[Libguestfs] [nbdkit PATCH 0/9] can_FOO caching, more filter validation

It's easy to use the sh script to demonstrate that nbdkit is inefficiently calling into .get_size, .can_fua, and friends more than necessary. We've also commented on the list in the past that it would be nice to ensure that when filters call into next_ops, they are not violating constraints (as we've have to fix several bugs in the past where we did not have such checking to protect ourselves). This series fixes both (well, we may still have buggy filters, but if so, now we'll get earlier assertion failures instead of random plugin behavior). Another benefit - this gives us a single point for adding any code for performing NBD_INFO_BLOCKSIZE support, where we'll have to hoist what the blocksize filter is currently doing into something that nbdkit does from any one backend calling into another with different sizing constraints. However, that is not done in this series. My pending work on fast zeroes was easy enough to rebase on top of this (in fact, I wrote this because I noticed that .can_zero was being called more than necessary in implementing .can_fast_zero). Eric Blake (9): server: Fewer dereferences in filter server: Consolidate common backend tasks into new backend.c server: Create new backend_* functions for central handling server: Rework storage of per-backend handle server: Cache per-connection size server: Cache per-connection can_FOO flags filters: Change semantics of .can_zero server: Move fallbacks from protocol.c to backend.c server: Move command validation from protocol.c to backend.c docs/nbdkit-filter.pod | 62 +++- docs/nbdkit-plugin.pod | 6 +- server/internal.h | 108 +++++- server/Makefile.am | 3 +- server/backend.c | 515 +++++++++++++++++++++++++++ server/connections.c | 51 +-- server/filters.c | 379 ++++++-------------- server/main.c | 2 +- server/plugins.c | 192 +++------- server/protocol-handshake-newstyle.c | 20 +- server/protocol-handshake-oldstyle.c | 3 +- server/protocol-handshake.c | 97 +++-- server/protocol.c | 201 +++-------- include/nbdkit-filter.h | 4 + filters/blocksize/blocksize.c | 2 - filters/cache/cache.c | 1 - filters/cow/cow.c | 16 +- filters/nozero/nozero.c | 9 +- tests/test-layers.c | 50 +-- 19 files changed, 963 insertions(+), 758 deletions(-) create mode 100644 server/backend.c -- 2.21.0