Gracefully deal with dangling symlinks

* Don't add files that are dangling symlinks
* When watching a directory, ignore dangling symlinks rather than treating them as forever-stale

Author: jeremy

lib/spring/test/watcher_test.rb

@@ -162,6 +162,33 @@ def assert_not_stale
         watcher.add './foobar'
         assert watcher.files.empty?
       end
+
+      test "add symlink" do
+        File.write("#{dir}/bar", "bar")
+        File.symlink("#{dir}/bar", "#{dir}/foo")
+        watcher.add './foo'
+        assert_equal ["#{dir}/bar"], watcher.files.to_a
+      end
+
+      test "add dangling symlink" do
+        File.symlink("#{dir}/bar", "#{dir}/foo")
+        watcher.add './foo'
+        assert watcher.files.empty?
+      end
+
+      test "add directory with dangling symlink" do
+        subdir = "#{@dir}/subdir"
+        FileUtils.mkdir(subdir)
+        File.symlink("dangling", "#{subdir}/foo")
+
+        watcher.add subdir
+        assert_not_stale
+
+        # Adding a new file should mark as stale despite the dangling symlink.
+        File.write("#{subdir}/new-file", "new")
+        watcher.check_stale
+        assert_stale
+      end
     end
   end
 end

lib/spring/watcher/abstract.rb

@@ -48,14 +48,30 @@ def add(*items)
           end
         end
 
-        items = items.select(&:exist?)
+        items = items.select do |item|
+          if item.symlink?
+            item.readlink.exist?.tap do |exists|
+              if !exists
+                debug { "add: ignoring dangling symlink: #{item.inspect} -> #{item.readlink.inspect}" }
+              end
+            end
+          else
+            item.exist?
+          end
+        end
 
         synchronize {
           items.each do |item|
             if item.directory?
               directories << item.realpath.to_s
             else
-              files << item.realpath.to_s
+              begin
+                files << item.realpath.to_s
+              rescue Errno::ENOENT
+                # Race condition. Ignore symlinks whose target was removed
+                # since the check above, or are deeply chained.
+                debug { "add: ignoring now-dangling symlink: #{item.inspect} -> #{item.readlink.inspect}" }
+              end
             end
           end
 

lib/spring/watcher/polling.rb

@@ -64,10 +64,24 @@ def subjects_changed
       private
 
       def compute_mtime
-        expanded_files.map { |f| File.mtime(f).to_f }.max || 0
-      rescue Errno::ENOENT
-        # if a file does no longer exist, the watcher is always stale.
-        Float::MAX
+        expanded_files.map do |f|
+          # Get the mtime of symlink targets. Ignore dangling symlinks.
+          if File.symlink?(f)
+            begin
+              File.mtime(f)
+            rescue Errno::ENOENT
+              0
+            end
+          # If a file no longer exists, treat it as changed.
+          else
+            begin
+              File.mtime(f)
+            rescue Errno::ENOENT
+              debug { "compute_mtime: no longer exists: #{f}" }
+              Float::MAX
+            end
+          end.to_f
+        end.max || 0
       end
 
       def expanded_files