cpp: deduplicate pages by canonical wiki page, not by name

The entries filter kept a process-wide list of entry names and returned no
entries for any page whose name had been seen before. Since a page without
entries is not stored at all, this silently dropped whole documents rather
than just index entries.

Unrelated pages legitimately share a name: every container's erase_if page
is titled "std::erase_if (std::<container>)", which normalises to plain
"std::erase_if", so only the first one crawled survived. Same for std::move
in <algorithm> vs <utility>, and std::optional::operator bool.

The deduplication exists because cppreference serves the same wiki page
under several URLs and the scraper stores pages under the requested URL
rather than the effective one. Key it on the canonical page name from the
"Retrieved from" footer instead, which identifies the underlying wiki page
exactly.

Fixes #2175
Fixes #2190
Fixes #2223

Author: simon04

lib/docs/filters/cpp/entries.rb

@@ -1,7 +1,7 @@
 module Docs
   class Cpp
     class EntriesFilter < Docs::EntriesFilter
-      @@duplicate_names = []
+      @@canonical_pages = []
 
       REPLACE_NAMES = {
         'Error directive' => '#error directive',
@@ -65,18 +65,34 @@ def format_name(name)
         REPLACE_NAMES[name] || name
       end
 
-      # Avoid duplicate pages, these duplicate page are the same page for
-      # multiple functions that are organized in the same page because provide
-      # similar behavior but have different name.
+      # Avoid duplicate pages: cppreference serves the same wiki page under
+      # several URLs (redirects), and since the scraper stores pages under the
+      # requested URL rather than the effective one, each alias would be stored
+      # as a separate copy of the same document.
+      #
+      # Deduplicate on the canonical wiki page name taken from the "Retrieved
+      # from" footer, not on the entry name -- unrelated pages legitimately
+      # share a name (e.g. std::erase_if for every container, std::move in both
+      # <algorithm> and <utility>) and keying on the name silently dropped them.
       def entries
-        entries = []
+        return [] if duplicate_page?
+        super
+      end
 
-        if !(@@duplicate_names.include?(name))
-          @@duplicate_names.push(name)
-          entries << default_entry if root_page? || include_default_entry?
-          entries.concat(additional_entries)
-          build_entries(entries)
-        end
+      def duplicate_page?
+        page = canonical_page
+        return false if page.nil?
+        return true if @@canonical_pages.include?(page)
+
+        @@canonical_pages.push(page)
+        false
+      end
+
+      # e.g. "cpp/container/unordered_map/erase_if", from the printfooter link
+      # to /mwiki/index.php?title=<page>&oldid=<revision>
+      def canonical_page
+        href = at_css('.printfooter a').try(:[], 'href')
+        href && href[/[?&]title=([^&]+)/, 1]
       end
 
     end