attempt to rename PID file when possible

This will preserve mtime on successful renames for comparisions.
While we're at it, avoid writing the new PID until the listeners are
inherited successfully.  This can be useful to avoid accidentally
clobbering a good PID if binding the listener or building the app
(preload_app==true) fails

lib/unicorn/http_server.rb

@@ -134,11 +134,22 @@ def start
     # Note that signals don't actually get handled until the #join method
     QUEUE_SIGS.each { |sig| trap(sig) { SIG_QUEUE << sig; awaken_master } }
     trap(:CHLD) { awaken_master }
-    self.pid = config[:pid]
+
+    # write pid early for Mongrel compatibility if we're not inheriting sockets
+    # This was needed for compatibility with some health checker a long time
+    # ago.  This unfortunately has the side effect of clobbering valid PID
+    # files.
+    self.pid = config[:pid] unless ENV["UNICORN_FD"]
 
     self.master_pid = $$
     build_app! if preload_app
     bind_new_listeners!
+
+    # Assuming preload_app==false, we drop the pid file after the app is ready
+    # to process requests.  If binding or build_app! fails with
+    # preload_app==true, we'll never get here and the parent will recover
+    self.pid = config[:pid] if ENV["UNICORN_FD"]
+
     spawn_missing_workers
     self
   end
@@ -180,6 +191,21 @@ def logger=(obj)
     Unicorn::HttpRequest::DEFAULTS["rack.logger"] = @logger = obj
   end
 
+  def clobber_pid(path)
+    unlink_pid_safe(@pid) if @pid
+    if path
+      fp = begin
+        tmp = "#{File.dirname(path)}/#{rand}.#$$"
+        File.open(tmp, File::RDWR|File::CREAT|File::EXCL, 0644)
+      rescue Errno::EEXIST
+        retry
+      end
+      fp.syswrite("#$$\n")
+      File.rename(fp.path, path)
+      fp.close
+    end
+  end
+
   # sets the path for the PID file of the master process
   def pid=(path)
     if path
@@ -194,18 +220,18 @@ def pid=(path)
                              "(or pid=#{path} is stale)"
       end
     end
-    unlink_pid_safe(pid) if pid
 
-    if path
-      fp = begin
-        tmp = "#{File.dirname(path)}/#{rand}.#$$"
-        File.open(tmp, File::RDWR|File::CREAT|File::EXCL, 0644)
-      rescue Errno::EEXIST
-        retry
+    # rename the old pid if posible
+    if @pid && path
+      begin
+        File.rename(@pid, path)
+      rescue Errno::ENOENT, Errno::EXDEV
+        # a user may have accidentally removed the original.
+        # Obviously cross-FS renames
+        clobber_pid(path)
       end
-      fp.syswrite("#$$\n")
-      File.rename(fp.path, path)
-      fp.close
+    else
+      clobber_pid(path)
     end
     @pid = path
   end