fix: directly write dump to file without buffering in memory

Resolves https://kolaente.dev/konrad/docker-db-backup/issues/5

save.go

@@ -1,14 +1,13 @@
 package main
 
 import (
-	"bytes"
 	"context"
 	"fmt"
-	"github.com/docker/docker/api/types"
-	"github.com/docker/docker/client"
-	"github.com/docker/docker/pkg/stdcopy"
 	"io"
 	"os"
+
+	"github.com/docker/docker/api/types"
+	"github.com/docker/docker/client"
 )
 
 func runAndSaveCommandInContainer(filename string, c *client.Client, container *types.ContainerJSON, command string, args ...string) error {
@@ -31,33 +30,20 @@ func runAndSaveCommandInContainer(filename string, c *client.Client, container *
 	}
 	defer resp.Close()
 
-	// read the output
-	var outBuf, errBuf bytes.Buffer
-	outputDone := make(chan error)
-
-	go func() {
-		// StdCopy demultiplexes the stream into two buffers
-		_, err = stdcopy.StdCopy(&outBuf, &errBuf, resp.Reader)
-		outputDone <- err
-	}()
-
-	err = <-outputDone
-	if err != nil {
-		fmt.Printf(errBuf.String())
-		return err
-	}
-
-	_, err = c.ContainerExecInspect(ctx, r.ID)
-	if err != nil {
-		return err
-	}
-
 	f, err := os.Create(filename)
 	if err != nil {
 		return err
 	}
 	defer f.Close()
 
-	_, err = io.Copy(f, &outBuf)
+	_, err = io.Copy(f, resp.Reader)
+	if err != nil {
+		return err
+	}
+
+	execInspect, err := c.ContainerExecInspect(ctx, r.ID)
+	if execInspect.ExitCode != 0 {
+		return fmt.Errorf("backup from container %s failed with exit code %d", container.Name, execInspect.ExitCode)
+	}
 	return err
 }