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Ian Jackson
5f0c4a7810
Previously, an application which had multiple libxl ctxs in multiple threads, would have to itself plumb SIGCHLD through to each ctx. Instead, permit multiple libxl ctxs to all share the SIGCHLD handler. We keep a list of all the ctxs which are interested in SIGCHLD and notify all of their self-pipes. In more detail: * sigchld_owner, the ctx* of the SIGCHLD owner, is replaced by sigchld_users, a list of SIGCHLD users. * Each ctx keeps track of whether it is on the users list, so that libxl__sigchld_needed and libxl__sigchld_notneeded now instead of idempotently installing and removing the handler, idempotently add or remove the ctx from the list. We ensure that we always have the SIGCHLD handler installed iff the sigchld_users list is nonempty. To make this a bit easier we make sigchld_installhandler_core and sigchld_removehandler_core idempotent. Specifically, the call sites for sigchld_installhandler_core and sigchld_removehandler_core are updated to manipulate sigchld_users and only call the install or remove functions as applicable. * In the signal handler we walk the list of SIGCHLD users and write to each of their self-pipes. That means that we need to arrange to defer SIGCHLD when we are manipulating the list (to avoid the signal handler interrupting our list manipulation); this is quite tiresome to arrange. The code as written will, on the first installation of the SIGCHLD handler, firstly install the real handler, then immediately replace it with the deferral handler. Doing it this way makes the code clearer as it makes the SIGCHLD deferral machinery much more self-contained (and hence easier to reason about). * The first part of libxl__sigchld_notneeded is broken out into a new function sigchld_user_remove (which is also needed during for postfork). And of course that first part of the function is now rather different, as explained above. * sigchld_installhandler_core no longer takes the gc argument, because it now deals with SIGCHLD for all ctxs. Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com> Cc: Jim Fehlig <jfehlig@suse.com> Cc: Ian Campbell <Ian.Campbell@citrix.com> Acked-by: Ian Campbell <ian.campbell@citrix.com> --- v3: Include bugfixes from "Fixup SIGCHLD sharing" patch: * Use a mutex for defer_sigchld, to guard against concurrency between the thread calling defer_sigchld and an instance of the primary signal handler on another thread. * libxl_sigchld_owner_libxl_always is incompatible with SIGCHLD sharing. Document this correctly. Fix "have have" error in comment. Move removal of newly unused variables to previous patch. v2.1: Provide feature test macro LIBXL_HAVE_SIGCHLD_SHARING