db: fix finalization problems

A `this%` expression used in a finalization callback implicitly
referred to `this`, since it's a dynamic reference to the object's
class. As a result, the finalizer for `this` refers to `this`, so
`this` never becomes collectable. The problem is fixed by
lifting the `this%` out of the `lambda`.

Less significantly, the finalizer thread in "prepared.rkt" captured
various parameters on creation, including the current namespace. If a
prepared statement is bound to a module-level variable, then the
finalizer thread refers through the namespace to the prepared
statement, so the prepared statement can never be finalized. Setting
the current namespace to a fresh empty one while creating the thread
avoids that specific problem. (Other parameters could cause similar
problems, but solving the namespace one works well enough for now.)

racket/collects/db/private/generic/prepared.rkt

@@ -102,18 +102,21 @@
 (define will-executor (make-will-executor))
 
 (define finalizer-thread
-  (thread/suspend-to-kill
+  ;; Set parameters for the new thread to avoid references to the
-   (lambda ()
+  ;; namespace, since prepared statements may be bound to globals, etc.
-     (let loop ()
+  (parameterize ([current-namespace (make-empty-namespace)])
-       (with-handlers
+    (thread/suspend-to-kill
-           ([(lambda (e) #t)
+     (lambda ()
-             (lambda (e)
+       (let loop ()
-               ((error-display-handler)
+         (with-handlers
-                (cond [(exn? e)
+             ([(lambda (e) #t)
-                       (format "prepared statement finalizer thread handled exception:\n~a"
+               (lambda (e)
-                               (exn-message e))]
+                 ((error-display-handler)
-                      [else
+                  (cond [(exn? e)
-                       "prepared statement finalizer thread handled non-exception"])
+                         (format "prepared statement finalizer thread handled exception:\n~a"
-                e))])
+                                 (exn-message e))]
-         (will-execute will-executor))
+                        [else
-       (loop)))))
+                         "prepared statement finalizer thread handled non-exception"])
+                  e))])
+           (will-execute will-executor))
+         (loop))))))

racket/collects/db/private/sqlite3/connection.rkt

@@ -374,16 +374,16 @@
       (handle-status* who full-s -db db-spec pst))
 
     ;; ----
-
     (super-new)
-    (register-finalizer-and-custodian-shutdown this
+    (register-finalizer-and-custodian-shutdown
-                        (lambda (obj)
+     this
-                          ;; Keep a reference to the class to keep all FFI callout objects
+     ;; Keep a reference to the class to keep all FFI callout objects
-                          ;; (eg, sqlite3_close) used by its methods from being finalized.
+     ;; (eg, sqlite3_close) used by its methods from being finalized.
-                          (let ([dont-gc this%])
+     (let ([dont-gc this%])
-                            (send obj disconnect)
+       (lambda (obj)
-                            ;; Dummy result to prevent reference from being optimized away
+         (send obj disconnect)
-                            dont-gc)))))
+         ;; Dummy result to prevent reference from being optimized away
+         dont-gc)))))
 
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