fix handling of tenative cursor presence if protection strategy doesn't use it (#714)

Before this PR, we would panic in the `check` phase of `endpoint.Send()`'s `TryBatchDestroy` call in the following cases: the current protection strategy does NOT produce a tentative replication cursor AND
  * `FromVersion` is a tentative cursor bookmark
  * `FromVersion` is a snapshot, and there exists a tentative cursor bookmark for that snapshot
  * `FromVersion` is a bookmark != tentative cursor bookmark, but there exists a tentative cursor bookmark for the same snapshot as the `FromVersion` bookmark

In those cases, the `check` concluded that we would delete `FromVersion`.
It came to that conclusion because the tentative cursor isn't part of `obsoleteAbs` if the protection strategy doesn't produce a tentative replication cursor.

The scenarios above can happen if the user changes the protection strategy from "with tentative cursor" to one "without tentative replication cursor", while there is a tentative replication cursor on disk.
The workaround was to rename the tentative cursor.

In all cases above, `TryBatchDestroy` would have destroyed the tentative cursor.

In case 1, that would fail the `Send` step and potentially break replication if the cursor is the last common bookmark. The `check` conclusion was correct.

In cases 2 and 3, deleting the tentative cursor would have been fine because `FromVersion` was a different entity than the tentative cursor. So, destroying the tentative cursor would be the right call.

The solution in this PR is as follows:
* add the `FromVersion` to the `liveAbs` set of live abstractions
* rewrite the `check` closure to use the full dataset path (`fullpath`) to identify the concrete ZFS object instead of the `zfs.FilesystemVersionEqualIdentity`, which is only identified by matching GUID.
  * Holds have no dataset path and are not the `FromVersion` in any case, so disregard them.

fixes #666

endpoint/endpoint.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"path"
+	"strings"
 
