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\chapter{Concurrent system programming}

The Scheme Shell provides the user with support for concurrent programming.
The interface consists of several parts:
\begin{itemize}
\item The thread system 
\item Synchronization vehicles 
\item Process state abstractions
\end{itemize}
Whereas the user deals with threads and synchronization explicitly,
the process state abstractions are built into the rest of the system,
almost transparent for the user. Section \ref{sec:ps_interac}
describes the interaction between process state and threads.

\section{Threads}

A thread can be thought of as a procedure that can run independently of
and concurrent to the rest of the system. The calling procedure fires
the thread up and forgets about it.

The current thread interface is completely taken from Scheme\ 48. This
documentation is an extension of the file \texttt{doc/threads.txt}.

The thread structure is named \texttt{threads}, it has to be opened explicitly.

\defun {spawn} {thunk [name]} \undefined 

Create and schedule a new thread that will execute \var{thunk}, a
procedure with no arguments. Note that Scsh's \ex{spawn} does
\textbf{not} return a reference to a thread object. The optional
argument \var{name} is used when printing the thread. 

The new thread will not inherit the values for the process state from
its parent, see the procedure \texttt{fork-thread} in Section
\ref{sec:ps_interac} for a way to create a thread with
semantics similar to process forking.

\defun {relinquish-timeslice} {} \undefined

Let other threads run for a while. 

\defun {sleep} {time} \undefined

Puts the current thread into sleep for \var{time} milliseconds. The
time at which the thread is run again may be longer of course.

\defun {terminate-current-thread} {} {does-not-return}

Kill the current thread. 

Mainly for debugging purposes, there is also an interface to the
internal representation of thread objects:

\defun {current-thread} {} {thread-object}

Return the object to which the current thread internally corresponds.
Note that this procedure is exported by the package
\texttt{threads-internal} only.

\defun {thread?} {thing} {\boolean}

Returns true iff \var{thing} is a  thread object. 

\defun {thread-name} {thread} {name}

\var{Name} corresponds to the second parameter that was given to
\ex{spawn} when \var{thread} was created.

\defun{thread-uid} {thread} {\integer}

Returns a unique identifier for the current thread.

\section{Locks}

Locks are a simple mean for mutual exclusion. They implement a concept
commonly known as \textit{semaphores}. Threads can obtain and release
locks. If a thread tries to obtain a lock which is held by another
thread, the first thread is blocked. To access the following
procedures, you must open the structure \texttt{locks}.

\defun{make-lock} {} {lock}

Creates a lock. 

\defun{lock?} {thing} {\boolean}

Returns true iff \var{thing} is a lock.

\defun{obtain-lock} {lock} {\undefined}

Obtain \var{lock}. Causes the thread to block if the lock is held by
a thread.

\defun{maybe-obtain-lock} {lock} {\boolean}

Tries to obtain \var{lock}, but returns false if the lock cannot be
obtained.

\defun{release-lock} {lock} {\boolean}

Releases \var{lock}. Returns true if the lock immediately got a new
owner, false otherwise.

\defun{lock-owner-uid} {lock} {\integer}

Returns the uid of the thread that currently holds \var{lock} or false
if the lock is free.

\section{Placeholders}
Placeholers combine synchronization with value delivery. They can be
thought of as special variables. After creation the value of the
placeholder is undefined. If a thread tries to read the placeholders
value this thread is blocked. Multiple threads are allowed to block on
a single placeholder. They will continue running after another thread
sets the value of the placeholder. Now all reading threads receive the
value and continue executing. Setting a placeholder to two different
values causes an error. The structure \texttt{placeholders} features
the following procedures:

\defun {make-placeholder} {} {placeholder}

Creates a new placeholder.

\defun {placeholder?} {thing} {\boolean}

Returns true iff \var{thing} is a placeholder.

\defun {placeholder-set!} {placeholder value} {\undefined}

Sets the placeholders value to \var{value}. If the placeholder is
already set to a \textit{different} value an exception is risen.

\defun {placeholder-value} {placeholder} {value}

Returns the value of the placeholder. If the placeholder is yet unset,
the current thread is blocked until another thread sets the value by
means of \ex{placeholder-set!}.

