Add --data-only to copy data without structure

This was prompted as a workaround for the issue presented in hellofresh#139.
By allowing data-only transfers to be run, we can use multiple config
files in order to extract multiple subsets from the same table. An
initial run, without `--data-only`, copies the DB structure and
anonmized users while a second run using `--data-only` and a different
config file copies unmodified versions of admin users to the same table.

It's likely useful in many other caeses as well.

cmd/steal.go

@@ -33,6 +33,7 @@ type (
 		concurrency int
 		readOpts    connOpts
 		writeOpts   connOpts
+		dataOnly    bool
 	}
 	connOpts struct {
 		timeout         time.Duration
@@ -76,6 +77,7 @@ func NewStealCmd() *cobra.Command {
 	persistentFlags.DurationVar(&opts.writeOpts.maxConnLifetime, "write-conn-lifetime", 0, "Sets the maximum amount of time a connection may be reused on the write database")
 	persistentFlags.IntVar(&opts.writeOpts.maxConns, "write-max-conns", 5, "Sets the maximum number of open connections to the write database")
 	persistentFlags.IntVar(&opts.writeOpts.maxIdleConns, "write-max-idle-conns", 0, "Sets the maximum number of connections in the idle connection pool for the write database")
+	persistentFlags.BoolVar(&opts.dataOnly, "data-only", false, "Only steal data; requires that the target database structure already exists")
 
 	return cmd
 }
@@ -122,7 +124,7 @@ func RunSteal(opts *StealOptions) (err error) {
 	defer close(done)
 
 	start := time.Now()
-	if err := target.Dump(done, opts.cfgTables, opts.concurrency); err != nil {
+	if err := target.Dump(done, opts.cfgTables, opts.concurrency, opts.dataOnly); err != nil {
 		return fmt.Errorf("error while dumping: %w", err)
 	}
 

features/mysql_test.go

@@ -53,7 +53,7 @@ func (s *MysqlTestSuite) TestExample() {
 
 	done := make(chan struct{})
 	defer close(done)
-	s.Require().NoError(dmp.Dump(done, config.Tables{}, 4), "Failed to dump")
+	s.Require().NoError(dmp.Dump(done, config.Tables{}, 4, false), "Failed to dump")
 
 	<-done
 

features/postgres_test.go

@@ -54,7 +54,7 @@ func (s *PostgresTestSuite) TestExample() {
 
 	done := make(chan struct{})
 	defer close(done)
-	s.Require().NoError(dmp.Dump(done, config.Tables{}, 4), "Failed to dump")
+	s.Require().NoError(dmp.Dump(done, config.Tables{}, 4, false), "Failed to dump")
 
 	<-done
 

pkg/dumper/dumper.go

@@ -22,7 +22,7 @@ type (
 	// A Dumper writes a database's structure to the provided stream.
 	Dumper interface {
 		// Dump executes the dump process.
-		Dump(chan<- struct{}, config.Tables, int) error
+		Dump(chan<- struct{}, config.Tables, int, bool) error
 		// Close closes the dumper resources and releases them.
 		Close() error
 	}

pkg/dumper/engine/engine.go

@@ -47,9 +47,11 @@ func New(rdr reader.Reader, dumper Dumper) dumper.Dumper {
 }
 
 // Dump executes the dump process.
-func (e *Engine) Dump(done chan<- struct{}, cfgTables config.Tables, concurrency int) error {
-	if err := e.readAndDumpStructure(); err != nil {
-		return err
+func (e *Engine) Dump(done chan<- struct{}, cfgTables config.Tables, concurrency int, dataOnly bool) error {
+	if !dataOnly {
+		if err := e.readAndDumpStructure(); err != nil {
+			return err
+		}
 	}
 
 	return e.readAndDumpTables(done, cfgTables, concurrency)

pkg/dumper/query/dumper.go

@@ -33,18 +33,20 @@ func NewDumper(output io.Writer, rdr reader.Reader) dumper.Dumper {
 }
 
 // Dump executes the dump stream process.
-func (d *textDumper) Dump(done chan<- struct{}, cfgTables config.Tables, concurrency int) error {
+func (d *textDumper) Dump(done chan<- struct{}, cfgTables config.Tables, concurrency int, dataOnly bool) error {
 	tables, err := d.reader.GetTables()
 	if err != nil {
 		return fmt.Errorf("failed to get tables: %w", err)
 	}
 
-	structure, err := d.reader.GetStructure()
-	if err != nil {
-		return fmt.Errorf("could not get database structure: %w", err)
-	}
-	if _, err := io.WriteString(d.output, structure); err != nil {
-		return fmt.Errorf("could not write structure to output: %w", err)
+	if !dataOnly {
+		structure, err := d.reader.GetStructure()
+		if err != nil {
+			return fmt.Errorf("could not get database structure: %w", err)
+		}
+		if _, err := io.WriteString(d.output, structure); err != nil {
+			return fmt.Errorf("could not write structure to output: %w", err)
+		}
 	}
 
 	var wg sync.WaitGroup