concurrency-tools: more synchronization primitives

Note: at present, these are implemented directly in terms of SBCL’s sb-thread, rather than a portable library like bordeaux-threads. I would accept PRs to make this library portable across implementations.

shared-cell

A mutex which is explicitly associated with the shared resource to which accesses are synchronized.

Construct them with (make-shared-cell OBJECT).

Access them with (with-shared-cell (ACCESSOR CELL) BODY...). Within the BODY forms, ACCESSOR will be bound as a symbol-macro to a reader and writer for the value stored in the CELL.

what not to do

Exfiltrating a mutable object from a shared cell is undefined behavior; that is, the contents of the cell must only ever be accessed via the with-shared-cell macro.

Common Lisp doesn’t provide us a mechanism for detecting or preventing these errors, so just don’t do it.

For example, the following invokes undefined behavior:

(defparameter *cell* (make-shared-cell (make-array 128 :initial-element 0)))

(defparameter *illegal-unsynchronized-reference*
  (with-shared-cell (value *cell*)
    value))

(setf (aref *illegal-unsynchronized-reference* 0)
      1)

Replacing a value in a shared cell and returning the old value is acceptable, though. The following is fine:

(defparameter *cell* (make-shared-cell (make-array 128 :initial-element 0)))

(defparameter *fine-unique-reference*
  (with-shared-cell (value *cell*)
    (prog1 value
      (setf value (make-array 64 :initial-element 1)))))

(setf (aref *fine-unique-reference* 0) 1)

once-cell

A thunk that is evaluated only once.

The implementation is not particularly optimized; particularly, it needlessly grabs a mutex on every read. It should be possible to test whether the cell is initialized without synchronizing, and fall back to waiting on a condition variable only if the cell is uninitialized.

constructing them

The function (make-once-cell FUNCTION), where FUNCTION is a function from no arguments to one value, constructs a once-cell.

The macro (once-cell BODY...) expands to (make-once-cell (lambda () ...BODY)).

accessing them

The function (once-cell-get CELL) returns the value stored in CELL, computing it or blocking as appropriate.

other macros

The macro (once BODY...) evaluates BODY once and returns its value subsequent times. BODY cannot close over a lexical environment, and likely should not depend on a particular dynamic environment.

rwlock

A shared-cell which allows multiple concurrent readers, or a single writer.

The interface is similar to that of shared-cell: (make-rwlock OBJECT) constructs a rwlock, (with-writing (ACCESSOR RWLOCK) BODY...) evaluates BODY with a unique lock held for writing, and (with-reading (ACCESSOR RWLOCK) BODY...) evaluated BODY with a shared lock held for reading.

Common Lisp provides no facility for detecting or preventing erroneous writes within a with-reading form, so just don’t do that.

As with shared-cell, also don’t leak references to objects contained in the rwlock.