Previously serve s3 buffered every part of a multipart upload in memory
(in the gofakes3 S3 library) and concatenated them when the upload
completed, so memory use grew with the size of the upload.
serve s3 now streams the parts, in part-number order, into a single
PutStream upload to the underlying remote, which performs its own upload
with bounded memory. The whole file is never held in memory - memory use
is bounded by the parts in flight. This works for any remote that
supports PutStream (nearly all, including crypt) and for any part size,
so clients that don't produce uniform-sized parts (for example
PostgreSQL backup tools such as pgBarman and pgBackRest) work too.
Parts must arrive in ascending, contiguous part-number order; parts
uploaded out of order are buffered until their turn, and there is no
per-part retry (a failure aborts the whole upload). These trade-offs are
documented.
Passing --disable-multipart-streaming, or using a remote without
PutStream, reverts to buffering the parts in memory (the previous
behaviour); a one-off NOTICE is logged the first time this happens.
Fixes#7453
Nick Craig-Wood
3d246a2aea