[tor-bugs] #16288 [Tor]: The open file limit is not checked before calling tor_accept_socket_nonblocking()

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#16288: The open file limit is not checked before calling
tor_accept_socket_nonblocking()
--------------------------+------------------------------------
 Reporter:  dgoulet       |          Owner:
     Type:  defect        |         Status:  new
 Priority:  major         |      Milestone:  Tor: 0.2.7.x-final
Component:  Tor           |        Version:
 Keywords:  026-backport  |  Actual Points:
Parent ID:                |         Points:
--------------------------+------------------------------------
 Tor keeps a buffer amount of file descriptor to avoid running out when
 trying to do critical operation such as updating crypto keys or saving the
 state file. See `ULIMIT_BUFFER` for this which is set to 32. For example,
 if the system running tor is allowed to open `4096` files, only `4064` of
 them will be used for network connections.

 The `tor_accept_socket_nonblocking()` call is done in
 `connection_handle_listener_read()` which creates a new file descriptor
 from a listener connection but the open file limit is currently _not_
 checked before. Because of that, we accept connection well beyond our
 limit (set in `ConnLimit_`) which is bad because if we ran out, we can't
 update for instance our ed25519 keys which results in a violent `exit()`
 :).

 I propose we put the open file limit just before we call `socket()`, in
 `tor_open_socket_with_extensions()` and in
 `tor_accept_socket_with_extensions()`. This way we centralize the limit
 check and avoid future issues when adding a new open/accept socket call.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/16288>
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