os: implement virtual filesystem support

This allows applications to mount filesystems in the os package. This is
useful for mounting external flash filesystems, for example.

src/os/filesystem.go

@@ -0,0 +1,85 @@
+package os
+
+import (
+	"strings"
+)
+
+// mounts lists the mount points currently mounted in the filesystem provided by
+// the os package. To resolve a path to a mount point, it is scanned from top to
+// bottom looking for the first prefix match.
+var mounts []mountPoint
+
+type mountPoint struct {
+	// prefix is a filesystem prefix, that always starts and ends with a forward
+	// slash. To denote the root filesystem, use a single slash: "/".
+	// This allows fast checking whether a path lies within a mount point.
+	prefix string
+
+	// filesystem is the Filesystem implementation that is mounted at this mount
+	// point.
+	filesystem Filesystem
+}
+
+// Filesystem provides an interface for generic filesystem drivers mounted in
+// the os package. The errors returned must be one of the os.Err* errors, or a
+// custom error if one doesn't exist. It should not be a *PathError because
+// errors will be wrapped with a *PathError by the filesystem abstraction.
+//
+// WARNING: this interface is not finalized and may change in a future version.
+type Filesystem interface {
+	// OpenFile opens the named file.
+	OpenFile(name string, flag int, perm FileMode) (FileHandle, error)
+
+	// Mkdir creates a new directoy with the specified permission (before
+	// umask). Some filesystems may not support directories or permissions.
+	Mkdir(name string, perm FileMode) error
+
+	// Remove removes the named file or (empty) directory.
+	Remove(name string) error
+}
+
+// FileHandle is an interface that should be implemented by filesystems
+// implementing the Filesystem interface.
+//
+// WARNING: this interface is not finalized and may change in a future version.
+type FileHandle interface {
+	// Read reads up to len(b) bytes from the file.
+	Read(b []byte) (n int, err error)
+
+	// Write writes up to len(b) bytes to the file.
+	Write(b []byte) (n int, err error)
+
+	// Close closes the file, making it unusable for further writes.
+	Close() (err error)
+}
+
+// findMount returns the appropriate (mounted) filesystem to use for a given
+// filename plus the path relative to that filesystem.
+func findMount(path string) (Filesystem, string) {
+	for i := len(mounts) - 1; i >= 0; i-- {
+		mount := mounts[i]
+		if strings.HasPrefix(path, mount.prefix) {
+			return mount.filesystem, path[len(mount.prefix)-1:]
+		}
+	}
+	if isOS {
+		// Assume that the first entry in the mounts slice is the OS filesystem
+		// at the root of the directory tree. Use it as-is, to support relative
+		// paths.
+		return mounts[0].filesystem, path
+	}
+	return nil, path
+}
+
+// Mount mounts the given filesystem in the filesystem abstraction layer of the
+// os package. It is not possible to unmount filesystems. Filesystems added
+// later will override earlier filesystems.
+//
+// The provided prefix must start and end with a forward slash. This is true for
+// the root directory ("/") for example.
+func Mount(prefix string, filesystem Filesystem) {
+	if prefix[0] != '/' || prefix[len(prefix)-1] != '/' {
+		panic("os.Mount: invalid prefix")
+	}
+	mounts = append(mounts, mountPoint{prefix, filesystem})
+}