buf_file: handle "Too many open files" error to keep buffer and metad…

…ata pair

This is not incomplete but keep buffer and metadata pair phase
as much as possible.

lib/fluent/plugin/buffer/file_chunk.rb

@@ -106,10 +106,20 @@ def enqueued!
 
           write_metadata(update: false) # re-write metadata w/ finalized records
 
-          file_rename(@chunk, @path, new_chunk_path, ->(new_io){ @chunk = new_io })
-          @path = new_chunk_path
+          begin
+            file_rename(@chunk, @path, new_chunk_path, ->(new_io) { @chunk = new_io })
+          rescue => e
+            raise "can't enqueue buffer file: path = #{@path}, error = #{e}"
+          end
+
+          begin
+            file_rename(@meta, @meta_path, new_meta_path, ->(new_io) { @meta = new_io })
+          rescue => e
+            file_rename(@chunk, new_chunk_path, @path, ->(new_io) { @chunk = new_io })
+            raise "can't enqueue buffer metadata: path = #{@meta_path}, error = #{e}"
+          end
 
-          file_rename(@meta, @meta_path, new_meta_path, ->(new_io){ @meta = new_io })
+          @path = new_chunk_path
           @meta_path = new_meta_path
 
           super
@@ -242,14 +252,24 @@ def file_rename(file, old_path, new_path, callback=nil)
         def create_new_chunk(path, perm)
           @path = self.class.generate_stage_chunk_path(path, @unique_id)
           @meta_path = @path + '.meta'
-          @chunk = File.open(@path, 'wb+', perm)
-          @chunk.set_encoding(Encoding::ASCII_8BIT)
-          @chunk.sync = true
-          @chunk.binmode
-          @meta = File.open(@meta_path, 'wb', perm)
-          @meta.set_encoding(Encoding::ASCII_8BIT)
-          @meta.sync = true
-          @meta.binmode
+          begin
+            @chunk = File.open(@path, 'wb+', perm)
+            @chunk.set_encoding(Encoding::ASCII_8BIT)
+            @chunk.sync = true
+            @chunk.binmode
+          rescue => e
+            raise BufferOverflowError, "can't create buffer file for #{path}. Stop creating buffer files: error = #{e}"
+          end
+          begin
+            @meta = File.open(@meta_path, 'wb', perm)
+            @meta.set_encoding(Encoding::ASCII_8BIT)
+            @meta.sync = true
+            @meta.binmode
+          rescue => e
+            @chunk.close
+            File.unlink(@path)
+            raise BufferOverflowError, "can't create buffer metadata for #{path}. Stop creating buffer files: error = #{e}"
+          end
 
           @state = :unstaged
           @bytesize = 0