Occasionally odd issues arise without a transparent or acceptable solution. Those issues are documented here.
Some libraries/applications may install signal handlers which conflict with signal handlers unicorn uses. Leaving "preload_app false" (the default) will allow unicorn to always override existing signal handlers.
Issues with FreeBSD jails can be worked around as documented by Tatsuya Ono: yhbt.net/unicorn-public/CAHBuKRj09FdxAgzsefJWotexw-7JYZGJMtgUp_dhjPz9VbKD6Q@mail.gmail.com/
PRNGs (pseudo-random number generators) loaded before forking (e.g. "preload_app true") may need to have their internal state reset in the after_fork hook. Starting with unicorn 3.6.1, we have builtin workarounds for Kernel#rand and OpenSSL::Random users, but applications may use other PRNGs.
For notes on sandboxing tools such as Bundler or Isolate, see the Sandbox page.
nginx with "sendfile on" under FreeBSD 8 is broken when uploads are buffered to disk. Disabling sendfile is required to work around this bug which should be fixed in newer versions of FreeBSD.
When using "preload_app true", with apps using background threads need to restart them in the after_fork hook because threads are never shared with child processes. Additionally, any synchronization primitives (Mutexes, Monitors, ConditionVariables) should be reinitialized in case they are held during fork time to avoid deadlocks. The core Ruby Logger class needlessly uses a MonitorMutex which can be disabled with a monkey patch
Under some versions of Ruby 1.8, it is necessary to call
srand in an after_fork hook to get correct random number generation. We have a builtin workaround for this starting with unicorn 3.6.1
On Ruby 1.8 prior to Ruby 1.8.7-p248, *BSD platforms have a broken stdio that causes failure for file uploads larger than 112K. Upgrade your version of Ruby or continue using unicorn 1.x/3.4.x.
Under Ruby 1.9.1, methods like Array#shuffle and Array#sample will segfault if called after forking. Upgrade to Ruby 1.9.2 or call "Kernel.rand" in your after_fork hook to reinitialize the random number generator.
Rails 2.3.2 bundles its own version of Rack. This may cause subtle bugs when simultaneously loaded with the system-wide Rack Rubygem which unicorn depends on. Upgrading to Rails 2.3.4 (or later) is strongly recommended for all Rails 2.3.x users for this (and security reasons). Rails 2.2.x series (or before) did not bundle Rack and are should be unnaffected. If there is any reason which forces your application to use Rails 2.3.2 and you have no other choice, then you may edit your unicorn gemspec and remove the Rack dependency.
ref: yhbt.net/unicorn-public/20091014221552.GA30624@dcvr.yhbt.net/ Note: the workaround described in the article above only made the issue more subtle and we didn't notice them immediately.
WONTFIX: code reloading and restarts with Sinatra 0.3.x (and likely older versions) apps is broken. The workaround is to force production mode to disable code reloading as well as disabling "run" in your Sinatra application:
set :env, :production set :run, false
Since this is no longer an issue with Sinatra 0.9.x apps, this will not be fixed on our end. Since unicorn is itself the application launcher, the at_exit handler used in old Sinatra always caused Mongrel to be launched whenever a unicorn worker was about to exit.
Also remember we're capable of replacing the running binary without dropping any connections regardless of framework :)
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