Fetch sums from S3 and compare

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## 8.16.4 (unreleased)
 ### Implementation changes and bug fixes
 - [PR #1584](https://github.com/rqlite/rqlite/pull/1584): Count Snapshot reaping failures.
+- [PR #1585](https://github.com/rqlite/rqlite/pull/1585): Avoid unnecessary auto-backups by storing sha256 sums in S3.
 
 ## 8.16.3 (January 9th 2024)
 ### Implementation changes and bug fixes

auto/backup/uploader.go

@@ -1,6 +1,7 @@
 package backup
 
 import (
+	"bytes"
 	"compress/gzip"
 	"context"
 	"expvar"
@@ -16,6 +17,9 @@ import (
 // StorageClient is an interface for uploading data to a storage service.
 type StorageClient interface {
 	Upload(ctx context.Context, reader io.Reader, sum []byte) error
+
+	LastSum(ctx context.Context) ([]byte, error)
+
 	fmt.Stringer
 }
 
@@ -73,6 +77,7 @@ type Uploader struct {
 	lastUploadTime     time.Time
 	lastUploadDuration time.Duration
 
+	lastSum      []byte    // The SHA256 sum of the data most-recently uploaded.
 	lastModified time.Time // The last-modified time of the data most-recently uploaded.
 }
 
@@ -125,6 +130,7 @@ func (u *Uploader) Stats() (map[string]interface{}, error) {
 		"compress":             u.compress,
 		"last_upload_time":     u.lastUploadTime.Format(time.RFC3339),
 		"last_upload_duration": u.lastUploadDuration.String(),
+		"last_sum":             fmt.Sprintf("%x", u.lastSum),
 		"last_modified":        u.lastModified.String(),
 	}
 	return status, nil
@@ -170,12 +176,23 @@ func (u *Uploader) upload(ctx context.Context) error {
 		return err
 	}
 
+	if u.lastSum == nil {
+		ls, err := u.storageClient.LastSum(ctx)
+		if err == nil {
+			u.lastSum = ls
+		}
+	} else if bytes.Equal(u.lastSum, sum) {
+		stats.Add(numUploadsSkipped, 1)
+		return nil
+	}
+
 	cr := progress.NewCountingReader(fd)
 	startTime := time.Now()
 	err = u.storageClient.Upload(ctx, cr, sum)
 	if err != nil {
 		stats.Add(numUploadsFail, 1)
 	} else {
+		u.lastSum = sum
 		u.lastModified = lm
 		stats.Add(numUploadsOK, 1)
 		stats.Add(totalUploadBytes, cr.Count())

auto/backup/uploader_test.go

@@ -287,7 +287,8 @@ func Test_UploaderStats(t *testing.T) {
 }
 
 type mockStorageClient struct {
-	uploadFn func(ctx context.Context, reader io.Reader, sum []byte) error
+	uploadFn  func(ctx context.Context, reader io.Reader, sum []byte) error
+	lastSumFn func(ctx context.Context) ([]byte, error)
 }
 
 func (mc *mockStorageClient) Upload(ctx context.Context, reader io.Reader, sum []byte) error {
@@ -297,6 +298,13 @@ func (mc *mockStorageClient) Upload(ctx context.Context, reader io.Reader, sum [
 	return nil
 }
 
+func (mc *mockStorageClient) LastSum(ctx context.Context) ([]byte, error) {
+	if mc.lastSumFn != nil {
+		return mc.lastSumFn(ctx)
+	}
+	return nil, nil
+}
+
 func (mc *mockStorageClient) String() string {
 	return "mockStorageClient"
 }

aws/s3.go

@@ -2,6 +2,7 @@ package aws
 
 import (
 	"context"
+	"encoding/hex"
 	"fmt"
 	"io"
 	"strings"
@@ -13,6 +14,10 @@ import (
 	"github.com/aws/aws-sdk-go/service/s3/s3manager"
 )
 
+const (
+	AWSS3SumKey = "x-rqlite-sum"
+)
+
 // S3Config is the subconfig for the S3 storage type
 type S3Config struct {
 	Endpoint        string `json:"endpoint,omitempty"`
@@ -89,7 +94,7 @@ func (s *S3Client) Upload(ctx context.Context, reader io.Reader, sum []byte) err
 
 	if sum != nil {
 		input.Metadata = map[string]*string{
-			"x-rqlite-sum": aws.String(fmt.Sprintf("%x", sum)),
+			AWSS3SumKey: aws.String(fmt.Sprintf("%x", sum)),
 		}
 	}
 	_, err = uploader.UploadWithContext(ctx, input)
@@ -100,6 +105,30 @@ func (s *S3Client) Upload(ctx context.Context, reader io.Reader, sum []byte) err
 	return nil
 }
 
+func (s *S3Client) LastSum(ctx context.Context) ([]byte, error) {
+	sess, err := s.createSession()
+	if err != nil {
+		return nil, err
+	}
+
+	svc := s3.New(sess)
+	input := &s3.HeadObjectInput{
+		Bucket: aws.String(s.bucket),
+		Key:    aws.String(s.key),
+	}
+
+	result, err := svc.HeadObjectWithContext(ctx, input)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get object head for %v: %w", s, err)
+	}
+
+	sumHex, ok := result.Metadata[AWSS3SumKey]
+	if !ok {
+		return nil, fmt.Errorf("sum metadata not found for %v", s)
+	}
+	return hex.DecodeString(*sumHex)
+}
+
 // Download downloads data from S3.
 func (s *S3Client) Download(ctx context.Context, writer io.WriterAt) error {
 	sess, err := s.createSession()