Re: [Libguestfs] [PATCH nbdkit] blocksize: Export block size constraints

This filter is a little unusual because it allows clients to send a wider range of request sizes than the underlying plugin allows. Therefore we advertise the widest possible minimum and maximum block size to clients. We still need to pick a suitable preferred block size assuming the plugin itself doesn't advertise one. --- filters/blocksize/blocksize.c | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) diff --git a/filters/blocksize/blocksize.c b/filters/blocksize/blocksize.c index a6fa00cb..46e759b0 100644 --- a/filters/blocksize/blocksize.c +++ b/filters/blocksize/blocksize.c @@ -156,6 +156,29 @@ blocksize_get_size (nbdkit_next *next, return ROUND_DOWN (size, minblock); } +/* Block size constraints. + * + * Note that the purpose of this filter is to allow clients to send a + * wider range of request sizes than the underlying plugin permits, + * and therefore this callback advertises the full availability of the + * filter, likely widening the constraints from the plugin. + */ +static int +blocksize_block_size (nbdkit_next *next, void *handle, + uint32_t *minimum, uint32_t *preferred, uint32_t *maximum) +{ + if (next->block_size (next, minimum, preferred, maximum) == -1) + return -1; + + if (*preferred == 0) + *preferred = MAX (4096, minblock); + + *minimum = 1; + *maximum = 0xffffffff; + + return 0; +} + static int blocksize_pread (nbdkit_next *next, void *handle, void *b, uint32_t count, uint64_t offs, @@ -432,6 +455,7 @@ static struct nbdkit_filter filter = { .config_complete = blocksize_config_complete, .config_help = blocksize_config_help, .get_size = blocksize_get_size, + .block_size = blocksize_block_size, .pread = blocksize_pread, .pwrite = blocksize_pwrite, .trim = blocksize_trim,