db: add support for checkpointing with remote files

Previously, if we couldn't read a remote file's DiskFileNum locally,
we'd error out on the Checkpoint() function even though the store
was valid and the file just happened to be remote (shared or external).
This change adds support for saving the catalog's state to the checkpoint
directory so references to remote files from the checkpoint can still
be resolved. To prevent unreffing of shared files, we need to add a dangling
reference to them that doesn't get deleted.

Fixes cockroachdb#3362.

checkpoint.go

@@ -268,6 +268,7 @@ func (d *DB) Checkpoint(
 	}
 
 	var excludedFiles map[deletedFileEntry]*fileMetadata
+	var remoteFiles []base.DiskFileNum
 	// Set of FileBacking.DiskFileNum which will be required by virtual sstables
 	// in the checkpoint.
 	requiredVirtualBackingFiles := make(map[base.DiskFileNum]struct{})
@@ -293,6 +294,21 @@ func (d *DB) Checkpoint(
 				}
 				requiredVirtualBackingFiles[fileBacking.DiskFileNum] = struct{}{}
 			}
+			meta, err := d.objProvider.Lookup(fileTypeTable, fileBacking.DiskFileNum)
+			if err != nil {
+				ckErr = err
+				return ckErr
+			}
+			if meta.IsRemote() {
+				// We don't copy remote files. This is desirable as checkpointing is
+				// supposed to be a fast operation, and references to remote files can
+				// always be resolved by any checkpoint readers by reading the object
+				// catalog. We don't add this file to excludedFiles either, as that'd
+				// cause it to be deleted in the second manifest entry which is also
+				// inaccurate.
+				remoteFiles = append(remoteFiles, meta.DiskFileNum)
+				continue
+			}
 
 			srcPath := base.MakeFilepath(fs, d.dirname, fileTypeTable, fileBacking.DiskFileNum)
 			destPath := fs.PathJoin(destDir, fs.PathBase(srcPath))
@@ -319,6 +335,12 @@ func (d *DB) Checkpoint(
 	if ckErr != nil {
 		return ckErr
 	}
+	if len(remoteFiles) > 0 {
+		ckErr = d.objProvider.CheckpointState(fs, destDir, fileTypeTable, remoteFiles)
+		if ckErr != nil {
+			return ckErr
+		}
+	}
 
 	// Copy the WAL files. We copy rather than link because WAL file recycling
 	// will cause the WAL files to be reused which would invalidate the

checkpoint_test.go

@@ -16,11 +16,12 @@ import (
 
 	"github.com/cockroachdb/datadriven"
 	"github.com/cockroachdb/pebble/internal/base"
+	"github.com/cockroachdb/pebble/objstorage/remote"
 	"github.com/cockroachdb/pebble/vfs"
 	"github.com/stretchr/testify/require"
 )
 
-func TestCheckpoint(t *testing.T) {
+func testCheckpointImpl(t *testing.T, ddFile string, createOnShared bool) {
 	dbs := make(map[string]*DB)
 	defer func() {
 		for _, db := range dbs {
@@ -32,17 +33,24 @@ func TestCheckpoint(t *testing.T) {
 
 	mem := vfs.NewMem()
 	var memLog base.InMemLogger
+	remoteMem := remote.NewInMem()
 	opts := &Options{
 		FS:                          vfs.WithLogging(mem, memLog.Infof),
 		FormatMajorVersion:          internalFormatNewest,
 		L0CompactionThreshold:       10,
 		DisableAutomaticCompactions: true,
 		Logger:                      testLogger{t},
 	}
+	opts.Experimental.RemoteStorage = remote.MakeSimpleFactory(map[remote.Locator]remote.Storage{
+		"": remoteMem,
+	})
+	if createOnShared {
+		opts.Experimental.CreateOnShared = remote.CreateOnSharedAll
+	}
 	opts.DisableTableStats = true
 	opts.private.testingAlwaysWaitForCleanup = true
 
