feat(rustfs): serve GET misses from the migration source
Wire the on-demand migration read-through into the GET path, after the
local read and the replication proxy have both missed (rustfs/backlog#2156).
A source HEAD supplies size, validators and metadata. Conditional headers
are evaluated locally against it and never forwarded, so a source 304/412
cannot be mistaken for a source failure. An object within inline_max_bytes
is teed: the primary streams to the client while the secondary commits the
local copy in a background task, so a client disconnect still stores the
whole object and a failed write-back never touches the client stream.
Range reads and larger objects stream straight through and queue a
background pull per policy. Concurrent misses of one key share the
singleflight slot: the leader tees, followers re-read local after it
commits or degrade to passthrough after first_byte_ms.
Version reads, partNumber reads, anti-loop marked requests and a respected
local delete marker keep their original 404. Source answers carry
x-rustfs-on-demand-migration: source; local hits are untouched, and the
local hit path gains no await or lock.
Zhengchao An
d04611ba38