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5.7 Speculatively Atomic Execution

The ffi/unsafe/try-atomic supports atomic execution that can be suspended and resumed in non-atomic mode if it takes too long or if some external event causes the attempt to be abandoned.

(call-as-nonatomic-retry-point thunk)  any
  thunk : (-> any)
Calls thunk in atomic mode (see start-atomic and end-atomic) while allowing thunk to use try-atomic. Any incomplete computations started with try-atomic are run non-atomically after thunk returns. The result of thunk is used as the result of call-as-nonatomic-retry-point.

(try-atomic thunk    
  [#:should-give-up? give-up-proc    
  #:keep-in-order? keep-in-order?])  any
  thunk : (-> any)
  default-val : any/c
  give-up-proc : (-> any/c) = run-200-milliseconds
  keep-in-order? : any/c = #t
Within the dynamic extent of a call-as-nonatomic-retry-point call, attempts to run thunk in the existing atomic mode. The give-up-proc procedure is called periodically to determine whether atomic mode should be abandoned; the default give-up-proc returns true after 200 milliseconds. If atomic mode is abandoned, the computation is suspended, and default-val is returned, instead. The computation is resumed later by the enclosing call-as-nonatomic-retry-point call.

If keep-in-order? is true, then if try-atomic is called after an earlier computation was suspended for the same call-as-nonatomic-retry-point call, then thunk is immediately enqueued for completion by call-as-nonatomic-retry-point and default-val is returned.

The give-up-proc callback is polled only at points where the level of atomic-mode nesting (see start-atomic, start-breakable-atomic, and call-as-atomic) is the same as at the point of calling try-atomic.

If thunk aborts the current continuation using (default-continuation-prompt-tag), the abort is suspended the resumed by the enclosing call-as-nonatomic-retry-point. Escapes to the context of the call to thunk using any other prompt tag or continuation are blocked (using dynamic-wind) and simply return (void) from thunk.