 	"github.com/kr/pretty"
 	"github.com/pkg/errors"
@@ -233,6 +234,21 @@ func (s *Sender) Send(ctx context.Context, r *pdu.SendReq) (*pdu.SendRes, io.Rea
 	//
 	// Note further that a resuming send, due to the idempotent nature of func CreateReplicationCursor and HoldStep,
 	// will never lose its step holds because we just (idempotently re-)created them above, before attempting the cleanup.
+	destroyTypes := AbstractionTypeSet{
+		AbstractionStepHold:                           true,
+		AbstractionTentativeReplicationCursorBookmark: true,
+	}
+	// The replication planner can also pick an endpoint zfs abstraction as FromVersion.
+	// Keep it, so that the replication will succeed.
+	//
+	// NB: there is no abstraction for snapshots, so, we only need to check bookmarks.
+	if sendArgs.FromVersion != nil && sendArgs.FromVersion.IsBookmark() {
+		dp, err := zfs.NewDatasetPath(sendArgs.FS)
+		if err != nil {
+			panic(err) // sendArgs is validated, this shouldn't happen
+		}
+		liveAbs = append(liveAbs, destroyTypes.ExtractBookmark(dp, sendArgs.FromVersion))
+	}
 	func() {
 		ctx, endSpan := trace.WithSpan(ctx, "cleanup-stale-abstractions")
 		defer endSpan()
@@ -245,35 +261,45 @@ func (s *Sender) Send(ctx context.Context, r *pdu.SendReq) (*pdu.SendRes, io.Rea
 			return keep
 		}
 		check := func(obsoleteAbs []Abstraction) {
-			// last line of defense: check that we don't destroy the incremental `from` and `to`
-			// if we did that, we might be about to blow away the last common filesystem version between sender and receiver
-			mustLiveVersions := []zfs.FilesystemVersion{sendArgs.ToVersion}
+			// Ensure that we don't delete `From` or `To`.
+			// Regardless of whether they are in AbstractionTypeSet or not.
+			// And produce a nice error message in case we do, to aid debugging the resulting panic.
+			//
+			// This is especially important for `From`. We could break incremental replication
+			// if we deleted the last common filesystem version between sender and receiver.
+			type Problem struct {
+				sendArgsWhat string
+				fullpath     string
+				obsoleteAbs  Abstraction
+			}
+			problems := make([]Problem, 0)
+			checkFullpaths := make(map[string]string, 2)
+			checkFullpaths["ToVersion"] = sendArgs.ToVersion.FullPath(sendArgs.FS)
 			if sendArgs.FromVersion != nil {
-				mustLiveVersions = append(mustLiveVersions, *sendArgs.FromVersion)
+				checkFullpaths["FromVersion"] = sendArgs.FromVersion.FullPath(sendArgs.FS)
 			}
-			for _, staleVersion := range obsoleteAbs {
-				for _, mustLiveVersion := range mustLiveVersions {
-					isSendArg := zfs.FilesystemVersionEqualIdentity(mustLiveVersion, staleVersion.GetFilesystemVersion())
-					stepHoldBasedGuaranteeStrategy := false
-					k := replicationGuaranteeStrategy.Kind()
-					switch k {
-					case ReplicationGuaranteeKindResumability:
-						stepHoldBasedGuaranteeStrategy = true
-					case ReplicationGuaranteeKindIncremental:
-					case ReplicationGuaranteeKindNone:
-					default:
-						panic(fmt.Sprintf("this is supposed to be an exhaustive match, got %v", k))
-					}
-					isSnapshot := mustLiveVersion.IsSnapshot()
-					if isSendArg && (!isSnapshot || stepHoldBasedGuaranteeStrategy) {
-						panic(fmt.Sprintf("impl error: %q would be destroyed because it is considered stale but it is part of of sendArgs=%s", mustLiveVersion.String(), pretty.Sprint(sendArgs)))
+			for _, a := range obsoleteAbs {
+				for what, fullpath := range checkFullpaths {
+					if a.GetFullPath() == fullpath && a.GetType().IsSnapshotOrBookmark() {
+						problems = append(problems, Problem{
+							sendArgsWhat: what,
+							fullpath:     fullpath,
+							obsoleteAbs:  a,
+						})
 					}
 				}
 			}
+			if len(problems) == 0 {
+				return
 			}
-		destroyTypes := AbstractionTypeSet{
-			AbstractionStepHold:                           true,
-			AbstractionTentativeReplicationCursorBookmark: true,
+			var msg strings.Builder
+			fmt.Fprintf(&msg, "cleaning up send stale would destroy send args:\n")
+			fmt.Fprintf(&msg, "  SendArgs: %s\n", pretty.Sprint(sendArgs))
+			for _, check := range problems {
+				fmt.Fprintf(&msg, "would delete %s %s because it was deemed an obsolete abstraction: %s\n",
+					check.sendArgsWhat, check.fullpath, check.obsoleteAbs)
+			}
+			panic(msg.String())
 		}
 		abstractionsCacheSingleton.TryBatchDestroy(ctx, s.jobId, sendArgs.FS, destroyTypes, keep, check)
 	}()

endpoint/endpoint_guarantees.go

@@ -89,6 +89,8 @@ func ReplicationGuaranteeFromKind(k ReplicationGuaranteeKind) ReplicationGuarant
 
 type ReplicationGuaranteeNone struct{}
 
+func (g ReplicationGuaranteeNone) String() string { return "none" }
+
 func (g ReplicationGuaranteeNone) Kind() ReplicationGuaranteeKind {
 	return ReplicationGuaranteeKindNone
 }
@@ -107,6 +109,8 @@ func (g ReplicationGuaranteeNone) SenderPostRecvConfirmed(ctx context.Context, j
 
 type ReplicationGuaranteeIncremental struct{}
 
+func (g ReplicationGuaranteeIncremental) String() string { return "incremental" }
+
 func (g ReplicationGuaranteeIncremental) Kind() ReplicationGuaranteeKind {
 	return ReplicationGuaranteeKindIncremental
 }
@@ -144,6 +148,8 @@ func (g ReplicationGuaranteeIncremental) SenderPostRecvConfirmed(ctx context.Con
 
 type ReplicationGuaranteeResumability struct{}
 
+func (g ReplicationGuaranteeResumability) String() string { return "resumability" }
+
 func (g ReplicationGuaranteeResumability) Kind() ReplicationGuaranteeKind {
 	return ReplicationGuaranteeKindResumability
 }

endpoint/endpoint_zfs_abstraction.go

@@ -31,7 +31,7 @@ const (
 	AbstractionReplicationCursorBookmarkV2        AbstractionType = "replication-cursor-bookmark-v2"
 )
 
