taildrop: add logic for resuming partial files (#9785)

We add the following API:
* type FileChecksums
* type Checksum
* func Manager.PartialFiles
* func Manager.HashPartialFile
* func ResumeReader

The Manager methods provide the ability to query for partial files
and retrieve a list of checksums for a given partial file.
The ResumeReader function is a helper that wraps an io.Reader
to discard content that is identical locally and remotely.
The FileChecksums type represents the checksums of a file
and is safe to JSON marshal and send over the wire.

Updates tailscale/corp#14772

Signed-off-by: Joe Tsai <joetsai@digital-static.net>
Co-authored-by: Rhea Ghosh <rhea@tailscale.com>

taildrop/resume_test.go

@@ -0,0 +1,63 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package taildrop
+
+import (
+	"bytes"
+	"io"
+	"math/rand"
+	"os"
+	"testing"
+	"testing/iotest"
+
+	"tailscale.com/util/must"
+)
+
+func TestResume(t *testing.T) {
+	oldBlockSize := blockSize
+	defer func() { blockSize = oldBlockSize }()
+	blockSize = 256
+
+	m := Manager{Logf: t.Logf, Dir: t.TempDir()}
+
+	rn := rand.New(rand.NewSource(0))
+	want := make([]byte, 12345)
+	must.Get(io.ReadFull(rn, want))
+
+	t.Run("resume-noop", func(t *testing.T) {
+		r := io.Reader(bytes.NewReader(want))
+		offset, r, err := ResumeReader(r, func(offset, length int64) (FileChecksums, error) {
+			return m.HashPartialFile("", "foo", offset, length)
+		})
+		must.Do(err)
+		must.Get(m.PutFile("", "foo", r, offset, -1))
+		got := must.Get(os.ReadFile(must.Get(m.joinDir("foo"))))
+		if !bytes.Equal(got, want) {
+			t.Errorf("content mismatches")
+		}
+	})
+
+	t.Run("resume-retry", func(t *testing.T) {
+		rn := rand.New(rand.NewSource(0))
+		for {
+			r := io.Reader(bytes.NewReader(want))
+			offset, r, err := ResumeReader(r, func(offset, length int64) (FileChecksums, error) {
+				return m.HashPartialFile("", "foo", offset, length)
+			})
+			must.Do(err)
+			numWant := rn.Int63n(min(int64(len(want))-offset, 1000) + 1)
+			if offset < int64(len(want)) {
+				r = io.MultiReader(io.LimitReader(r, numWant), iotest.ErrReader(io.ErrClosedPipe))
+			}
+			if _, err := m.PutFile("", "foo", r, offset, -1); err == nil {
+				break
+			}
+		}
+		got := must.Get(os.ReadFile(must.Get(m.joinDir("foo"))))
+		if !bytes.Equal(got, want) {
+			t.Errorf("content mismatches")
+		}
+	})
+
+}