[functional tests] start thinking about a block cache interface.

I want to use FFI to link with a C library containing the actual block
cache, io engine and crc32 code.

functional-tests/block-cache.scm

@@ -0,0 +1,83 @@
+(library
+  (block-cache)
+  (exports)
+  (imports (chezscheme))
+
+  (define (cache-open path block-size nr-cache-blocks)
+    ...
+    )
+
+  (define (cache-read-lock cache index)
+    
+    )
+
+  (define (cache-write-lock cache index))
+  (define (cache-zero-lock cache index))
+
+  ;; The super block is the one that should be written last.  Unlocking this
+  ;; block triggers the following events:
+  ;;
+  ;; i)  synchronous write of all dirty blocks _except_ the superblock.
+  ;;
+  ;; ii)  synchronous write of superblock
+  ;;
+  ;; If any locks are held at the time of the superblock being unlocked then an
+  ;; error will be raised.
+  (define (cache-superblock-lock cache index))
+  (define (cache-superblock-zero))
+
+  (define (cache-unlock b)
+    )
+
+  (define-syntax with-block
+    (syntax-rules ()
+      ((_ (var b) body ...)
+       (let ((var b))
+        (dynamic-wind
+         (lambda () #f)
+         (lambda () body ...)
+         (lambda () (block-put var)))))))
+
+  ;;--------------------------------------------
+
+  (define-record-type ro-spine (fields cache parent child))
+
+  (define (ro-spine-begin cache)
+    (make-ro-spine cache #f #f))
+
+  (define (ro-spine-end spine)
+    (define (unlock bl)
+      (if bl (cache-unlock) #f))
+
+    (unlock (ro-spine-parent spine))
+    (unlock (ro-spine-child spine))
+    (ro-spine-parent-set! spine #f)
+    (ro-spind-child-set! spine #f))
+
+  (define (ro-spine-step spine index)
+    (define (push b)
+      (cond
+        ((ro-spine-child spine)
+         (let ((grandparent (ro-spine-parent spine)))
+          (ro-spine-parent-set! spine (ro-spine-child spine))
+          (ro-spine-child-set! spine b)))
+        ((ro-spine-parent spine)
+         (ro-spine-child-set! spine b))
+        (else
+          (ro-spine-parent-set! spine b))))
+
+    (push (cache-read-lock (ro-spine-cache spine) index)))
+
+  (define-syntax with-ro-spine
+    (syntax-rules ()
+      ((_ (n cache) body ...)
+
+       (let ((n (ro-spine-begin cache)))
+        (dynamic-wind
+         (lambda () #f)
+         (lambda () body ...)
+         (lambda () (ro-spine-end)))))))
+
+  )
+
+