Upgrader should reserve and release allocations if transient size is unknown #130
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| func (u *Upgrader) refetch(ctx context.Context, into *os.File) error { | ||
| log.Debugw("actually refetching", "shard", u.key, "path", into.Name()) | ||
| func (u *Upgrader) refetch(ctx context.Context, outpath string) error { | ||
| log.Debugw("actually refetching", "shard", u.key) |
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Any reason why not to preserve the path here?
| // Use this when automated GC is disabled. | ||
| type SimpleDownloader struct{} | ||
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| func (s *SimpleDownloader) Download(ctx context.Context, underlying Mount, outpath string) error { |
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Download as a name implies that this is going to be fetched from the network. I think CopyLocal or CloneLocal might work best here as a more generic term that is consistent with scenarios like copying from a USB key into the local filesystem.
| // Reserve attempts to reserve `toReserve` bytes for the transient and returns the number | ||
| // of bytes actually reserved or any error encountered when trying to make the reservation. | ||
| // The `nPrevReservations` param denotes the number of previous successful reservations | ||
| // the caller has made for an ongoing download. | ||
| // Note: reserved != 0 if and only if err == nil. | ||
| Reserve(ctx context.Context, k shard.Key, nPrevReservations int64, toReserve int64) (reserved int64, err error) |
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This interface forces the caller to keep track of state that it shouldn't, concretely nPrevReservations. In fact, this is a bit leaky because not all allocators may need this information, and other allocators may require tracking other state.
I recommend changing the mechanics of this interface to be mediated by a Reservation object:
type TransientAllocator interface {
ReservationFor(k shard.Key) Reservation
}
type Reservation interface {
/// Reserves the specified number of bytes, returning the total number of bytes reserved.
Reserve(ctx context.Context, required uint64) (current uint64, err error)
/// Done releases the reservation, reporting the final size used to the allocator.
/// Calling Reserve after calling Done will panic.
Done(ctx context.Context, final uint64) error
}| // TransientDownloader manages the process of downloading a transient locally from a Mount | ||
| // when we are upgrading a Mount that does NOT have random access capabilities. | ||
| type TransientDownloader interface { | ||
| // Download copies the transient from the given underlying Mount to the given output path. | ||
| Download(ctx context.Context, underlying Mount, outpath string) error | ||
| } |
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Does this need to be pluggable? What kinds of different downloader logic do you foresee here?
For #134 .
PR 1 for the Automated GC work.
Upgrader should reserve and release allocations if the transient size is unknown before downloading a file from a remote mount that does not support random access.