Frogger 🐸

Simple file system implementation using the 9P protocol

Bind

The P9 protocol for file systems requires a unique feature called bind, which allows for flexible control over the namespace (file hierarchy). The bind operation maps a file, directory, or another namespace tree into a new location in the namespace. It supports three binding modes: Before, After, and Replace. Each mode provides different behaviors for resolving file lookups when multiple resources are mapped to the same namespace.

Replace Binding: frg bind src mountpoint

This mode replaces whatever was previously mounted at the mountpoint with the src. Only the new src is visible at the specified mountpoint.

• Behavior: The src completely overrides any existing content at the mountpoint.
• Example use cases:
  • Temporarily replacing a default configuration directory with a test or alternative version

    frg bind /test/config /etc
  

  After this, processes see /test/config contents instead of the original /etc.
  • Redirecting access to /bin to a custom toolchain directory for development:

    frg bind /custom/tools/bin /bin
  

Before binding frg bind -b src mountpoint

In this mode the src is placed before the existing contents of the mountpoint. When a lookup occurs, a Plan9 file system searches src first, and if the file isn't found there, it searches the original mountpoint.

• Behavior: Adds src at a higher priority, leaving the existing content accessible as a fallback.
• Use case:
  • Overlaying new tools or files over existing directories without completely replacing them. For example, adding custom binaries that take precedence over system binaries:

    frg bind -b /custom/bin /bin
  

  In this case, /custom/bin/ls will be used instead of /bin/ls if both exist.
• Example: Temporarily prioritizing a new set of libraries or data over the default paths for testing or debugging.

After binding frg bind -a src mountpoint

This mode appends the src to the mountpoint's search path. Plan 9 resolves lookups by searching the original mountpoint first, and if the file isn't found there, it checks the src.

• Behavior: Adds src as a fallback while maintaining the existing content's priority.
• Use case:
  • Adding extra directories to extend a namespace without interfering with its current operation. For example, appending a directory with additional fonts:

    frg bind -a /extra/fonts /fonts
  

  Here, /fonts will use default system fonts first and fall back to /extra/fonts if needed.
• Example: Supplementing a default configuration directory with additional files:

    frg bind -a /additional/config /etc
  

  This ensures /etc retains its default behavior but gains the additional configuration files if the defaults don’t exist.

Union directories

Using frg bind -b and frg bind -a, you can create union directories where files from multiple sources appear merged. For example:

frg bind -b /local/bin /bin
frg bind -a /backup/bin /bin

This setup prioritizes /local/bin, followed by /bin, and finally /backup/bin.

Custom environments

For isolating environments, such as creating chroot-like environments or managing per-process views of namespaces.