Mysqld.GetSchema: tolerate tables being dropped while inspecting sche…

…ma (vitessio#12641)

* Mysqld.GetSchema: tolerate tables being dropped while inspecting schema

Signed-off-by: Shlomi Noach <2607934+shlomi-noach@users.noreply.github.com>

* lack of primary key columns in STATISTICS does not mean table is dropped. It can also mean the table does not have PRIMARY KEY

Signed-off-by: Shlomi Noach <2607934+shlomi-noach@users.noreply.github.com>

* populate validTds rather than rely on nil hints

Signed-off-by: Shlomi Noach <2607934+shlomi-noach@users.noreply.github.com>

* re-introdce earlier check

Signed-off-by: Shlomi Noach <2607934+shlomi-noach@users.noreply.github.com>

* use validTds, sync

Signed-off-by: Shlomi Noach <2607934+shlomi-noach@users.noreply.github.com>

* due to many tests consistently failing, trying a different approach: we keep the table, but with empty column/key/fields info

Signed-off-by: Shlomi Noach <2607934+shlomi-noach@users.noreply.github.com>

* grammar

Signed-off-by: Shlomi Noach <2607934+shlomi-noach@users.noreply.github.com>

---------

Signed-off-by: Shlomi Noach <2607934+shlomi-noach@users.noreply.github.com>

go/vt/mysqlctl/schema.go

@@ -24,6 +24,7 @@ import (
  "strings"
  "sync"
 
+ "vitess.io/vitess/go/mysql"
  "vitess.io/vitess/go/sqltypes"
  "vitess.io/vitess/go/vt/concurrency"
  "vitess.io/vitess/go/vt/vterrors"
@@ -109,6 +110,15 @@ func (mysqld *Mysqld) GetSchema(ctx context.Context, dbName string, request *tab
 
  fields, columns, schema, err := mysqld.collectSchema(ctx, dbName, td.Name, td.Type, request.TableSchemaOnly)
  if err != nil {
+ // There's a possible race condition: it could happen that a table was dropped in between reading
+ // the list of tables (collectBasicTableData(), earlier) and the point above where we investigate
+ // the table.
+ // This is fine. We identify the situation and keep the table without any fields/columns/key information
+ sqlErr, isSQLErr := mysql.NewSQLErrorFromError(err).(*mysql.SQLError)
+ if isSQLErr && sqlErr != nil && sqlErr.Number() == mysql.ERNoSuchTable {
+ return
+ }
+
  allErrors.RecordError(err)
  cancel()
  return
@@ -121,6 +131,8 @@ func (mysqld *Mysqld) GetSchema(ctx context.Context, dbName string, request *tab
  }
 
  // Get primary columns concurrently.
+ // The below runs a single query on `INFORMATION_SCHEMA` and does not interact with the actual tables.
+ // It is therefore safe to run even if some tables are dropped in the interim.
  colMap := map[string][]string{}
  if len(tableNames) > 0 {
  wg.Add(1)