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connections: downstream: harden listener and architecture [Backport to 4.2] #12054

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cosmo0920 wants to merge 2 commits into from
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connections: downstream: harden listener and architecture [Backport to 4.2] #12054

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6c680d9
connection: add drop notification callback
edsiper Apr 23, 2026
00eadc5
downstream: notify before destroying connections
edsiper Apr 23, 2026
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14 changes: 14 additions & 0 deletions include/fluent-bit/flb_connection.h
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Expand Up @@ -58,12 +58,26 @@ struct flb_net_setup;
struct flb_upstream;
struct flb_downstream;
struct flb_tls_session;
struct flb_connection;

typedef void (*flb_connection_drop_notification_callback)(
struct flb_connection *connection);

/* Base network connection */
struct flb_connection {
struct mk_event event;

void *user_data;
/*
* Optional notification invoked from prepare_destroy_conn() while the
* connection is still linked on busy_queue and before the event is
* deregistered and the file descriptor is closed.
*
* Callers may detach external state here, but must not free, destroy or
* unlink the connection because prepare_destroy_conn() performs the final
* teardown immediately after the callback returns.
*/
flb_connection_drop_notification_callback drop_notification_callback;

/* Socket */
flb_sockfd_t fd;
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2 changes: 1 addition & 1 deletion src/flb_connection.c
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Expand Up @@ -254,4 +254,4 @@ void flb_connection_unset_io_timeout(struct flb_connection *connection)
assert(connection != NULL);

connection->ts_io_timeout = -1;
}
}
4 changes: 4 additions & 0 deletions src/flb_downstream.c
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Expand Up @@ -209,6 +209,10 @@ static int prepare_destroy_conn(struct flb_connection *connection)
flb_trace("[downstream] destroy connection #%i to %s",
connection->fd, flb_connection_get_remote_address(connection));

if (connection->drop_notification_callback != NULL) {
connection->drop_notification_callback(connection);
}
Comment on lines +212 to +214

@chatgpt-codex-connector chatgpt-codex-connector Bot Jul 6, 2026

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P2 Badge Wire the drop callback for downstream owners

This call does not currently notify any owner because the new field is never assigned anywhere in the tree (rg "drop_notification_callback\\s*=" finds no setters). In the HTTP server path, sessions still retain session->connection/connection->user_data when the connection is attached (src/http_server/flb_http_server.c:275-281), and later unconditionally release that pointer during session destroy (src/http_server/flb_http_server.c:512-513); if flb_downstream_conn_timeouts() drops the downstream connection first (src/flb_downstream.c:515-516), the session is left with a stale connection pointer. Please register the callback where downstream sessions are attached, or the added notification mechanism will not prevent the timeout/use-after-free case it is meant to handle.

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if (MK_EVENT_IS_REGISTERED((&connection->event))) {
mk_event_del(connection->evl, &connection->event);
}
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