export: use io.Writer instead of file

This allows use to use STDOUT directly without having to call open
again, also this makes the export API endpoint much more performant
since it no longer needs to copy to a temp file.
I noticed that there was no export API test so I added one.

And lastly opening /dev/stdout will not work on windows.

Fixes #16870

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

cmd/podman/containers/export.go

@@ -43,13 +43,14 @@ var (
 
 var (
 	exportOpts entities.ContainerExportOptions
+	outputFile string
 )
 
 func exportFlags(cmd *cobra.Command) {
 	flags := cmd.Flags()
 
 	outputFlagName := "output"
-	flags.StringVarP(&exportOpts.Output, outputFlagName, "o", "", "Write to a specified file (default: stdout, which must be redirected)")
+	flags.StringVarP(&outputFile, outputFlagName, "o", "", "Write to a specified file (default: stdout, which must be redirected)")
 	_ = cmd.RegisterFlagCompletionFunc(outputFlagName, completion.AutocompleteDefault)
 }
 
@@ -67,14 +68,24 @@ func init() {
 }
 
 func export(cmd *cobra.Command, args []string) error {
-	if len(exportOpts.Output) == 0 {
+	if len(outputFile) == 0 {
 		file := os.Stdout
 		if term.IsTerminal(int(file.Fd())) {
 			return errors.New("refusing to export to terminal. Use -o flag or redirect")
 		}
-		exportOpts.Output = "/dev/stdout"
+		exportOpts.Output = file
-	} else if err := parse.ValidateFileName(exportOpts.Output); err != nil {
+	} else {
-		return err
+		if err := parse.ValidateFileName(outputFile); err != nil {
+			return err
+		}
+		// open file here with WRONLY since on MacOS it can fail to open /dev/stderr in read mode for example
+		// https://github.com/containers/podman/issues/16870
+		file, err := os.OpenFile(outputFile, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0644)
+		if err != nil {
+			return err
+		}
+		defer file.Close()
+		exportOpts.Output = file
 	}
 	return registry.ContainerEngine().ContainerExport(context.Background(), args[0], exportOpts)
 }

libpod/container_api.go

@@ -462,7 +462,7 @@ func (c *Container) Unpause() error {
 
 // Export exports a container's root filesystem as a tar archive
 // The archive will be saved as a file at the given path
-func (c *Container) Export(path string) error {
+func (c *Container) Export(out io.Writer) error {
 	if !c.batched {
 		c.lock.Lock()
 		defer c.lock.Unlock()
@@ -477,7 +477,7 @@ func (c *Container) Export(path string) error {
 	}
 
 	defer c.newContainerEvent(events.Mount)
-	return c.export(path)
+	return c.export(out)
 }
 
 // AddArtifact creates and writes to an artifact file for the container

libpod/container_internal.go

@@ -745,7 +745,7 @@ func (c *Container) removeConmonFiles() error {
 	return nil
 }
 
-func (c *Container) export(path string) error {
+func (c *Container) export(out io.Writer) error {
 	mountPoint := c.state.Mountpoint
 	if !c.state.Mounted {
 		containerMount, err := c.runtime.store.Mount(c.ID(), c.config.MountLabel)
@@ -765,13 +765,7 @@ func (c *Container) export(path string) error {
 		return fmt.Errorf("reading container directory %q: %w", c.ID(), err)
 	}
 
-	outFile, err := os.Create(path)
+	_, err = io.Copy(out, input)
-	if err != nil {
-		return fmt.Errorf("creating file %q: %w", path, err)
-	}
-	defer outFile.Close()
-
-	_, err = io.Copy(outFile, input)
 	return err
 }
 

pkg/api/handlers/compat/containers_export.go

@@ -3,7 +3,6 @@ package compat
 import (
 	"fmt"
 	"net/http"
-	"os"
 
 	"github.com/containers/podman/v4/libpod"
 	"github.com/containers/podman/v4/pkg/api/handlers/utils"
@@ -18,25 +17,14 @@ func ExportContainer(w http.ResponseWriter, r *http.Request) {
 		utils.ContainerNotFound(w, name, err)
 		return
 	}
-	tmpfile, err := os.CreateTemp("", "api.tar")
+
-	if err != nil {
+	// set the correct header
-		utils.Error(w, http.StatusInternalServerError, fmt.Errorf("unable to create tempfile: %w", err))
+	w.Header().Set("Content-Type", "application/x-tar")
+	// NOTE: As described in w.Write() it automatically sets the http code to
+	// 200 on first write if no other code was set.
+
+	if err := con.Export(w); err != nil {
+		utils.Error(w, http.StatusInternalServerError, fmt.Errorf("failed to export container: %w", err))
 		return
 	}
-	defer os.Remove(tmpfile.Name())
-	if err := tmpfile.Close(); err != nil {
-		utils.Error(w, http.StatusInternalServerError, fmt.Errorf("unable to close tempfile: %w", err))
-		return
-	}
-	if err := con.Export(tmpfile.Name()); err != nil {
-		utils.Error(w, http.StatusInternalServerError, fmt.Errorf("failed to save image: %w", err))
-		return
-	}
-	rdr, err := os.Open(tmpfile.Name())
-	if err != nil {
-		utils.Error(w, http.StatusInternalServerError, fmt.Errorf("failed to read the exported tarfile: %w", err))
-		return
-	}
-	defer rdr.Close()
-	utils.WriteResponse(w, http.StatusOK, rdr)
 }

pkg/domain/entities/containers.go

@@ -181,7 +181,7 @@ type CommitReport struct {
 }
 
 type ContainerExportOptions struct {
-	Output string
+	Output io.Writer
 }
 
 type CheckpointOptions struct {

pkg/domain/infra/tunnel/containers.go

@@ -355,17 +355,7 @@ func (ic *ContainerEngine) ContainerCommit(ctx context.Context, nameOrID string,
 }
 
 func (ic *ContainerEngine) ContainerExport(ctx context.Context, nameOrID string, options entities.ContainerExportOptions) error {
-	var (
+	return containers.Export(ic.ClientCtx, nameOrID, options.Output, nil)
-		err error
-		w   io.Writer
-	)
-	if len(options.Output) > 0 {
-		w, err = os.Create(options.Output)
-		if err != nil {
-			return err
-		}
-	}
-	return containers.Export(ic.ClientCtx, nameOrID, w, nil)
 }
 
 func (ic *ContainerEngine) ContainerCheckpoint(ctx context.Context, namesOrIds []string, opts entities.CheckpointOptions) ([]*entities.CheckpointReport, error) {

test/apiv2/25-containersMore.at

@@ -51,6 +51,13 @@ like "$output" ".*merged" "Check container mount"
 # Unmount the container
 t POST libpod/containers/foo/unmount 204
 
+# export the container fs to tarball
+
+t GET libpod/containers/foo/export 200
+like "$(<$WORKDIR/curl.headers.out)" ".*"Content-Type\: application/x-tar".*"
+tar_tf=$(tar tf $WORKDIR/curl.result.out)
+like "$tar_tf" ".*bin/cat.*" "fetched tarball: contains bin/cat path"
+
 t DELETE libpod/containers/foo?force=true 200
 
 # Create 3 stopped containers to test containers prune