Passenger 6.0.27 – spawn failure with errno=24, very high number of open FDs

[Munrok]

We’re running Passenger on a shared hosting environment, where users run their own applications via Passenger. On Passenger 6.0.27 we’re seeing cases where an application fails to start. From passenger.log:

[ E 2025-12-14 12:58:08.0616 58412/Tjw3 age/Cor/App/Implementation.cpp:218 ]:
Could not spawn process for application <REDACTED_APP_PATH>:
An operating system error occurred while preparing to spawn an application process:
Error looking up OS group account 1000: Too many open files (errno=24)

At the same time, PassengerAgent ends up with a very large number of open file descriptors (around 80k+). System limits are high (openfiles 3771288), so it doesn’t look like a ulimit problem.

On Passenger 6.0.22 this setup was working fine: PassengerAgent typically had around 200 open files, not tens of thousands.

After attaching truss to PassengerAgent, the process crashed, and after restart the problem temporarily disappeared.

[CamJN]

Between release-6.0.22 and release-6.0.27 these files had changes that involve fds:

• src/agent/Core/ApplicationPool/Socket.h now connection.fd = state.getFd().detach(); is set differently, was connection.fd = connectToServer(address, __FILE__, __LINE__);
• src/agent/Core/ApplicationPool/Session.h virtual bool initiate() override no longer sets the fd as non-blocking and therefore doesn't have a fdguard
• src/agent/SpawnEnvSetupper/SpawnEnvSetupperMain.cpp changed from boost bind boost::bind(reopenStdout, fileno(f))); to closure int fd = fileno(f); runCommand(command, info, true, true, [=]() { reopenStdout(fd); });
• src/agent/Core/Controller/CheckoutSession.cpp onSessionSocketConnected and initiateSession have new uses of fds
• src/cxx_supportlib/IOTools/IOUtils.* NConnect_State::getFd() is used to get fd instead of connectToServer(NConnect_State &state) interestingly connectToServer used to return a bool...

[CamJN]

You can also set PassengerFileDescriptorLogFile to log fd activity, which might help gather info.