I've been trying figure out a problem we are seeing in passenger module where by the full POST data isnt passed to the underlying application.

After much digging I believe this may actually be a problem with the http_upstream / sendfile support in the core nginx code.

The following log snippet shows a trace of this happening:-

2010/12/24 18:04:08 [debug] 27169#0: *21 connected
2010/12/24 18:04:08 [debug] 27169#0: *21 http upstream connect: 0
2010/12/24 18:04:08 [debug] 27169#0: *21 http upstream send request
2010/12/24 18:04:08 [debug] 27169#0: *21 chain writer buf fl:0 s:2405
2010/12/24 18:04:08 [debug] 27169#0: *21 chain writer buf fl:0 s:15939
2010/12/24 18:04:08 [debug] 27169#0: *21 chain writer buf fl:1 s:23658449
2010/12/24 18:04:08 [debug] 27169#0: *21 chain writer in: 0000000801885F78
2010/12/24 18:04:08 [debug] 27169#0: *21 sendfile() sent only 18344 bytes (35: Resource temporarily unavailable)
2010/12/24 18:04:08 [debug] 27169#0: *21 sendfile: -1, 1, 0, @0 18344:23658449
2010/12/24 18:04:08 [debug] 27169#0: *21 chain writer out: 0000000801885F98
2010/12/24 18:04:08 [debug] 27169#0: *21 event timer add: 23: 600000:1293214448446
2010/12/24 18:04:08 [debug] 27169#0: *21 kevent set event: 23: ft:-2 fl:0025
2010/12/24 18:04:08 [debug] 27169#0: *21 http run request: "/uploadfile"
2010/12/24 18:04:08 [debug] 27169#0: *21 http upstream check client, write event:1, "/uploadfile"
2010/12/24 18:06:08 [debug] 27169#0: *21 http run request: "/uploadfile"

The last line is where the 2 minute request timeout triggered.

It seems that even though EAGAIN is returned there is never an attempt to try to send the remaining data.

Any ideas?