racket-index: fix document links for layered installations

When a layer renders new documentation that refers to a previous
layer, the redirection script was not generated as needed to reach
the previous layer.

The documentation currently implements less sharing that would be
possible across layers. For each installation layer, `raco setup`
duplicates the documentation database in "docindex.sqlite", and it
duplicates the search index, and it duplicates the local-redirection
index. That duplication avoids having to join across an an arbitrary
number of SQLite databases or chain the search and local-redirection
indexes.

pkgs/racket-doc/scribblings/raco/setup.scrbl

@@ -545,6 +545,10 @@ Optional @filepath{info.rkt} fields trigger additional actions by
      @item{@racket['no-search] : Build the document without a search
            box.}
 
+     @item{@racket['every-main-layer] : With @racket['main-doc],
+           indicates that the document should be rendered separately
+           at every installation layer (see @secref["layered-install"]).}
+
     ]
 
     The @racket[_category] list specifies how to show the document in

pkgs/racket-index/scribblings/main/info.rkt

@@ -1,10 +1,9 @@
 #lang info
 
 (define scribblings
-  '(("start.scrbl"
-     (main-doc-root depends-all-main no-depend-on) (omit))
-    ("search.scrbl"         (depends-all-main no-depend-on) (omit) "search" 1 10)
-    ("local-redirect.scrbl" (depends-all-main no-depend-on) (omit) "local-redirect" 1 10)
+  '(("start.scrbl"   (main-doc-root depends-all-main no-depend-on) (omit))
+    ("search.scrbl"  (depends-all-main no-depend-on every-main-layer) (omit) "search" 1 10)
+    ("local-redirect.scrbl" (depends-all-main no-depend-on every-main-layer) (omit) "local-redirect" 1 10)
     ("license.scrbl" () (omit))
     ("acks.scrbl"    () (omit))
     ("release.scrbl" (depends-all-main no-depend-on) (omit))))

pkgs/racket-index/scribblings/main/private/local-redirect.rkt

@@ -284,7 +284,8 @@
                [i (in-naturals)])
            (define name (extract-name e))
            (fprintf o (if (zero? i) "\n" ",\n"))
-           (fprintf o " [~s, ~s]" name (if user?
+           (fprintf o " [~s, ~s]" name (if (or user?
+                                               (not (immediately-in-doc-dir? e)))
                                            (url->string (path->url e))
                                            (format "../~a" name))))
          (fprintf o "];\n\n")
@@ -305,4 +306,8 @@
                (js-addition (string->url "local-user-redirect.js"))
                (js-addition
                 (string->bytes/utf-8 search-code))))
-    null)))
+     null)))
+
+(define (immediately-in-doc-dir? path)
+  (define-values (base name dir?) (split-path path))
+  (equal? (path->directory-path (find-doc-dir)) base))

pkgs/racket-index/setup/private/doc-path.rkt

@@ -23,12 +23,13 @@
              (memq 'main-doc flags)
              (pair? (path->main-collects-relative dir)))
          (and main?
-              ;; check for existing directory in search path before
-              ;; assuming the main doc directory
-              (or (for/or ([dir (get-doc-search-dirs)])
-                    (define p (build-path dir name))
-                    (and (directory-exists? p)
-                         p))
+              ;; check for existing directory in search path (unless
+              ;; 'every-main-layer) before assuming the main doc directory
+              (or (and (not (memq 'every-main-layer flags))
+                       (for/or ([dir (get-doc-extra-search-dirs)])
+                         (define p (build-path dir name))
+                         (and (directory-exists? p)
+                              p)))
                   (build-path (find-doc-dir) name)))]
         [else 
          (and (not (eq? 'never user-doc-mode))

pkgs/racket-index/setup/scribble.rkt

@@ -158,7 +158,8 @@
   (define (scribblings-flag? sym)
     (memq sym '(main-doc main-doc-root user-doc-root user-doc multi-page
                          depends-all depends-all-main depends-all-user
-                         no-depend-on always-run keep-style no-search)))
+                         no-depend-on always-run keep-style no-search
+                         every-main-layer)))
   (define (validate-scribblings-infos infos)
     (define (validate path [flags '()] [cat '(library)] [name #f] [out-count 1] [order-hint 0])
       (and (string? path) (relative-path? path)