feat: add experimental 'mount' command

Add experimental support for rootless mount of SIF images.

internal/app/siftool/mount.go

@@ -0,0 +1,20 @@
+// Copyright (c) 2022, Sylabs Inc. All rights reserved.
+// This software is licensed under a 3-clause BSD license. Please consult the
+// LICENSE file distributed with the sources of this project regarding your
+// rights to use or distribute this software.
+
+package siftool
+
+import (
+	"context"
+
+	"github.com/sylabs/sif/v2/internal/pkg/exp"
+)
+
+// Mount mounts the primary system partition of the SIF file at path into mountPath.
+func (a *App) Mount(ctx context.Context, path, mountPath string) error {
+	return exp.Mount(ctx, path, mountPath,
+		exp.OptMountStdout(a.opts.out),
+		exp.OptMountStderr(a.opts.err),
+	)
+}

internal/pkg/exp/mount.go

@@ -0,0 +1,104 @@
+// Copyright (c) 2022, Sylabs Inc. All rights reserved.
+// This software is licensed under a 3-clause BSD license. Please consult the
+// LICENSE file distributed with the sources of this project regarding your
+// rights to use or distribute this software.
+
+// Package exp contains experimental functionality that is not sufficiently mature to be exported
+// as part of the module API.
+package exp
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"path/filepath"
+
+	"github.com/sylabs/sif/v2/pkg/sif"
+)
+
+// mountSquashFS mounts the SquashFS filesystem from path at offset into mountPath.
+func mountSquashFS(ctx context.Context, offset int64, path, mountPath string, mo mountOpts) error {
+	args := []string{
+		"-o", fmt.Sprintf("ro,offset=%d", offset),
+		filepath.Clean(path),
+		filepath.Clean(mountPath),
+	}
+
+	cmd := exec.CommandContext(ctx, "squashfuse", args...)
+	cmd.Stdout = mo.stdout
+	cmd.Stderr = mo.stderr
+
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("failed to mount: %w", err)
+	}
+
+	return nil
+}
+
+// mountOpts accumulates mount options.
+type mountOpts struct {
+	stdout io.Writer
+	stderr io.Writer
+}
+
+// MountOpt are used to specify mount options.
+type MountOpt func(*mountOpts) error
+
+// OptMountStdout writes standard output to w.
+func OptMountStdout(w io.Writer) MountOpt {
+	return func(mo *mountOpts) error {
+		mo.stdout = w
+		return nil
+	}
+}
+
+// OptMountStderr writes standard error to w.
+func OptMountStderr(w io.Writer) MountOpt {
+	return func(mo *mountOpts) error {
+		mo.stderr = w
+		return nil
+	}
+}
+
+var errUnsupportedFSType = errors.New("unrecognized filesystem type")
+
+// Mount mounts the primary system partition of the SIF file at path into mountPath.
+//
+// Mount may start one or more underlying processes. By default, stdout and stderr of these
+// processes is discarded. To modify this behavior, consider using OptMountStdout and/or
+// OptMountStderr.
+func Mount(ctx context.Context, path, mountPath string, opts ...MountOpt) error {
+	mo := mountOpts{}
+
+	for _, opt := range opts {
+		if err := opt(&mo); err != nil {
+			return fmt.Errorf("%w", err)
+		}
+	}
+
+	f, err := sif.LoadContainerFromPath(path, sif.OptLoadWithFlag(os.O_RDONLY))
+	if err != nil {
+		return fmt.Errorf("failed to load image: %w", err)
+	}
+	defer func() { _ = f.UnloadContainer() }()
+
+	d, err := f.GetDescriptor(sif.WithPartitionType(sif.PartPrimSys))
+	if err != nil {
+		return fmt.Errorf("failed to get partition descriptor: %w", err)
+	}
+
+	fs, _, _, err := d.PartitionMetadata()
+	if err != nil {
+		return fmt.Errorf("failed to get partition metadata: %w", err)
+	}
+
+	switch fs {
+	case sif.FsSquash:
+		return mountSquashFS(ctx, d.Offset(), path, mountPath, mo)
+	default:
+		return errUnsupportedFSType
+	}
+}

pkg/siftool/mount.go

@@ -0,0 +1,27 @@
+// Copyright (c) 2022, Sylabs Inc. All rights reserved.
+// This software is licensed under a 3-clause BSD license. Please consult the
+// LICENSE file distributed with the sources of this project regarding your
+// rights to use or distribute this software.
+
+package siftool
+
+import (
+	"github.com/spf13/cobra"
+)
+
+// getMount returns a command that mounts the primary system partition of a SIF image.
+func (c *command) getMount() *cobra.Command {
+	return &cobra.Command{
+		Use:     "mount <sif_path> <mount_path>",
+		Short:   "Mount primary system partition",
+		Long:    "Mount the primary system partition of a SIF image",
+		Example: c.opts.rootPath + " mount image.sif path/",
+		Args:    cobra.ExactArgs(2),
+		PreRunE: c.initApp,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			return c.app.Mount(cmd.Context(), args[0], args[1])
+		},
+		DisableFlagsInUseLine: true,
+		Hidden:                true, // hide while command is experimental
+	}
+}

pkg/siftool/siftool.go

@@ -76,5 +76,9 @@ func AddCommands(cmd *cobra.Command, opts ...CommandOpt) error {
 		c.getSetPrim(),
 	)
 
+	if c.opts.experimental {
+		cmd.AddCommand(c.getMount())
+	}
+
 	return nil
 }

pkg/siftool/siftool_test.go

@@ -115,6 +115,11 @@ func TestAddCommands(t *testing.T) {
 			name: "SetPrim",
 			args: []string{"help", "setprim"},
 		},
+		{
+			name: "Mount",
+			opts: []CommandOpt{OptWithExperimental(true)},
+			args: []string{"help", "mount"},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

pkg/siftool/testdata/TestAddCommands/Mount/out.golden

@@ -0,0 +1,10 @@
+Mount the primary system partition of a SIF image
+
+Usage:
+  siftool mount <sif_path> <mount_path>
+
+Examples:
+siftool mount image.sif path/
+
+Flags:
+  -h, --help   help for mount