-var AbstractionTypesAll = map[AbstractionType]bool{
+var AbstractionTypesAll = AbstractionTypeSet{
 	AbstractionStepHold:                           true,
 	AbstractionLastReceivedHold:                   true,
 	AbstractionTentativeReplicationCursorBookmark: true,
@@ -181,6 +181,38 @@ func (s AbstractionTypeSet) Validate() error {
 	return nil
 }
 
+// Use the `BookmarkExtractor()` method of each abstraction type in this set
+// to try extract an abstraction from the given FilesystemVersion.
+//
+// Abstraction types in this set that don't have a bookmark extractor are skipped.
+//
+// Panics if more than one abstraction type matches.
+func (s AbstractionTypeSet) ExtractBookmark(dp *zfs.DatasetPath, v *zfs.FilesystemVersion) Abstraction {
+	matched := make(AbstractionTypeSet, 1)
+	var matchedAbs Abstraction
+	for absType := range s {
+		extractor := absType.BookmarkExtractor()
+		if extractor == nil {
+			continue
+		}
+		abstraction := extractor(dp, *v)
+		if abstraction != nil {
+			matched[absType] = true
+			matchedAbs = abstraction
+		}
+	}
+	if len(matched) == 0 {
+		return nil
+	}
+	if len(matched) == 1 {
+		if matchedAbs == nil {
+			panic("loop above should always set matchedAbs if there is a match")
+		}
+		return matchedAbs
+	}
+	panic(fmt.Sprintf("abstraction types extractors should not overlap: %s", matched))
+}
+
 type BookmarkExtractor func(fs *zfs.DatasetPath, v zfs.FilesystemVersion) Abstraction
 
 // returns nil if the abstraction type is not bookmark-based
@@ -238,6 +270,23 @@ func (t AbstractionType) BookmarkNamer() func(fs string, guid uint64, jobId JobI
 	}
 }
 
+func (t AbstractionType) IsSnapshotOrBookmark() bool {
+	switch t {
+	case AbstractionTentativeReplicationCursorBookmark:
+		return true
+	case AbstractionReplicationCursorBookmarkV1:
+		return true
+	case AbstractionReplicationCursorBookmarkV2:
+		return true
+	case AbstractionStepHold:
+		return false
+	case AbstractionLastReceivedHold:
+		return false
+	default:
+		panic(fmt.Sprintf("unimpl: %q", t))
+	}
+}
+
 type ListZFSHoldsAndBookmarksQuery struct {
 	FS ListZFSHoldsAndBookmarksQueryFilesystemFilter
 	// What abstraction types should match (any contained in the set)

platformtest/tests/generated_cases.go

@@ -20,6 +20,7 @@ var Cases = []Case{BatchDestroy,
 	ReplicationIncrementalCleansUpStaleAbstractionsWithCacheOnSecondReplication,
 	ReplicationIncrementalCleansUpStaleAbstractionsWithoutCacheOnSecondReplication,
 	ReplicationIncrementalDestroysStepHoldsIffIncrementalStepHoldsAreDisabledButStepHoldsExist,
+	ReplicationIncrementalHandlesFromVersionEqTentativeCursorCorrectly,
 	ReplicationIncrementalIsPossibleIfCommonSnapshotIsDestroyed,
 	ReplicationInitialAll,
 	ReplicationInitialFail,

