More documentation and add CapDefault

CapDefault has all the capabilities supported in the current release
and should not be changed until a new major release.

Also changed Capabilities of mount to return the common capabilities
from both filesystems.

Signed-off-by: Javi Fontan <[email protected]>

Author: jfontan

fs.go

@@ -13,7 +13,10 @@ var (
 	ErrCrossedBoundary = errors.New("chroot boundary crossed")
 )
 
-// Capability holds the supported features of a filesystem.
+// Capability holds the supported features of a billy filesystem. This does
+// not mean that the capability has to be supported by the underlying storage.
+// For example, a billy filesystem may support CapWrite but the storage be
+// mounted in read only mode.
 type Capability uint64
 
 const (
@@ -30,6 +33,12 @@ const (
 	// CapLock is the ability to lock a file.
 	CapLock
 
+	// CapDefault lists all capable features supported by filesystems without
+	// Capability interface. This list should not be changed until a major
+	// version is released.
+	CapDefault Capability = CapWrite | CapRead | CapReadAndWrite |
+		CapSeek | CapTruncate | CapLock
+
 	// CapAll lists all capable features.
 	CapAll Capability = CapWrite | CapRead | CapReadAndWrite |
 		CapSeek | CapTruncate | CapLock
@@ -177,7 +186,7 @@ type Capable interface {
 func Capabilities(fs Basic) Capability {
 	capable, ok := fs.(Capable)
 	if !ok {
-		return CapAll
+		return CapDefault
 	}
 
 	return capable.Capabilities()

helper/mount/mount.go

@@ -169,7 +169,7 @@ func (h *Mount) Underlying() billy.Basic {
 
 // Capabilities implements the Capable interface.
 func (fs *Mount) Capabilities() billy.Capability {
-	return billy.Capabilities(fs.underlying)
+	return billy.Capabilities(fs.underlying) & billy.Capabilities(fs.source)
 }
 
 func (fs *Mount) getBasicAndPath(path string) (billy.Basic, string) {

memfs/memory_test.go

@@ -27,5 +27,5 @@ func (s *MemorySuite) TestCapabilities(c *C) {
 	c.Assert(ok, Equals, true)
 
 	caps := billy.Capabilities(s.FS)
-	c.Assert(caps, Equals, billy.CapAll&^billy.CapLock)
+	c.Assert(caps, Equals, billy.CapDefault&^billy.CapLock)
 }

osfs/os.go

@@ -129,7 +129,7 @@ func (fs *OS) Readlink(link string) (string, error) {
 
 // Capabilities implements the Capable interface.
 func (fs *OS) Capabilities() billy.Capability {
-	return billy.CapAll
+	return billy.CapDefault
 }
 
 // file is a wrapper for an os.File which adds support for file locking.