Address kou's review comments: extract safe_mtime helper, cleanups

* Add RDoc.safe_mtime(file) module method that wraps File.mtime with an
  explicit rescue SystemCallError. Previously, five call sites used
  `File.mtime(file) rescue nil`, which silently swallows every
  StandardError. The narrower rescue lets unrelated bugs surface.

* Use the helper at the five sites that needed the same pattern:
  rdoc.rb's rbs_signature_mtimes, and four spots in server.rb
  (file_mtimes_for, the change-detection loop, the post-parse mtime
  record, and the rbs reload block).

* Anchor the remove_unparseable file extension regex with \z instead
  of $ so the match only succeeds at end-of-string. Same change for
  the tags regex on the next line.

* Use the existing rbs_signature_file? predicate in two more spots
  instead of re-inlining File.extname(file) == '.rbs'.

Author: st0012

lib/rdoc.rb

@@ -151,6 +151,16 @@ def self.home
     end
   end
 
+  ##
+  # Returns +File.mtime(file)+, or +nil+ if the file cannot be stat'd
+  # (missing, permission denied, etc.).
+
+  def self.safe_mtime(file)
+    File.mtime(file)
+  rescue SystemCallError
+    nil
+  end
+
   autoload :RDoc,           "#{__dir__}/rdoc/rdoc"
 
   autoload :CrossReference, "#{__dir__}/rdoc/cross_reference"

lib/rdoc/rdoc.rb

@@ -430,8 +430,8 @@ def parse_files(files)
 
   def remove_unparseable(files)
     files.reject do |file, *|
-      file =~ /\.(?:class|eps|erb|rbs|scpt\.txt|svg|ttf|yml)$/i or
-        (file =~ /tags$/i and
+      file =~ /\.(?:class|eps|erb|rbs|scpt\.txt|svg|ttf|yml)\z/i or
+        (file =~ /tags\z/i and
          /\A(\f\n[^,]+,\d+$|!_TAG_)/.match?(File.binread(file, 100)))
     end
   end
@@ -582,7 +582,7 @@ def rbs_signature_files
 
   def rbs_signatures_changed?
     current = rbs_signature_mtimes
-    previous = @last_modified.select { |file, _| File.extname(file) == '.rbs' }
+    previous = @last_modified.select { |file, _| rbs_signature_file?(file) }
 
     return true unless (previous.keys - current.keys).empty?
 
@@ -597,7 +597,7 @@ def rbs_signatures_changed?
   # and the live server watcher can see signature-only edits.
 
   def record_rbs_signature_mtimes
-    @last_modified.reject! { |file, _| File.extname(file) == '.rbs' }
+    @last_modified.reject! { |file, _| rbs_signature_file?(file) }
     @last_modified.merge! rbs_signature_mtimes
   end
 
@@ -614,7 +614,7 @@ def rbs_signature_file?(file) # :nodoc:
 
   def rbs_signature_mtimes # :nodoc:
     rbs_signature_files.each_with_object({}) do |file, mtimes|
-      mtime = File.mtime(file) rescue nil
+      mtime = RDoc.safe_mtime(file)
       mtimes[file] = mtime if mtime
     end
   end

lib/rdoc/server.rb

@@ -310,7 +310,7 @@ def start_watcher(source_files)
 
   def file_mtimes_for(files)
     files.each_with_object({}) do |f, h|
-      h[f] = File.mtime(f) rescue nil
+      h[f] = RDoc.safe_mtime(f)
     end
   end
 
@@ -334,7 +334,7 @@ def check_for_changes
         next
       end
 
-      current_mtime = File.mtime(file) rescue nil
+      current_mtime = RDoc.safe_mtime(file)
       next unless current_mtime
       next unless old_mtime.nil? || current_mtime > old_mtime
 
@@ -397,7 +397,7 @@ def reparse_and_refresh(changed_files, removed_files, rbs_changed: false)
             begin
               @store.clear_file_contributions(relative, keep_position: true)
               @rdoc.parse_file(f)
-              @file_mtimes[f] = File.mtime(f) rescue nil
+              @file_mtimes[f] = RDoc.safe_mtime(f)
             rescue => e
               $stderr.puts "Error parsing #{f}: #{e.message}"
             end
@@ -415,7 +415,7 @@ def reparse_and_refresh(changed_files, removed_files, rbs_changed: false)
           @rdoc.load_rbs_signatures
           @rdoc.record_rbs_signature_mtimes
           @rdoc.rbs_signature_files.each do |file|
-            @file_mtimes[file] = File.mtime(file) rescue nil
+            @file_mtimes[file] = RDoc.safe_mtime(file)
           end
         end
         $stderr.puts "Reloaded RBS signatures (#{duration_ms}ms)" if rbs_changed