platformtest/tests/replication.go

@@ -248,7 +248,10 @@ func implReplicationIncrementalCleansUpStaleAbstractions(ctx *platformtest.Conte
 		require.NoError(ctx, err)
 		snap2Hold, err := endpoint.HoldStep(ctx, sfs, snap2, jobId) // no shadow
 		require.NoError(ctx, err)
-		return []endpoint.Abstraction{snap2Cursor, snap1Hold, snap2Hold}
+		// create artificial tentative cursor
+		snap3TentativeCursor, err := endpoint.CreateTentativeReplicationCursor(ctx, sfs, snap3, jobId)
+		require.NoError(ctx, err)
+		return []endpoint.Abstraction{snap2Cursor, snap1Hold, snap2Hold, snap3TentativeCursor}
 	}
 	createArtificalStaleAbstractions(sjid)
 	ojidSendAbstractions := createArtificalStaleAbstractions(ojid)
@@ -333,21 +336,29 @@ func implReplicationIncrementalCleansUpStaleAbstractions(ctx *platformtest.Conte
 		require.NoError(ctx, err)
 		snap2OjidCursorName, err := endpoint.ReplicationCursorBookmarkName(sfs, snap2.Guid, ojid)
 		require.NoError(ctx, err)
+		snap3SjidTentativeCursorName, err := endpoint.TentativeReplicationCursorBookmarkName(sfs, snap3.Guid, sjid)
+		require.NoError(ctx, err)
+		snap3OjidTentativeCursorName, err := endpoint.TentativeReplicationCursorBookmarkName(sfs, snap3.Guid, ojid)
+		require.NoError(ctx, err)
 		var bmNames []string
 		for _, bm := range sBms {
 			bmNames = append(bmNames, bm.Name)
 		}
 
 		if invalidateCacheBeforeSecondReplication {
-			require.Len(ctx, sBms, 3)
+			require.Len(ctx, sBms, 4)
 			require.Contains(ctx, bmNames, snap5SjidCursorName)
 			require.Contains(ctx, bmNames, snap2OjidCursorName)
+			require.Contains(ctx, bmNames, snap3OjidTentativeCursorName)
 			require.Contains(ctx, bmNames, "2")
 		} else {
-			require.Len(ctx, sBms, 4)
+			require.Len(ctx, sBms, 6)
+			ctx.Logf("%s", pretty.Sprint(sBms))
 			require.Contains(ctx, bmNames, snap5SjidCursorName)
 			require.Contains(ctx, bmNames, snap2SjidCursorName)
 			require.Contains(ctx, bmNames, snap2OjidCursorName)
+			require.Contains(ctx, bmNames, snap3SjidTentativeCursorName)
+			require.Contains(ctx, bmNames, snap3OjidTentativeCursorName)
 			require.Contains(ctx, bmNames, "2")
 		}
 	}
@@ -370,6 +381,84 @@ func implReplicationIncrementalCleansUpStaleAbstractions(ctx *platformtest.Conte
 