\section{The event interface to interrupts}
\label{sec:event-interf-interr}
Scsh provides an synchronous interface to the asynchronous signals
delivered by the operation system\footnote{Olin's paper ``Automatic
  management of operation-system resources'' describes this system in
  detail.}.  The key element in this system is an object called
\textit{sigevent} which corresponds to the single occurrence of a
signal. A sigevent has two fields: the Unix signal that occurred and a
pointer to the sigevent that happened or will happen. That is, events
are kept in a linked list in increasing-time order. Scsh's structure
\texttt{sigevents} provides various procedures to access this list:

\defun {most-recent-sigevent} {} {sigevent}

Returns the most recent sigevent --- the head of the sigevent
list.

\defun {sigevent?} {object} {\boolean}

The predicate for sigevents.

\defun {next-sigevent} {pre-event type} {event} 

Returns the next sigevent of type \texttt{type} after sigevent
\texttt{pre-event}. If no such event exists, the procedure blocks.

\defun {next-sigevent-set} {pre-event set} {event}

Returns the next sigevent whose type is in \texttt{set} after
\texttt{pre-event}. If no such event exists, the procdure blocks. 

\defun {next-sigevent/no-wait} {pre-event type} {event or \sharpf}

Same as \texttt{next-sigevent}, but returns \sharpf if no appropriate
event exists.

\defun {next-sigevent-set/no-wait} {set pre-event} {event or \sharpf}

Same as \texttt{next-sigevent-set}, but returns \sharpf if no appropriate
event exists.

As a small example, consider this piece of code that toggles the
variable \texttt{state} by USR1 and USR2:
\begin{code}
(define state #t)

(let lp ((sigevent (most-recent-sigevent)))
  (let ((next (next-sigevent sigevent interrupt/usr1)))
    (set! state #f)
    (let ((next (next-sigevent next interrupt/usr2)))
      (set! state #t)
      (lp next))))
\end{code}

\textbf{Warning:} The current version of scsh also defines
asynchronous handlers for interrupts (See Section
\ref{sec:int_handlers}). The default action of some of these handlers
is to terminate the process in which case you will most likely not see
an effect of the synchronous event interface described here. It is
therefore recommended to disable the corresponding interrupt handler
using \texttt{(set-interrupt-handler interrupt \#f)}.

\section{Interaction between threads and process state}
\label{sec:ps_interac}

In Unix, a number of resources are global to the process: signal
handlers, working directory, umask, environment, user and group ids.
Modular programming is difficult in the context of this global state
and for concurrent programming things get even worse. Section
\ref{sec:event-interf-interr} presents how scsh turns
the global, asynchronous signals handlers into modular, synchronous
sigevents. Concurrent programming also benefit from sigevents as every
thread may chase down the sigevent chain separately.

Scsh treats the working directory, umask, environment, and the
effective user/group ID as thread-local resources. The initial value
of the resources is determined by the way a thread is started:
\texttt{spawn} assigns the initial values whereas \texttt{fork-thread}
adopts the values of its parent. Here is a detailed description of the
whole facility:

\begin{itemize}
\item The procedures to access and modify the resources remain as
  described in the previous chapters (\texttt{cwd} and \texttt{chdir},
  \texttt{umask} and \texttt{set-umask}, \texttt{getenv} and
  \texttt{putenv}).
\item Every thread receives its own copy of each resource.
\item If \texttt{spawn} is used to start a new thread, the values of
  the resources are the same as they where at the start of scsh.
\item The procedure 

\defun {fork-thread} {thunk} \undefined 

from the structure \texttt{thread-fluids} starts a thread which
inherits the values of all resources from its parent. This behaviour
is similar to what happens at process forking.
\item The actual process state is updated only when necessary, i.e. on
  access or modification but not on context switch from one thread
  to another.
\end{itemize}

\defun{spoon} {thunk} \undefined

This is just an alias for \ex{fork-thread} suggested by Alan Bawden.

For user and group identities arbitrary changing is not possible.
Therefore they remain global process state: If a thread changes one of
these values, all other threads see the new value. Consequently, scsh
does not provide \texttt{with-uid} and friends.

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