When libcurl discards a connection there are two phases this may go
through: "shutdown" and "closing". If a connection is aborted, the
shutdown phase is skipped and it is closed right away.

The connection filters attached to the connection implement the phases
in their `do_shutdown()` and `do_close()` callbacks. Filters carry now a
`shutdown` flags next to `connected` to keep track of the shutdown
operation.

Filters are shut down from top to bottom. If a filter is not connected,
its shutdown is skipped. Notable filters that *do* something during
shutdown are HTTP/2 and TLS. HTTP/2 sends the GOAWAY frame. TLS sends
its close notify and expects to receive a close notify from the server.

As sends and receives may EAGAIN on the network, a shutdown is often not
successful right away and needs to poll the connection's socket(s). To
facilitate this, such connections are placed on a new shutdown list
inside the connection cache.

Since managing this list requires the cooperation of a multi handle,
only the connection cache belonging to a multi handle is used. If a
connection was in another cache when being discarded, it is removed
there and added to the multi's cache. If no multi handle is available at
that time, the connection is shutdown and closed in a one-time,
best-effort attempt.

When a multi handle is destroyed, all connection still on the shutdown
list are discarded with a final shutdown attempt and close. In curl
debug builds, the environment variable `CURL_GRACEFUL_SHUTDOWN` can be
set to make this graceful with a timeout in milliseconds given by the
variable.

The shutdown list is limited to the max number of connections configured
for a multi cache. Set via CURLMOPT_MAX_TOTAL_CONNECTIONS. When the
limit is reached, the oldest connection on the shutdown list is
discarded.

- In multi_wait() and multi_waitfds(), collect all connection caches
  involved (each transfer might carry its own) into a temporary list.
  Let each connection cache on the list contribute sockets and
  POLLIN/OUT events it's connections are waiting for.

- in multi_perform() collect the connection caches the same way and let
  them peform their maintenance. This will make another non-blocking
  attempt to shutdown all connections on its shutdown list.

- for event based multis (multi->socket_cb set), add the sockets and
  their poll events via the callback. When `multi_socket()` is invoked
  for a socket not known by an active transfer, forward this to the
  multi's cache for processing. On closing a connection, remove its
  socket(s) via the callback.

TLS connection filters MUST NOT send close nofity messages in their
`do_close()` implementation. The reason is that a TLS close notify
signals a success. When a connection is aborted and skips its shutdown
phase, the server needs to see a missing close notify to detect
something has gone wrong.

A graceful shutdown of FTP's data connection is performed implicitly
before regarding the upload/download as complete and continuing on the
control connection. For FTP without TLS, there is just the socket close
happening. But with TLS, the sent/received close notify signals that the
transfer is complete and healthy. Servers like `vsftpd` verify that and
reject uploads without a TLS close notify.

- added test_19_* for shutdown related tests
- test_19_01 and test_19_02 test for TCP RST packets
  which happen without a graceful shutdown and should
  no longer appear otherwise.
- add test_19_03 for handling shutdowns by the server
- add test_19_04 for handling shutdowns by curl
- add test_19_05 for event based shutdowny by server
- add test_30_06/07 and test_31_06/07 for shutdown checks
  on FTP up- and downloads.

Closes #13976

2024-06-26 08:33:17 +02:00

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SPDX-License-Identifier

Title

Section

Source

See-also

Protocol

curl

libcurl-env-dbg

libcurl

libcurl-env (3)

All

NAME

libcurl-env-dbg - environment variables libcurl DEBUGBUILD understands

DESCRIPTION

This is a set of variables only recognized and used if libcurl was built "debug enabled", which should never be true for a library used in production. These variables are intended for internal use only, subject to change and have many effects on the behavior of libcurl. Refer to the source code to determine how exactly they are being used.

CURL_ALTSVC_HTTP

Bypass the AltSvc HTTPS protocol restriction if this variable exists.

CURL_DBG_SOCK_RBLOCK

The percentage of recv() calls that should be answered with a EAGAIN at random. For TCP/UNIX sockets.

CURL_DBG_SOCK_RMAX

The maximum data that shall be received from the network in one recv() call. For TCP/UNIX sockets. This is applied to every recv.

Example: CURL_DBG_SOCK_RMAX=400 means recv buffer size is limited to a maximum of 400 bytes.

CURL_DBG_SOCK_WBLOCK

The percentage of send() calls that should be answered with a EAGAIN at random. For TCP/UNIX sockets.

CURL_DBG_SOCK_WPARTIAL

The percentage of data that shall be written to the network. For TCP/UNIX sockets. This is applied to every send.

Example: CURL_DBG_SOCK_WPARTIAL=80 means a send with 1000 bytes would only send 800.

CURL_DBG_QUIC_WBLOCK

The percentage of send() calls that should be answered with EAGAIN at random. QUIC only.

CURL_DEBUG

Trace logging behavior as an alternative to calling curl_global_trace(3).

Example: CURL_DEBUG=http/2 means trace details about HTTP/2 handling.

CURL_DEBUG_SIZE

Fake the size returned by CURLINFO_HEADER_SIZE and CURLINFO_REQUEST_SIZE.

CURL_GETHOSTNAME

Fake the local machine's unqualified hostname for NTLM and SMTP.

CURL_HSTS_HTTP

Bypass the HSTS HTTPS protocol restriction if this variable exists.

CURL_FORCETIME

A time of 0 is used for AWS signatures and NTLM if this variable exists.

CURL_ENTROPY

A fixed faked value to use instead of a proper random number so that functions in libcurl that are otherwise getting random outputs can be tested for what they generate.

CURL_SMALLREQSEND

An alternative size of HTTP data to be sent at a time only if smaller than the current.

CURL_SMALLSENDS

An alternative size of socket data to be sent at a time only if smaller than the current.

CURL_TIME

Fake unix timestamp to use for AltSvc, HSTS and CURLINFO variables that are time related.

This variable can also be used to fake the data returned by some CURLINFO variables that are not time-related (such as CURLINFO_LOCAL_PORT), and in that case the value is not a timestamp.

CURL_TRACE

LDAP tracing is enabled if this variable exists and its value is 1 or greater.

OpenLDAP tracing is separate. Refer to CURL_OPENLDAP_TRACE.

CURL_NTLM_WB_FILE

Debug-version of the ntlm-wb executable.

CURL_OPENLDAP_TRACE

OpenLDAP tracing is enabled if this variable exists and its value is 1 or greater. There is a number of debug levels, refer to openldap.c comments.

CURL_WS_CHUNK_SIZE

Used to influence the buffer chunk size used for WebSocket encoding and decoding.

CURL_FORBID_REUSE

Used to set the CURLOPT_FORBID_REUSE flag on each transfer initiated by the curl command line tool. The value of the environment variable does not matter.

CURL_GRACEFUL_SHUTDOWN

Make a blocking, graceful shutdown of all remaining connections when a multi handle is destroyed. This implicitly triggers for easy handles that are run via easy_perform. The value of the environment variable gives the shutdown timeout in milliseconds.

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