After updating to Rails 3.1.0.rc5 Passenger with nginx has intermittent 502 errors

It appears that only one passenger instance serves requests, the others just serve 502 errors. This causes an intermittent error pattern because only the requests that are directed to the second instance fail.

~$ rvmsudo passenger-status
----------- General information -----------
max      = 4
count    = 2
active   = 0
inactive = 2
Waiting on global queue: 0

----------- Application groups -----------
/u/apps/pixie.strd6.com/current:
  App root: /u/apps/pixie.strd6.com/current
  * PID: 3179    Sessions: 0    Processed: 121     Uptime: 3m 57s
  * PID: 3762    Sessions: 0    Processed: 0       Uptime: 2s

This happened after updating to Rails 3.1.0 rc5

2011/07/27 21:37:37 [error] 3125#0: *608 upstream prematurely closed connection while reading response header from upstream, client: 68.226.71.148, server: pixieengine.com, request: "GET /chats/recent HTTP/1.1", upstream: "passenger:unix:/passenger_helper_server:", host: "pixieengine.com", referrer: "http://pixieengine.com/projects/426/ide"
2011/07/27 21:38:31 [error] 3125#0: *596 upstream prematurely closed connection while reading response header from upstream, client: 76.102.14.57, server: pixieengine.com, request: "GET /chats/recent HTTP/1.1", upstream: "passenger:unix:/passenger_helper_server:", host: "pixieengine.com", referrer: "http://pixieengine.com/pixel-editor"
2011/07/27 21:39:12 [error] 3125#0: *576 upstream prematurely closed connection while reading response header from upstream, client: 68.8.173.234, server: pixieengine.com, request: "GET /chats/recent HTTP/1.1", upstream: "passenger:unix:/passenger_helper_server:", host: 开发者_运维技巧"pixieengine.com", referrer: "http://pixieengine.com/community/forums/1"
2011/07/27 21:39:12 [error] 3125#0: *687 upstream prematurely closed connection while reading response header from upstream, client: 201.231.103.247, server: pixieengine.com, request: "GET /chats/active_users HTTP/1.1", upstream: "passenger:unix:/passenger_helper_server:", host: "pixieengine.com", referrer: "http://pixieengine.com/projects/demo/ide"
2011/07/27 21:39:12 [error] 3125#0: *686 upstream prematurely closed connection while reading response header from upstream, client: 201.231.103.247, server: pixieengine.com, request: "GET /chats/recent HTTP/1.1", upstream: "passenger:unix:/passenger_helper_server:", host: "pixieengine.com", referrer: "http://pixieengine.com/projects/demo/ide"

I solved this by switching to Unicorn.

I wasn't actually able to figure out how to fix passenger, but I was able to verify that it broke because of the transition from Rails 3.0.9 to 3.1.0.rc5.

The temporary fix is to use PassengerSpawnMethod conservative in passenger, which disables forking running processes.