doc/signals.rdoc: new document work-in-progress

We need a longer document to inform users of caveats
related to Signal.trap usage.  This is still incomplete,
and we can fill in and edit other bits as needed.

* doc/signals.rdoc: new document
  [ruby-core:85107] [Misc #14222]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62083 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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+= Caveats for implementing Signal.trap callbacks
+
+As with implementing signal handlers in C or most other languages,
+all code passed to Signal.trap must be reentrant.  If you are not
+familiar with reentrancy, you need to read up on it at
+{Wikipedia}[https://en.wikipedia.org/wiki/Reentrancy_(computing)] or
+elsewhere before reading the rest of this document.
+
+Most importantly, "thread-safety" does not guarantee reentrancy;
+and methods such as Mutex#lock and Mutex#synchronize which are
+commonly used for thread-safety even prevent reentrancy.
+
+== An implementation detail of the Ruby VM
+
+The Ruby VM defers Signal.trap callbacks from running until it is safe
+for its internal data structures, but it does not know when it is safe
+for data structures in YOUR code.  Ruby implements deferred signal
+handling by registering short C functions with only
+{async-signal-safe functions}[http://man7.org/linux/man-pages/man7/signal-safety.7.html] as
+signal handlers.  These short C functions only do enough tell the VM to
+run callbacks registered via Signal.trap later in the main VM loop.
+
+== Unsafe methods to call in Signal.trap blocks
+
+When in doubt, consider anything not listed as safe below as being
+unsafe.
+
+* Mutex#lock, Mutex#synchronize and any code using them are explicitly
+  unsafe.  This includes Monitor in the standard library which uses
+  Mutex to provide reentrancy.
+
+* Dir.chdir with block
+
+* any IO write operations when IO#sync is false;
+  including IO#write, IO#write_nonblock, IO#puts.
+  Pipes and sockets default to `IO#sync = true', so it is safe to
+  write to them unless IO#sync was disabled.
+
+* File#flock, as the underlying flock(2) call is not specified by POSIX
+
+== Commonly safe operations inside Signal.trap blocks
+
+* Assignment and retrieval of local, instance, and class variables
+
+* Most object allocations and initializations of common types
+  including Array, Hash, String, Struct, Time.
+
+* Common Array, Hash, String, Struct operations which do not execute a block
+  are generally safe; but beware if iteration is occuring elsewhere.
+
+* Hash#[], Hash#[]= (unless Hash.new was given an unsafe block)
+
+* Thread::Queue#push and Thread::SizedQueue#push (since Ruby 2.1)
+
+* Creating a new Thread via Thread.new/Thread.start can used to get
+  around the unusability of Mutexes inside a signal handler
+
+* Signal.trap is safe to use inside blocks passed to Signal.trap
+
+* arithmetic on Integer and Float (`+', `-', '%', '*', '/')
+
+  Additionally, signal handlers do not run between two successive
+  local variable accesses, so shortcuts such as `+=' and `-=' will
+  not trigger a data race when used on Integer and Float classes in
+  signal handlers.
+
+== System call wrapper methods which are safe inside Signal.trap
+
+Since Ruby has wrappers around many
+{async-signal-safe C functions}[http://man7.org/linux/man-pages/man7/signal-safety.7.html]
+the corresponding wrappers for many IO, File, Dir, and Socket methods
+are safe.
+
+(Incomplete list)
+
+* Dir.chdir (without block arg)
+* Dir.mkdir
+* Dir.open
+* File#truncate
+* File.link
+* File.open
+* File.readlink
+* File.rename
+* File.stat
+* File.symlink
+* File.truncate
+* File.unlink
+* File.utime
+* IO#close
+* IO#dup
+* IO#fsync
+* IO#read
+* IO#read_nonblock
+* IO#stat
+* IO#sysread
+* IO#syswrite
+* IO.select
+* IO.pipe
+* Process.clock_gettime
+* Process.exit!
+* Process.fork
+* Process.kill
+* Process.pid
+* Process.ppid
+* Process.waitpid
+...