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bug#35550: Installer: wpa_supplicant fails to start

From: Ludovic Courtès
Subject: bug#35550: Installer: wpa_supplicant fails to start
Date: Sun, 05 May 2019 22:26:01 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.2 (gnu/linux)

Ludovic Courtès <address@hidden> skribis:

> You would expect wpa_supplicant to create its PID file atomically: write
> it under a different name, then rename(2)… but no:

Regarding this PID file atomicity issue, a few other data points: Tor,
syslogd (GNU Inetutils), sshd (OpenSSH), and dbus-daemon all do the
exact same non-atomic PID file creation as wpa_supplicant.

Libdaemon (used by Avahi) does something more fancy: it acquires an
exclusive write lock (fcntl with F_SETLKW) on the PID file right after
opening it, but since shepherd does not attempt to acquire locks on PID
file, it has no effect.

systemd and GNU Pies don’t take any precautions while reading PID
files.  The assumption seems to be that if the file exists, then it must
contain a valid PID.

So perhaps the problem we see with ‘wpa_supplicant’ is elsewhere.


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