-	datadriven.RunTest(t, "testdata/checkpoint", func(t *testing.T, td *datadriven.TestData) string {
+	datadriven.RunTest(t, ddFile, func(t *testing.T, td *datadriven.TestData) string {
 		switch td.Cmd {
 		case "batch":
 			if len(td.CmdArgs) != 1 {
@@ -192,6 +200,12 @@ func TestCheckpoint(t *testing.T) {
 				return err.Error()
 			}
 			dbs[dir] = d
+			if len(dbs) == 1 && createOnShared {
+				// This is the first db. Set a creator ID.
+				if err := d.SetCreatorID(1); err != nil {
+					return err.Error()
+				}
+			}
 			return memLog.String()
 
 		case "scan":
@@ -216,6 +230,15 @@ func TestCheckpoint(t *testing.T) {
 	})
 }
 
+func TestCheckpoint(t *testing.T) {
+	t.Run("shared=false", func(t *testing.T) {
+		testCheckpointImpl(t, "testdata/checkpoint", false /* createOnShared */)
+	})
+	t.Run("shared=true", func(t *testing.T) {
+		testCheckpointImpl(t, "testdata/checkpoint_shared", true /* createOnShared */)
+	})
+}
+
 func TestCheckpointCompaction(t *testing.T) {
 	fs := vfs.NewMem()
 	d, err := Open("", &Options{FS: fs, Logger: testLogger{t: t}})

objstorage/objstorage.go

@@ -298,6 +298,11 @@ type Provider interface {
 	// directory does not exist.
 	IsNotExistError(err error) bool
 
+	// CheckpointState saves any saved state on local disk to the specified
+	// directory on the specified VFS. A new Pebble instance instantiated at that
+	// path should be able to resolve references to the specified files.
+	CheckpointState(fs vfs.FS, dir string, fileType base.FileType, fileNums []base.DiskFileNum) error
+
 	// Metrics returns metrics about objstorage. Currently, it only returns metrics
 	// about the shared cache.
 	Metrics() sharedcache.Metrics

objstorage/objstorageprovider/provider.go

@@ -455,6 +455,30 @@ func (p *provider) Metrics() sharedcache.Metrics {
 	return sharedcache.Metrics{}
 }
 
+// CheckpointState is part of the objstorage.Provider interface.
+func (p *provider) CheckpointState(
+	fs vfs.FS, dir string, fileType base.FileType, fileNums []base.DiskFileNum,
+) error {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	for i := range fileNums {
+		if _, ok := p.mu.knownObjects[fileNums[i]]; !ok {
+			return errors.Wrapf(
+				os.ErrNotExist,
+				"file %s (type %d) unknown to the objstorage provider",
+				fileNums[i], errors.Safe(fileType),
+			)
+		}
+		// Prevent this object from deletion, at least for the life of this instance.
+		p.mu.protectedObjects[fileNums[i]] = p.mu.protectedObjects[fileNums[i]] + 1
+	}
+
+	if p.remote.catalog != nil {
+		return p.remote.catalog.Checkpoint(fs, dir)
+	}
+	return nil
+}
+
 func (p *provider) addMetadata(meta objstorage.ObjectMetadata) {
 	p.mu.Lock()
 	defer p.mu.Unlock()

objstorage/objstorageprovider/remoteobjcat/catalog.go

@@ -8,6 +8,7 @@ import (
 	"cmp"
 	"fmt"
 	"io"
+	"path/filepath"
 	"slices"
 	"sync"
 
@@ -373,6 +374,28 @@ func (c *Catalog) createNewCatalogFileLocked() (outErr error) {
 	return nil
 }
 
