Fix #3991: Loading post #1604318 is broken.

Fix exception in Pool#previous_post_id when the post id is not contained
in the pool.

This can happen when a post's pool_string lists the post as being in a
certain pool, but the post is not included in the pool's post_ids. Such
pool_string / post_ids inconsistencies exist because of past bugs.

app/models/pool.rb

@@ -268,19 +268,25 @@ class Pool < ApplicationRecord
   end
 
   def first_post?(post_id)
-    page_number(post_id) == 1
+    post_id == post_ids.first
+  end
+
+  def last_post?(post_id)
+    post_id == post_ids.last
   end
 
   # XXX finds wrong post when the pool contains multiple copies of the same post (#2042).
   def previous_post_id(post_id)
+    return nil if first_post?(post_id) || !contains?(post_id)
+
     n = post_ids.index(post_id) - 1
-    return nil if n < 0
     post_ids[n]
   end
 
   def next_post_id(post_id)
+    return nil if last_post?(post_id) || !contains?(post_id)
+
     n = post_ids.index(post_id) + 1
-    return nil if n >= post_ids.size
     post_ids[n]
   end
 

app/views/posts/partials/show/_pool_list_item.html.erb

@@ -1,5 +1,5 @@
 <%= content_tag :li, id: "nav-link-for-pool-#{pool.id}", class: "pool-category-#{pool.category} pool-selected-#{selected}" do -%>
-  <% if !pool.first_post?(post.id) -%>
+  <% if !pool.first_post?(post.id) && pool.post_ids.first -%>
     <%= link_to("&laquo;".html_safe, post_path(pool.post_ids.first, pool_id: pool.id), class: "first", title: "to page 1") %>
   <% else -%>
     <span class="first">&laquo;</span>
@@ -25,7 +25,7 @@
     <% end -%>
   <% end -%>
 
-  <% if post.id != pool.post_ids.last -%>
+  <% if !pool.last_post?(post.id) && pool.post_ids.last -%>
     <%= link_to("&raquo;".html_safe, post_path(pool.post_ids.last, pool_id: pool.id), class: "last", title: "to page #{pool.post_count}") -%>
   <% else -%>
     <span class="last">&raquo;</span>