Simple file system implementation using the 9P protocol

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.

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

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.

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.

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.

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