racket/base: add `impersonator-ephemeron'

collects/scribblings/reference/chaperones.scrbl

@@ -110,6 +110,16 @@ Otherwise, all chaperones of @racket[v2] must be intact in
 from @racket[v1] through one of the chaperone constructors (e.g.,
 @racket[chaperone-procedure]).}
 
+@defproc[(impersonator-ephemeron [v any/c]) ephemeron?]{
+
+Produces an @tech{ephemeron} that can be used to connect the
+reachability of @racket[v] (in the sense of garbage collection; see
+@secref["gc-model"]) with the reachability of any value for which
+@racket[v] is an @tech{impersonator}. That is, the value @racket[v]
+will be considered reachable as long as the result ephemeron is
+reachable in addition to any value that @racket[v] impersonates
+(including itself).}
+
 @; ------------------------------------------------------------
 @section{Impersonator Constructors}
 

collects/scribblings/reference/memory.scrbl

@@ -13,7 +13,7 @@ A @deftech{weak box} is similar to a normal box (see
 only reachable via weak references, the content of the weak box is
 replaced with @racket[#f]. A @defterm{@tech{weak reference}} is a
 reference through a weak box, through a key reference in a weak hash
-table (see @secref["hashtables"]), through a value in an ephemeron
+table (see @secref["hashtables"]), through a value in an @tech{ephemeron}
 where the value can be replaced by @racket[#f] (see
 @secref["ephemerons"]), or through a custodian (see
 @secref["custodians"]).
@@ -56,7 +56,9 @@ are the same.
 One particularly common use of ephemerons is to combine them with a
 weak hash table (see @secref["hashtables"]) to produce a mapping where
 the memory manager can reclaim key--value pairs even when the value
-refers to the key.
+refers to the key. A related use is to retain a reference to a value
+as long as any value for which it is an @tech{impersonator} is
+reachable; see @racket[impersonator-ephemeron].
 
 More precisely,
 @itemize[

collects/tests/racket/chaperone.rktl

@@ -1314,6 +1314,29 @@
      o)
     (test #"(a #0=(a #0#))" get-output-bytes o)))
 
+;; ----------------------------------------
+;; Impersonators and ephemerons:
+
+(let ()
+  (define stuff
+    (for/list ([n 100])
+      (define v (vector n))
+      (define c (chaperone-vector v
+                                  (lambda (b i v) v)
+                                  (lambda (b i v) v)))
+      (define e (impersonator-ephemeron c))
+      (test c ephemeron-value e)
+      ;; hold on to every other vector:
+      (cons e (if (even? n) v #f))))
+  (collect-garbage)
+  (define n (for/fold ([n 0]) ([p stuff])
+              (+ n
+                 ;; add 1 if should-retained != is-retained
+                 (if (ephemeron-value (car p))
+                     (if (vector? (cdr p)) 0 1)
+                     (if (vector? (cdr p)) 1 0)))))
+  (test #t < n 50))
+
 ;; ----------------------------------------
 
 (report-errs)

doc/release-notes/racket/HISTORY.txt

@@ -1,3 +1,6 @@
+Version 5.3.3.4
+Added impersonator-ephemeron
+
 Version 5.3.3.3
 ffi/unsafe: added _size, _ssize, _ptrdiff, _intmax, _uintmax
 ffi/vector: added f8vectors

src/racket/src/list.c

@@ -145,6 +145,7 @@ static Scheme_Object *weak_boxp(int argc, Scheme_Object *argv[]);
 static Scheme_Object *make_ephemeron(int argc, Scheme_Object *argv[]);
 static Scheme_Object *ephemeron_value(int argc, Scheme_Object *argv[]);
 static Scheme_Object *ephemeronp(int argc, Scheme_Object *argv[]);
+static Scheme_Object *impersonator_ephemeron(int argc, Scheme_Object *argv[]);
 
 static Scheme_Object *make_graph(int argc, Scheme_Object *argv[]);
 static Scheme_Object *make_placeholder(int argc, Scheme_Object *argv[]);
@@ -693,6 +694,11 @@ scheme_init_list (Scheme_Env *env)
 						      "ephemeron?",
 						      1, 1, 1),
 			     env);
+  scheme_add_global_constant("impersonator-ephemeron",
+			     scheme_make_immed_prim(impersonator_ephemeron,
+                                                    "impersonator-ephemeron",
+                                                    1, 1),
+			     env);
 
   scheme_add_global_constant("make-reader-graph",
 			     scheme_make_prim_w_arity(make_graph,
@@ -3551,6 +3557,19 @@ static Scheme_Object *ephemeronp(int argc, Scheme_Object *argv[])
 	  : scheme_false);
 }
 
+static Scheme_Object *impersonator_ephemeron(int argc, Scheme_Object *argv[])
+{
+  Scheme_Object *obj = argv[0];
+
+  if (SCHEME_CHAPERONEP(obj)) {
+    return scheme_make_ephemeron(SCHEME_CHAPERONE_VAL(obj), obj);
+  } else {
+    /* This is a useless ephemeron, but we create one for consistency
+       with the case that we have an impersonator: */
+    return scheme_make_ephemeron(obj, obj);
+  }
+}
+
 #ifndef MZ_PRECISE_GC
 
 int scheme_propagate_ephemeron_marks()

src/racket/src/schminc.h

@@ -14,7 +14,7 @@
 
 #define USE_COMPILED_STARTUP 1
 
-#define EXPECTED_PRIM_COUNT 1086
+#define EXPECTED_PRIM_COUNT 1087
 #define EXPECTED_UNSAFE_COUNT 100
 #define EXPECTED_FLFXNUM_COUNT 69
 #define EXPECTED_EXTFL_COUNT 45

src/racket/src/schvers.h

@@ -13,12 +13,12 @@
    consistently.)
 */
 
-#define MZSCHEME_VERSION "5.3.3.3"
+#define MZSCHEME_VERSION "5.3.3.4"
 
 #define MZSCHEME_VERSION_X 5
 #define MZSCHEME_VERSION_Y 3
 #define MZSCHEME_VERSION_Z 3
-#define MZSCHEME_VERSION_W 3
+#define MZSCHEME_VERSION_W 4
 
 #define MZSCHEME_VERSION_MAJOR ((MZSCHEME_VERSION_X * 100) + MZSCHEME_VERSION_Y)
 #define MZSCHEME_VERSION_MINOR ((MZSCHEME_VERSION_Z * 1000) + MZSCHEME_VERSION_W)