[Python-checkins] peps: PEP 475

[victor.stinner]

http://hg.python.org/peps/rev/2a6de9d30b91
changeset:   5511:2a6de9d30b91
user:        Victor Stinner <victor.stinner at gmail.com>
date:        Tue Jul 29 22:10:53 2014 +0200
summary:
  PEP 475

files:
  pep-0475.txt |  35 +++++++++++++++++++++--------------
  1 files changed, 21 insertions(+), 14 deletions(-)


diff --git a/pep-0475.txt b/pep-0475.txt
--- a/pep-0475.txt
+++ b/pep-0475.txt
@@ -36,15 +36,17 @@
 * 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 "signal-safe" functions
-can be called.  For example, ``printf()`` and ``malloc()`` are not
-signal-safe. When a signal is sent to a process calling a system
-call, the system call fails with the ``EINTR`` error to give the
-program an opportunity to handle the signal without the restriction on
-signal safe functions.
+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 C
-library retries some the system call instead of failing with
+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
@@ -85,9 +87,12 @@
 * ``socketserver``
 * ``subprocess``
 
+Other programming languages like Perl, Java and Go already retry
+system calls failing with ``EINTR``.
 
-Use Case 1: Don't Bother of Signals
------------------------------------
+
+Use Case 1: Don't Bother With Signals
+-------------------------------------
 
 In most cases, you don't want to be interrupted by signals and you
 don't expect to get ``InterruptedError`` exceptions. For example, do
@@ -192,10 +197,12 @@
 with ``InterruptedError`` will hang. The authors of this PEP don't
 think that such application exist.
 
-If such applications exist, they must be fixed to handle signals
-differently, to have a reliable behaviour on all platforms and all
-Python versions.  For example, use a signal handle which raises an
-exception, or use a wakeup file descriptor.
+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 handle which raises an exception, or use a
+wakeup file descriptor.
 
 Applications should not call ``signal.siginterrupt(signum, False)``
 anymore, since this call will raise an exception in Python 3.6.

-- 
Repository URL: http://hg.python.org/peps