[Bug debuginfod/27917] protect against federation loops

["mark at klomp dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27917

Mark Wielaard <mark at klomp dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
                 CC|                            |mark at klomp dot org
             Status|NEW                         |RESOLVED

--- Comment #3 from Mark Wielaard <mark at klomp dot org> ---
commit d3f914023abcd6ae76b168da97518e5e7dbd761a
Author: Di Chen <dichen@redhat.com>
Date:   Fri Aug 20 13:03:21 2021 +0800

    debuginfod: PR27917 - protect against federation loops

    If someone misconfigures a debuginfod federation to have loops, and
    a nonexistent buildid lookup is attempted, bad things will happen,
    as is documented.

    This patch aims to reduce the risk by adding an option to debuginfod
    that functions kind of like an IP packet's TTL: a limit on the length of
    XFF: header that debuginfod is willing to process. If X-Forwarded-For:
    exceeds N hops, it will not delegate a local lookup miss to upstream
    debuginfods.

    Commit ab38d167c40c99 causes federation loops for non-existent resources
    to result in multiple temporary deadlocks, each lasting for
    $DEBUGINFOD_TIMEOUT seconds. Since concurrent requests for each unique
    resource are now serialized, federation loops can result in one server
    thread waiting to acquire a lock while the server thread holding the
    lock waits for the first thread to respond to an http request.

    This PR can help protect against the above multiple temporary deadlocks
    behaviour. Ex. if --forwarded-ttl-limit=0 then the timeout behaviour of
    local loops should be avoided.

    https://sourceware.org/bugzilla/show_bug.cgi?id=27917

    Signed-off-by: Di Chen <dichen@redhat.com>

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