FIX: Clobbering plugin files on startup is problematic

On startup, (including when starting a rails console) we manipule a
collection of plugin files. Writing these files is done in multiple
observable steps, which presents opportunities for race conditions and
causes temporary corruption.

This commit uses the write, fsync and rename trick to atomically
overwrite these files instead, but reads them first to avoid unnecessary
writes.

c457d3bf was a previous attempt to fix the same problem.

lib/discourse.rb

@@ -45,6 +45,39 @@ module Discourse
       logs.join("\n".freeze)
     end
 
+    def self.atomic_write_file(destination, contents)
+      begin
+        return if File.read(destination) == contents
+      rescue Errno::ENOENT
+      end
+
+      FileUtils.mkdir_p(File.join(Rails.root, 'tmp'))
+      temp_destination = File.join(Rails.root, 'tmp', SecureRandom.hex)
+
+      File.open(temp_destination, "w") do |fd|
+        fd.write(contents)
+        fd.fsync()
+      end
+
+      File.rename(temp_destination, destination)
+
+      nil
+    end
+
+    def self.atomic_ln_s(source, destination)
+      begin
+        return if File.readlink(destination) == source
+      rescue Errno::ENOENT, Errno::EINVAL
+      end
+
+      FileUtils.mkdir_p(File.join(Rails.root, 'tmp'))
+      temp_destination = File.join(Rails.root, 'tmp', SecureRandom.hex)
+      execute_command('ln', '-s', source, temp_destination)
+      File.rename(temp_destination, destination)
+
+      nil
+    end
+
     private
 
     class CommandRunner