roachtest: allow for missing system table column during MVT fingerprinting

Previously, the MVT backup framework could not unconditionally elide
fingerprinting a column introduced in a migration, even if the test writer
wrapped the column in the `WithSentinel` custom fingerprint handler.
`WithSentinel` used to assume that the column will exist in the backed up
cluster and the restored cluster. This patch relaxes that assumption.

Fixes #147562

Release note: none

Author: msbutler

pkg/cmd/roachtest/tests/mixed_version_backup.go

@@ -693,7 +693,9 @@ func (sr *systemTableRow) skip() bool {
 // callback will be passed the value associated with that column, if
 // found, and its return value overwrites the value for that
 // column. The callback is not called if the column name is invalid.
-func (sr *systemTableRow) processColumn(column string, fn func(interface{}) interface{}) {
+func (sr *systemTableRow) processColumn(
+	column string, skipMissing bool, fn func(interface{}) interface{},
+) {
 	if sr.skip() {
 		return
 	}
@@ -702,6 +704,9 @@ func (sr *systemTableRow) processColumn(column string, fn func(interface{}) inte
 	hasCol := colIdx < len(sr.columns) && sr.columns[colIdx] == column
 
 	if !hasCol {
+		if skipMissing {
+			return
+		}
 		sr.err = fmt.Errorf("could not find column %s on %s", column, sr.table)
 		return
 	}
@@ -712,19 +717,19 @@ func (sr *systemTableRow) processColumn(column string, fn func(interface{}) inte
 // Matches allows the caller to only apply certain changes if the
 // value of a certain column matches a given value.
 func (sr *systemTableRow) Matches(column string, value interface{}) *systemTableRow {
-	sr.processColumn(column, func(actualValue interface{}) interface{} {
+	sr.processColumn(column, false /*skipMissing */, func(actualValue interface{}) interface{} {
 		sr.matches = reflect.DeepEqual(actualValue, value)
 		return actualValue
 	})
 
 	return sr
 }
 
-// WithSentinel replaces the contents of the given columns with a
-// fixed sentinel value.
-func (sr *systemTableRow) WithSentinel(columns ...string) *systemTableRow {
+// WithSentinelIfExists replaces the contents of the given columns with a
+// fixed sentinel value, if the column exists.
+func (sr *systemTableRow) WithSentinelIfExists(columns ...string) *systemTableRow {
 	for _, column := range columns {
-		sr.processColumn(column, func(value interface{}) interface{} {
+		sr.processColumn(column, true /* skipMissing */, func(value interface{}) interface{} {
 			return systemTableSentinel
 		})
 	}
@@ -819,7 +824,7 @@ func (sc *systemTableContents) settingsHandler(
 		// `name` column equals 'version'
 		Matches("name", "version").
 		// use the sentinel value for every column in the settings table
-		WithSentinel(columns...).
+		WithSentinelIfExists(columns...).
 		Values()
 }
 
@@ -837,7 +842,7 @@ func (sc *systemTableContents) scheduledJobsHandler(
 	values []interface{}, columns []string,
 ) ([]interface{}, error) {
 	return newSystemTableRow(sc.table, values, columns).
-		WithSentinel("next_run", "schedule_details", "schedule_state").
+		WithSentinelIfExists("next_run", "schedule_details", "schedule_state").
 		Values()
 }
 
@@ -848,15 +853,15 @@ func (sc *systemTableContents) usersHandler(
 	values []interface{}, columns []string,
 ) ([]interface{}, error) {
 	return newSystemTableRow(sc.table, values, columns).
-		WithSentinel("estimated_last_login_time").
+		WithSentinelIfExists("estimated_last_login_time").
 		Values()
 }
 
 func (sc *systemTableContents) commentsHandler(
 	values []interface{}, columns []string,
 ) ([]interface{}, error) {
 	return newSystemTableRow(sc.table, values, columns).
-		WithSentinel("object_id"). // object_id is rekeyed
+		WithSentinelIfExists("object_id"). // object_id is rekeyed
 		Values()
 }