcut.
Some signals are not interesting and should not interrupt the the application. There are two options to only interrupt an application on some signals:
- Raise an exception in the signal handler, like KeyboardInterrupt for SIGINT
- Use a I/O multiplexing function like select() with the Python signal "wakeup" file descriptor: see the function signal.set_wakeup_fd() .
If a system call fails with EINTR , Python must call signal handlers: call PyErr_CheckSignals() . If a signal handler raises an exception, the Python function fails with the exception. Otherwise, the system call is retried. If the system call takes a timeout parameter, the timeout is recomputed.
Example of functions that need to be modified:
- os.read() , io.FileIO.read() , io.FileIO.readinto()
- os.write() , io.FileIO.write()
- socket.recv() , socket.recv_into()
- selectors.SelectSelector.select() and other selector classes
Note: The selector module already retries on InterruptedError , but it doesn't recompute the timeout yet.
Since interrupted system calls are automatically retried, the InterruptedError exception should not occur anymore. The code handling InterruptedError can be removed from in the standard library to simply the code.
Applications relying on the fact that system calls are interrupted with InterruptedError will hang. The authors of this PEP don't think that such application exist.
If such applications exist, they are not portable and are subject to race conditions (deadlock if the signal comes before the system call). These applications must be fixed to handle signals differently, to have a reliable behaviour on all platforms and all Python versions. For example, use a signal handler which raises an exception, or use a wakeup file descriptor.
For applications using event loops, signal.set_wakeup_fd() is the recommanded option to handle signals. The signal handler writes signal numbers into the file descriptor and the event loop is awaken to read them. The event loop can handle these signals without the restriction of signal handlers.
Since Python 3.3, signal.set_wakeup_fd() writes the signal number into the file descriptor, whereas it only wrote a null byte before. It becomes possible to handle different signals using the wakeup file descriptor.
Linux has a signalfd() which provides more information on each signal. For example, it's possible to know the pid and uid who sent the signal. This function is not exposed in Python yet (see the issue 12304 ).
On Unix, the asyncio module uses the wakeup file descriptor to wake up its event loop.
A C signal handler can be called from any thread, but the Python signal handler should only be called in the main thread.
Python has a PyErr_SetInterrupt() function which calls the SIGINT signal handler to interrupt the Python main thread.
Windows uses "control events":
- CTRL_BREAK_EVENT : Break ( SIGBREAK )
- CTRL_CLOSE_EVENT : Close event
- CTRL_C_EVENT : CTRL+C ( SIGINT )
- CTRL_LOGOFF_EVENT : Logoff
- CTRL_SHUTDOWN_EVENT : Shutdown
The SetConsoleCtrlHandler() function can be used to install a control handler.
The CTRL_C_EVENT and CTRL_BREAK_EVENT events can be sent to a process using the GenerateConsoleCtrlEvent() function . This function is exposed in Python as os.kill() .
The following signals are supported on Windows:
- SIGBREAK ( CTRL_BREAK_EVENT ): signal only available on Windows
- SIGINT ( CTRL_C_EVENT )
The default Python signal handler for SIGINT sets a Windows event object: sigint_event .
time.sleep() is implemented with WaitForSingleObjectEx() , it waits for the sigint_event object using time.sleep() parameter as the timeout. So the sleep can be interrupted by SIGINT .
_winapi.WaitForMultipleObjects() automatically adds sigint_event to the list of watched handles, so it can also be interrupted.
PyOS_StdioReadline() also used sigint_event when fgets() failed to check if Ctrl-C or Ctrl-Z was pressed.
This document has been placed in the public domain.