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backend: factor out connection limiting and parameter validation

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The SemaphoreBackend now uniformly enforces the limit of concurrent
backend operations. In addition, it unifies the parameter validation.

The List() methods no longer uses a semaphore. Restic already never runs
multiple list operations in parallel.

By managing the semaphore in a wrapper backend, the sections that hold a
semaphore token grow slightly. However, the main bottleneck is IO, so
this shouldn't make much of a difference.

The key insight that enables the SemaphoreBackend is that all of the
complex semaphore handling in `openReader()` still happens within the
original call to `Load()`. Thus, getting and releasing the semaphore
tokens can be refactored to happen directly in `Load()`. This eliminates
the need for wrapping the reader in `openReader()` to release the token.
This commit is contained in:
Michael Eischer 2023-04-07 23:02:35 +02:00
parent 8b5ab5b59f
commit 8e1e3844aa
13 changed files with 126 additions and 384 deletions
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11
internal/backend/utils.go
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@ -6,7 +6,6 @@ import (
"fmt"
"io"
"github.com/cenkalti/backoff/v4"
"github.com/restic/restic/internal/debug"
"github.com/restic/restic/internal/errors"
"github.com/restic/restic/internal/restic"
@ -63,15 +62,7 @@ func LimitReadCloser(r io.ReadCloser, n int64) *LimitedReadCloser {
func DefaultLoad(ctx context.Context, h restic.Handle, length int, offset int64,
openReader func(ctx context.Context, h restic.Handle, length int, offset int64) (io.ReadCloser, error),
fn func(rd io.Reader) error) error {
if err := h.Valid(); err != nil {
return backoff.Permanent(err)
}
if offset < 0 {
return errors.New("offset is negative")
}
if length < 0 {
return errors.Errorf("invalid length %d", length)
}
rd, err := openReader(ctx, h, length, offset)
if err != nil {
return err