 }
 
+func ReplicationIncrementalHandlesFromVersionEqTentativeCursorCorrectly(ctx *platformtest.Context) {
+
+	platformtest.Run(ctx, platformtest.PanicErr, ctx.RootDataset, `
+		CREATEROOT
+		+  "sender"
+		+  "sender@1"
+		+  "receiver"
+		R  zfs create -p "${ROOTDS}/receiver/${ROOTDS}"
+	`)
+
+	sjid := endpoint.MustMakeJobID("sender-job")
+	rjid := endpoint.MustMakeJobID("receiver-job")
+
+	sfs := ctx.RootDataset + "/sender"
+	rfsRoot := ctx.RootDataset + "/receiver"
+
+	rep := replicationInvocation{
+		sjid:    sjid,
+		rjid:    rjid,
+		sfs:     sfs,
+		rfsRoot: rfsRoot,
+		// It doesn't really matter what guarantee we use here, as the second replication will configure another.
+		// But, in the real world, the only way for a stale tentative cursor to appear is if the guarantee is set to
+		// incremental replication and we crash before converting the tentative cursor into a regular cursor.
+		guarantee: pdu.ReplicationConfigProtectionWithKind(pdu.ReplicationGuaranteeKind_GuaranteeIncrementalReplication),
+	}
+
+	// Do initial replication to set up the test.
+	rep1 := rep.Do(ctx)
+	ctx.Logf("\n%s", pretty.Sprint(rep1))
+	sfsDs := mustDatasetPath(sfs)
+	snap1_sender := mustGetFilesystemVersion(ctx, sfs+"@1")
+	snap1_replicationCursor_name, err := endpoint.ReplicationCursorBookmarkName(sfs, snap1_sender.Guid, sjid)
+	require.NoError(ctx, err)
+	snap1_replicationCursor := mustGetFilesystemVersion(ctx, sfs+"#"+snap1_replicationCursor_name)
+
+	// The second replication will be done with a guarantee kind that doesn't create tentative cursors by itself.
+	// So, it would generally be right to clean up any tentative cursors on sfs since they're stale abstractions.
+	// However, if the cursor is used as the `from` version in any send step, we must not destroy it, as that
+	// would break incremental replication.
+	// NB: we only need to test the first step as all subsequent steps will be snapshot->snapshot.
+	rep.guarantee = pdu.ReplicationConfigProtectionWithKind(pdu.ReplicationGuaranteeKind_GuaranteeNothing)
+	// create the artificial cursor
+	snap1_tentativeCursor, err := endpoint.CreateTentativeReplicationCursor(ctx, sfs, snap1_sender, sjid)
+	require.NoError(ctx, err)
+	endpoint.AbstractionsCacheInvalidate(sfs)
+	// remove other bookmarks of snap1, and snap1 itself, to force the replication planner to use the tentative cursor
+	err = zfs.ZFSDestroyFilesystemVersion(ctx, sfsDs, &snap1_sender)
+	require.NoError(ctx, err)
+	err = zfs.ZFSDestroyFilesystemVersion(ctx, sfsDs, &snap1_replicationCursor)
+	require.NoError(ctx, err)
+	versions, err := zfs.ZFSListFilesystemVersions(ctx, sfsDs, zfs.ListFilesystemVersionsOptions{})
+	require.NoError(ctx, err)
+	require.Len(ctx, versions, 1)
+	require.Equal(ctx, versions[0].Guid, snap1_tentativeCursor.GetFilesystemVersion().Guid)
+	// create another snapshot so that replication does one incremental step `tentative_cursor` -> `@2`
+	mustSnapshot(ctx, sfs+"@2")
+	mustGetFilesystemVersion(ctx, sfs+"@2")
+	// do the replication
+	rep2 := rep.Do(ctx)
+	ctx.Logf("\n%s", pretty.Sprint(rep2))
+
+	// Ensure that the tentative cursor was used.
+	require.Len(ctx, rep2.Attempts, 1)
+	require.Equal(ctx, rep2.Attempts[0].State, report.AttemptDone)
+	require.Len(ctx, rep2.Attempts[0].Filesystems, 1)
+	require.Nil(ctx, rep2.Attempts[0].Filesystems[0].Error())
+	require.Len(ctx, rep2.Attempts[0].Filesystems[0].Steps, 1)
+	require.EqualValues(ctx, rep2.Attempts[0].Filesystems[0].CurrentStep, 1)
+	require.Len(ctx, rep2.Attempts[0].Filesystems[0].Steps, 1)
+	require.Equal(ctx, rep2.Attempts[0].Filesystems[0].Steps[0].Info.From, snap1_tentativeCursor.GetFilesystemVersion().RelName())
+
+	// Ensure that the tentative cursor was destroyed as part of SendPost.
+	_, err = zfs.ZFSGetFilesystemVersion(ctx, snap1_replicationCursor.FullPath(sfs))
+	_, ok := err.(*zfs.DatasetDoesNotExist)
+	require.True(ctx, ok)
+}
+
 type PartialSender struct {
 	*endpoint.Sender
 	failAfterByteCount int64