monads, gexp: Prevent redefinition of syntax parameters.

Fixes <https://bugs.gnu.org/27476>.

This fixes multi-threaded compilation of this code where syntax
parameters could end up being redefined and where a race condition could
lead a thread to see the "wrong" value of the syntax parameter.

* guix/monads.scm (define-syntax-parameter-once): New macro.
(>>=, return): Use it.
* guix/gexp.scm (define-syntax-parameter-once): New macro.
(current-imported-modules, current-imported-extensions): Use it.

guix/gexp.scm

@@ -920,7 +920,18 @@ and in the current monad setting (system type, etc.)"
               (simple-format #f "~a:~a" line column)))
         "<unknown location>")))
 
-(define-syntax-parameter current-imported-modules
+(define-syntax-rule (define-syntax-parameter-once name proc)
+  ;; Like 'define-syntax-parameter' but ensure the top-level binding for NAME
+  ;; does not get redefined.  This works around a race condition in a
+  ;; multi-threaded context with Guile <= 2.2.4: <https://bugs.gnu.org/27476>.
+  (eval-when (load eval expand compile)
+    (define name
+      (if (module-locally-bound? (current-module) 'name)
+          (module-ref (current-module) 'name)
+          (make-syntax-transformer 'name 'syntax-parameter
+                                   (list proc))))))
+
+(define-syntax-parameter-once current-imported-modules
   ;; Current list of imported modules.
   (identifier-syntax '()))
 
@@ -931,7 +942,7 @@ environment."
                          (identifier-syntax modules)))
     body ...))
 
-(define-syntax-parameter current-imported-extensions
+(define-syntax-parameter-once current-imported-extensions
   ;; Current list of extensions.
   (identifier-syntax '()))
 

guix/monads.scm

@@ -274,12 +274,23 @@ more optimizations."
                    (_
                     #'generic-name))))))))))
 
-(define-syntax-parameter >>=
+(define-syntax-rule (define-syntax-parameter-once name proc)
+  ;; Like 'define-syntax-parameter' but ensure the top-level binding for NAME
+  ;; does not get redefined.  This works around a race condition in a
+  ;; multi-threaded context with Guile <= 2.2.4: <https://bugs.gnu.org/27476>.
+  (eval-when (load eval expand compile)
+    (define name
+      (if (module-locally-bound? (current-module) 'name)
+          (module-ref (current-module) 'name)
+          (make-syntax-transformer 'name 'syntax-parameter
+                                   (list proc))))))
+
+(define-syntax-parameter-once >>=
   ;; The name 'bind' is already taken, so we choose this (obscure) symbol.
   (lambda (s)
     (syntax-violation '>>= ">>= (bind) used outside of 'with-monad'" s)))
 
-(define-syntax-parameter return
+(define-syntax-parameter-once return
   (lambda (s)
     (syntax-violation 'return "return used outside of 'with-monad'" s)))