+// Checkpoint copies catalog state to a file in the specified directory
+func (c *Catalog) Checkpoint(fs vfs.FS, dir string) error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	// NB: Every write to recWriter is flushed. We don't need to worry about
+	// this new file descriptor not getting all the saved catalog entries.
+	existingCatalogFilepath := filepath.Join(c.dirname, c.mu.catalogFilename)
+	destPath := filepath.Join(dir, c.mu.catalogFilename)
+	if err := vfs.CopyAcrossFS(c.fs, existingCatalogFilepath, fs, destPath); err != nil {
+		return err
+	}
+	catalogMarker, _, err := atomicfs.LocateMarker(fs, dir, catalogMarkerName)
+	if err != nil {
+		return err
+	}
+	if err := catalogMarker.Move(c.mu.catalogFilename); err != nil {
+		return err
+	}
+	return catalogMarker.Close()
+}
+
 func writeRecord(ve *VersionEdit, file vfs.File, recWriter *record.Writer) error {
 	w, err := recWriter.Next()
 	if err != nil {

testdata/checkpoint

@@ -17,6 +17,7 @@ close: db/marker.manifest.000001.MANIFEST-000001
 sync: db
 open-dir: db
 open-dir: db
+open-dir: db
 sync: db/MANIFEST-000001
 create: db/000002.log
 sync: db
@@ -281,6 +282,7 @@ open: checkpoints/checkpoint1/MANIFEST-000001
 close: checkpoints/checkpoint1/MANIFEST-000001
 open-dir: checkpoints/checkpoint1
 open-dir: checkpoints/checkpoint1
+open-dir: checkpoints/checkpoint1
 open: checkpoints/checkpoint1/OPTIONS-000003
 close: checkpoints/checkpoint1/OPTIONS-000003
 open: checkpoints/checkpoint1/000006.log
@@ -347,6 +349,7 @@ open: checkpoints/checkpoint2/MANIFEST-000001
 close: checkpoints/checkpoint2/MANIFEST-000001
 open-dir: checkpoints/checkpoint2
 open-dir: checkpoints/checkpoint2
+open-dir: checkpoints/checkpoint2
 open: checkpoints/checkpoint2/OPTIONS-000003
 close: checkpoints/checkpoint2/OPTIONS-000003
 open: checkpoints/checkpoint2/000006.log
@@ -388,6 +391,7 @@ open: checkpoints/checkpoint3/MANIFEST-000001
 close: checkpoints/checkpoint3/MANIFEST-000001
 open-dir: checkpoints/checkpoint3
 open-dir: checkpoints/checkpoint3
+open-dir: checkpoints/checkpoint3
 open: checkpoints/checkpoint3/OPTIONS-000003
 close: checkpoints/checkpoint3/OPTIONS-000003
 open: checkpoints/checkpoint3/000006.log
@@ -525,6 +529,7 @@ open: checkpoints/checkpoint4/MANIFEST-000001
 close: checkpoints/checkpoint4/MANIFEST-000001
 open-dir: checkpoints/checkpoint4
 open-dir: checkpoints/checkpoint4
+open-dir: checkpoints/checkpoint4
 open: checkpoints/checkpoint4/OPTIONS-000003
 close: checkpoints/checkpoint4/OPTIONS-000003
 open: checkpoints/checkpoint4/000008.log
@@ -635,6 +640,7 @@ open: checkpoints/checkpoint5/MANIFEST-000001
 close: checkpoints/checkpoint5/MANIFEST-000001
 open-dir: checkpoints/checkpoint5
 open-dir: checkpoints/checkpoint5
+open-dir: checkpoints/checkpoint5
 open: checkpoints/checkpoint5/OPTIONS-000003
 close: checkpoints/checkpoint5/OPTIONS-000003
 open: checkpoints/checkpoint5/000008.log
@@ -731,6 +737,7 @@ open: checkpoints/checkpoint6/MANIFEST-000001
 close: checkpoints/checkpoint6/MANIFEST-000001
 open-dir: checkpoints/checkpoint6
 open-dir: checkpoints/checkpoint6
+open-dir: checkpoints/checkpoint6
 open: checkpoints/checkpoint6/OPTIONS-000003
 close: checkpoints/checkpoint6/OPTIONS-000003
 open: checkpoints/checkpoint6/000008.log