thread: add notes on the behavior of futures

racket/src/thread/README.txt

@@ -6,3 +6,40 @@ Core engine support must be provided by a more primitive layer. The
 more primitive layer must also provide `break-enabled-key` and special
 handling for looking up a mark with that key so that an egine-specific
 default thread cell is produced.
+
+Futures
+-------
+
+The implementation of futures here has different behavior than in the
+original Racket implementation:
+
+ - Many more primitive operations are "safe" in the sense of allowing
+   a future to keep running in parallel. For example, operations on
+   mutable `eq?` and `eqv?` hash tables are safe, as are `equal?` and
+   `equal-hash-code`.
+
+   Any attempt to enter atomic mode at the level of Racket threads (as
+   implemented in this layer) is treated as unsafe and will block the
+   future. For example, attempting to use a port will invariably go
+   into atomic mode, so port operations are still "unsafe". Any
+   attempt to use a semaphore or channel will also involve atomic
+   mode, so those are unsafe in the sense of blocking a fututre.
+
+   Attempting to use any operation that depends on the current
+   continuation or the current thread will also block, as it must.
+   However, a continuation-sensitive operation can succeed if it is
+   delimited by a prompt that is shallower in the continuation than
+   the future's start; that's an improvement over the original Racket
+   implementation.
+
+ - A blocked future becomes permanently ineligible for running in
+   parallel. That is, if a future hits an unsafe operation, the future
+   won't switch to the main thread just long enough to perform the
+   operation and then switch back to parallel mode.
+
+   To put is another way, there are no operations that trigger a 'sync
+   logging output instead of a 'block output. That's main because so
+   many more operations are safe. A second reason is that the futures
+   implementation has a finer-grained view of a computation so that it
+   doesn't see operations like `fprintf`; it sees only `start-atomic`
+   as an unsafe step inside `fprintf`.

racket/src/thread/future.rkt

@@ -18,6 +18,9 @@
          "future-lock.rkt"
          "future-logging.rkt")
 
+;; See "README.txt" for some general information about this
+;; implementation of futures.
+
 (provide init-future-place!
          futures-enabled?
          future