db: improved locking

Tests suggest new locking is faster, but primary benefit is
detecting when thread holding lock is killed.

collects/db/private/generic/interfaces.rkt

@@ -1,5 +1,6 @@
 #lang racket/base
-(require racket/class)
+(require racket/class
+         ffi/unsafe/atomic)
 (provide connection<%>
          dbsystem<%>
          prepared-statement<%>
@@ -188,27 +189,33 @@
 
 ;; Connection base class (locking)
 
-;; Disabled for now, because this is an 80% solution. Unfortunately, I
-;; think a 100% solution would require an auxiliary kill-safe thread
-;; with multiple thread switches *per lock acquisition*. At that
-;; point, might as well just use kill-safe connection.
-(define USE-LOCK-HOLDER? #f)
-
 (define locking%
   (class object%
 
     ;; == Communication locking
 
-    (define lock (make-semaphore 1))
+    ;; Goal: we would like to be able to detect if a thread has
-
-    ;; Ideally, we would like to be able to detect if a thread has
     ;; acquired the lock and then died, leaving the connection
-    ;; permanently locked. Roughly, we would like this: if lock is
+    ;; permanently locked.
-    ;; held by thread th, then lock-holder = (thread-dead-evt th), 
+    ;;
-    ;; and if lock is not held, then lock-holder = never-evt.
+    ;; lock-holder=(thread-dead-evt thd) iff thd has acquired inner-lock
-    ;; Unfortunately, there are intervals when this is not true.
+    ;;  - lock-holder, inner-lock always modified together within
-    ;; Also, since lock-holder changes, reference might be stale, so
+    ;;    atomic block
-    ;; need to double-check.
+    ;;
+    ;; Thus if (thread-dead-evt thd) is ready, thd died holding
+    ;; inner-lock, so hopelessly locked.
+    ;;
+    ;; outer-sema = inner-lock
+    ;;  - outer-sema, inner-lock always modified together within atomic
+    ;;
+    ;; The outer-lock just prevents threads from spinning polling
+    ;; inner-lock. If a thread gets past outer-lock and dies before
+    ;; acquiring inner-lock, ok, because outer-lock still open at that
+    ;; point, so other threads can enter outer-lock and acquire inner-lock.
+
+    (define outer-sema (make-semaphore 1))
+    (define outer-lock (semaphore-peek-evt outer-sema))
+    (define inner-lock (make-semaphore 1))
     (define lock-holder never-evt)
 
     ;; Delay async calls (eg, notice handler) until unlock
@@ -220,21 +227,32 @@
       (call-with-lock* who proc #f #t))
 
     (define/public-final (call-with-lock* who proc hopeless require-connected?)
-      (let* ([me (thread-dead-evt (current-thread))]
+      (let ([me (thread-dead-evt (current-thread))]
-             [result (sync lock lock-holder)])
+            [result (sync outer-lock lock-holder)])
-        (cond [(eq? result lock)
+        (cond [(eq? result outer-lock)
+               ;; Got past outer stage
+               (let ([proceed?
+                      (begin (start-atomic)
+                             (let ([proceed? (semaphore-try-wait? inner-lock)])
+                               (when proceed?
+                                 (set! lock-holder me)
+                                 (semaphore-wait outer-sema))
+                               (end-atomic)
+                               proceed?))])
+                 (cond [proceed?
                         ;; Acquired lock
-               (when USE-LOCK-HOLDER? (set! lock-holder me))
+                        ;;  - lock-holder = me, and outer-lock is closed again
                         (when (and require-connected? (not (connected?)))
-                 (semaphore-post lock)
+                          (unlock)
                           (error/not-connected who))
                         (with-handlers ([values (lambda (e) (unlock) (raise e))])
                           (begin0 (proc) (unlock)))]
+                       [else
+                        ;; Didn't acquire lock; retry
+                        (call-with-lock* who proc hopeless require-connected?)]))]
               [(eq? result lock-holder)
                ;; Thread holding lock is dead
-               (if hopeless
+               (if hopeless (hopeless) (error/hopeless who))]
-                   (hopeless)
-                   (error/hopeless who))]
               [else
                ;; lock-holder was stale; retry
                (call-with-lock* who proc hopeless require-connected?)])))
@@ -242,8 +260,11 @@
     (define/private (unlock)
       (let ([async-calls (reverse delayed-async-calls)])
         (set! delayed-async-calls null)
-        (when USE-LOCK-HOLDER? (set! lock-holder never-evt))
+        (start-atomic)
-        (semaphore-post lock)
+        (set! lock-holder never-evt)
+        (semaphore-post inner-lock)
+        (semaphore-post outer-sema)
+        (end-atomic)
         (for-each call-with-continuation-barrier async-calls)))
 
     ;; needs overriding