Support local filesystem mount of remote device for iterative build-install-test workflows

[raballew]

Use Case

When doing iterative development targeting physical hardware managed by Jumpstarter, I want to avoid the full flash-and-reboot cycle for every change. Instead, I want to:

1. Flash a base image once
2. Mount the device's filesystem locally
3. Build and install directly to the mount
4. Run tests

This dramatically shortens the edit-build-test loop by eliminating the reboot, which can take minutes depending on the hardware.

Desired Workflow

(rcar_s4) $ j storage flash --file ./my-remote-image.raw
(rcar_s4) $ mkdir -p /tmp/device-mnt
(rcar_s4) $ j mount /tmp/device-mnt
(rcar_s4) $ make
(rcar_s4) $ make install DESTDIR=/tmp/device-mnt
(rcar_s4) $ j tmt run --all

Why This Matters

For kernel and driver development on embedded targets (e.g., R-Car S4), a typical iteration involves:

• Changing a few source files
• Building (seconds with ccache)
• Re-flashing (only seconds with simg)
• Rebooting the device (minutes depending on the hardware) - this is the bottleneck
• Running tests

With j mount, the install step becomes a direct file copy to the device's filesystem.

Expected Behavior for j mount

• Exposes the storage as a locally mountable filesystem via sshfs
• Supports read-write access
• Works with standard tools (make install, cp, rsync)
• Unmounts cleanly on session end or explicit j umount

[mangelajo]

This can rely on a ref: to an ssh driver, and then use sshfs to make the mount.

[ambient-code]

Implementation Plan for j mount / j umount

Based on the maintainer comment about using sshfs with a ref to the SSH driver, here is my approach:

Changes

Modified: jumpstarter_driver_ssh/client.py

• Changed the SSH driver CLI from a single ssh command to a command group with subcommands: shell, mount, and umount
• The existing SSH shell functionality is now under j ssh shell (was j ssh)
• Added mount() method that:
  • Checks for sshfs availability with helpful install instructions
  • Creates the mountpoint directory if needed
  • Supports both direct TCP and port-forwarded connections (matching existing SSH behavior)
  • Handles SSH identity files (keys) for authentication
  • Automatically retries without allow_other if FUSE config doesn't allow it
• Added umount() method that:
  • Uses fusermount3/fusermount on Linux, falls back to umount on macOS
  • Supports lazy unmount via --lazy/-l flag
• Refactored _run_sshfs into smaller methods (_build_sshfs_args, _retry_sshfs_without_allow_other, _create_temp_identity_file) to meet complexity requirements

Usage

# Inside a jmp shell session:
j ssh mount /tmp/device-mnt                        # Mount remote / to local dir
j ssh mount /tmp/device-mnt -r /home/user          # Mount specific remote path
j ssh mount /tmp/device-mnt --direct                # Use direct TCP (no port forwarding)
j ssh mount /tmp/device-mnt -o "-o reconnect"       # Pass extra sshfs options

# Run your build workflow
make && make install DESTDIR=/tmp/device-mnt

# Unmount when done
j ssh umount /tmp/device-mnt
j ssh umount /tmp/device-mnt --lazy                 # Lazy unmount if busy

Tests Added

• 10 new unit tests covering: sshfs not installed, successful mount, mount with identity, allow_other fallback, fusermount umount, lazy umount, umount failure, and CLI command registration

PR incoming.

[ambient-code]

PR created: #434