preserve imperative changes to metas

pollen/private/main-base.rkt

@@ -53,6 +53,9 @@
             ;; and we want root to see those changes
             (begin0
               (parse xs PARSER-MODE ROOT-ID)
+              ;; pick up any imperative changes to current-metas by tag functions
+              (set! METAS-ID-CALLER (current-metas))
+              ;; restore previous value of metas
               (current-metas prev-metas))) 
           (module METAS-ID racket/base
             (provide METAS-ID)
@@ -60,9 +63,9 @@
           (require POLLEN/TOP (submod "." METAS-ID))
           (provide ALL-DEFINED-OUT; implicitly picks up METAS-ID-CALLER
                    DOC-ID)
-          (define prev-metas (current-metas))
-          (define METAS-ID-CALLER METAS-ID)
-          (and (current-metas METAS-ID) "") ; because empty strings get stripped, voids don't
           ;; we set current-metas imperatively rather than using `splicing-parameterize`
-          ;; so that we can unset it in the post processor, rather than out here
+          ;; so that we can restore it in the post-processor, rather than out here
+          (define METAS-ID-CALLER METAS-ID) ; grab the new metas
+          (define prev-metas (current-metas)) ; stash the old metas
+          (and (current-metas METAS-ID-CALLER) "") ; because empty strings get stripped, voids don't
           (begin . EXPRS)))])) 

pollen/private/ts.rktd

@@ -1 +1 @@
-1578891880
+1578951924

pollen/test/test-meta.rkt

@@ -8,3 +8,17 @@
 
 (require rackunit 'metatest)
 (check-equal? val "value")
+
+;; check that exported metas are a copy of final state of current-metas
+(module metatest2 pollen
+  (define-meta key "value")
+  (define (tag . xs)
+    (current-metas (hash-set (current-metas) 'key "reset"))
+    "")
+  (tag "hello")
+  (define val (hash-ref metas 'key))
+  (provide val))
+
+(require (prefix-in 2: 'metatest2))
+(check-equal? val "value")
+(check-equal? (hash-ref 2:metas 'key) "reset")