Change xref var to get-xref function; delete xref-ready-evt

The ready event, although not used, wasn't implemented correctly.

Although moot now, better to set this up to work correctly if needed
someday.

racket/commands/help.rkt

@@ -28,6 +28,7 @@
   (->identifier how str stx->uri-string))
 
 (define (stx->uri-string stx)
+  (define xref (get-xref))
   (match (and xref (xref-binding->definition-tag xref stx 0))
     [(? tag? tag)
      (define-values (path anchor) (xref-tag->path+anchor xref tag))

racket/scribble.rkt

@@ -44,7 +44,8 @@
 
 (define/contract (binding->path+anchor stx)
   (-> identifier? (or/c #f (cons/c path-string? (or/c #f string?))))
-  (let* ([tag (xref-binding->definition-tag xref stx 0)]
+  (let* ([xref (get-xref)]
+         [tag (xref-binding->definition-tag xref stx 0)]
          [p+a (and tag (tag->path+anchor xref tag))])
     p+a))
 
@@ -72,7 +73,7 @@
 
 (define/contract (identifier->bluebox stx)
   (-> identifier? (or/c #f string?))
-  (match (xref-binding->definition-tag xref stx 0)
+  (match (xref-binding->definition-tag (get-xref) stx 0)
     [(? tag? tag) (get-bluebox-string tag)]
     [_ #f]))
 
@@ -169,7 +170,7 @@
              (let ([ht (exported-index-desc*-extras desc)])
                (or (hash-ref ht 'hidden? #f)
                    (hash-ref ht 'constructor? #f))))))
-  (for* ([entry (in-list (xref-index xref))]
+  (for* ([entry (in-list (xref-index (get-xref)))]
          [desc (in-value (entry-desc entry))]
          #:when desc
          #:unless (hide-desc? desc)
@@ -181,7 +182,7 @@
   root)
 
 (define (doc-trie-value desc term tag)
-  (define-values (path anchor) (xref-tag->path+anchor xref tag))
+  (define-values (path anchor) (xref-tag->path+anchor (get-xref) tag))
   (define (method-what)
     (cond
       [(method-tag? tag)

racket/xref.rkt

@@ -5,9 +5,13 @@
 
 (require setup/xref)
 
-(provide xref
-         xref-ready-evt)
+(provide get-xref)
 
-(define xref-ready-evt never-evt)
-(define xref (begin0 (load-collections-xref)
-               (set! xref-ready-evt always-evt)))
+;; A single xref instance for all our modules to share.
+;;
+;; Will block safely until ready, if used from e.g. delay/thread or
+;; delay/idle (which, although we're not doing now, we've done before,
+;; and might do again someday).
+(define sema (make-semaphore 1))
+(define xref (call-with-semaphore sema load-collections-xref))
+(define (get-xref) (call-with-semaphore sema (λ () xref)))