From 387657e7b69396503ab4fc59fbc6cc444e947989 Mon Sep 17 00:00:00 2001
From: Bogdan Kanivets <bkanivets@apple.com>
Date: Thu, 1 Jun 2023 10:53:41 -0700
Subject: [PATCH] server: add fields to migrate from 3.5 to 3.4

Signed-off-by: Bogdan Kanivets <bkanivets@apple.com>
---
 etcdutl/etcdutl/migrate_command.go   |  4 ++--
 server/storage/schema/schema.go      | 11 +++++++++++
 server/storage/schema/schema_test.go | 10 ++++------
 tests/e2e/utl_migrate_test.go        | 23 +++++++++++++++++++++++
 4 files changed, 40 insertions(+), 8 deletions(-)

diff --git a/etcdutl/etcdutl/migrate_command.go b/etcdutl/etcdutl/migrate_command.go
index 521cf8ba80c8..4fc7f4644bfb 100644
--- a/etcdutl/etcdutl/migrate_command.go
+++ b/etcdutl/etcdutl/migrate_command.go
@@ -87,8 +87,8 @@ func (o *migrateOptions) Config() (*migrateConfig, error) {
 	if err != nil {
 		return nil, fmt.Errorf("failed to parse target version: %v", err)
 	}
-	if c.targetVersion.LessThan(version.V3_5) {
-		return nil, fmt.Errorf(`target version %q not supported. Minimal "3.5"`, storageVersionToString(c.targetVersion))
+	if c.targetVersion.LessThan(version.V3_4) {
+		return nil, fmt.Errorf(`target version %q not supported. Minimal "3.4"`, storageVersionToString(c.targetVersion))
 	}
 
 	dbPath := datadir.ToBackendFileName(o.dataDir)
diff --git a/server/storage/schema/schema.go b/server/storage/schema/schema.go
index e2e7fe7e5667..e9f661146013 100644
--- a/server/storage/schema/schema.go
+++ b/server/storage/schema/schema.go
@@ -131,8 +131,19 @@ var (
 		version.V3_6: {
 			addNewField(Meta, MetaStorageVersionName, emptyStorageVersion),
 		},
+		version.V3_5: {
+			// 'migrate' command from v3.4 to v3.5 isn't supported and will fail with `missing confstate information`
+			// these fields are added for v3.5 -> v3.4 downgrade only
+			//
+			// note about emptyValue:
+			// even if these fields are used for upgrade, it's safe to set emptyValue
+			// UnsafeReadConsistentIndex and UnsafeConfStateFromBackend checks for zero-length values
+			addNewField(Meta, MetaTermKeyName, emptyValue),
+			addNewField(Meta, MetaConfStateName, emptyValue),
+		},
 	}
 	// emptyStorageVersion is used for v3.6 Step for the first time, in all other version StoragetVersion should be set by migrator.
 	// Adding a addNewField for StorageVersion we can reuse logic to remove it when downgrading to v3.5
 	emptyStorageVersion = []byte("")
+	emptyValue          = []byte("")
 )
diff --git a/server/storage/schema/schema_test.go b/server/storage/schema/schema_test.go
index 6b0024ba7476..4fba3960e3b6 100644
--- a/server/storage/schema/schema_test.go
+++ b/server/storage/schema/schema_test.go
@@ -192,12 +192,10 @@ func TestMigrate(t *testing.T) {
 			expectErrorMsg: "cannot downgrade storage, WAL contains newer entries",
 		},
 		{
-			name:           "Downgrading v3.5 to v3.4 is not supported as schema was introduced in v3.6",
-			version:        version.V3_5,
-			targetVersion:  version.V3_4,
-			expectVersion:  nil,
-			expectError:    true,
-			expectErrorMsg: `cannot create migration plan: version "3.5.0" is not supported`,
+			name:          "Downgrading v3.5 to v3.4 is supported",
+			version:       version.V3_5,
+			targetVersion: version.V3_4,
+			expectVersion: nil,
 		},
 	}
 	for _, tc := range tcs {
diff --git a/tests/e2e/utl_migrate_test.go b/tests/e2e/utl_migrate_test.go
index 1513d34b4e1b..df25f335f24f 100644
--- a/tests/e2e/utl_migrate_test.go
+++ b/tests/e2e/utl_migrate_test.go
@@ -34,6 +34,11 @@ import (
 	"go.etcd.io/etcd/tests/v3/framework/e2e"
 )
 
+type bucketKey struct {
+	bucket backend.Bucket
+	key    []byte
+}
+
 func TestEtctlutlMigrate(t *testing.T) {
 	lastReleaseBinary := e2e.BinPath.EtcdLastRelease
 
@@ -45,6 +50,8 @@ func TestEtctlutlMigrate(t *testing.T) {
 
 		expectLogsSubString  string
 		expectStorageVersion *semver.Version
+
+		expectNonFoundKeys []bucketKey
 	}{
 		{
 			name:                 "Invalid target version string",
@@ -70,6 +77,16 @@ func TestEtctlutlMigrate(t *testing.T) {
 			expectLogsSubString:  `Error: wrong target version format, expected "X.Y", got "3.6.0"`,
 			expectStorageVersion: &version.V3_6,
 		},
+		{
+			name:                "Downgrade v3.5 to v3.4 should work",
+			clusterVersion:      e2e.LastVersion,
+			targetVersion:       "3.4",
+			expectLogsSubString: "updated storage version\t" + `{"new-storage-version": "3.4.0"}`,
+			expectNonFoundKeys: []bucketKey{
+				{bucket: schema.Meta, key: schema.MetaTermKeyName},
+				{bucket: schema.Meta, key: schema.MetaConfStateName},
+			},
+		},
 		{
 			name:                "Migrate v3.5 to v3.5 is no-op",
 			clusterVersion:      e2e.LastVersion,
@@ -170,6 +187,12 @@ func TestEtctlutlMigrate(t *testing.T) {
 
 			ver := schema.ReadStorageVersion(be.ReadTx())
 			assert.Equal(t, tc.expectStorageVersion, ver)
+
+			for _, bk := range tc.expectNonFoundKeys {
+				_, vs := be.ReadTx().UnsafeRange(bk.bucket, bk.key, nil, 0)
+				assert.Zero(t, len(vs))
+			}
+
 		})
 	}
 }