Handle windows-layer mounts somewhat like bind-mounts

TBBle · TBBle · commit 9dca2dbda65c · 2020-01-06T23:35:59.000+11:00
windows-layer mounts (the only mount type supported by Windows) do not
currently use the `target` parameter to `mount.All`.

So instead, we treat them like bind-mounts and access the activated
layer in-place. Unlike the bind-mounts, we still need to Mount and
Unmount the layer.

Since we don't own the directory, we must not delete it after we unmount
it.

Signed-off-by: Paul "TBBle" Hampson &lt;Paul.Hampson@Pobox.com&gt;
diff --git a/snapshot/localmounter.go b/snapshot/localmounter.go
@@ -47,6 +47,16 @@ func (lm *localMounter) Mount() (string, error) {
 		lm.release = release
 	}
 
+	// Windows mounts don't use the target parameter of mount.All, they activate in-place.
+	if len(lm.mounts) == 1 && lm.mounts[0].Type == "windows-layer" {
+		// TODO: How to handle read-only? A local copy would be awfully inefficient.
+		if err := mount.All(lm.mounts, ""); err != nil {
+			return "", errors.Wrapf(err, "failed to mount in-place: %+v", lm.mounts)
+		}
+		lm.target = lm.mounts[0].Source
+		return lm.mounts[0].Source, nil
+	}
+
 	if len(lm.mounts) == 1 && (lm.mounts[0].Type == "bind" || lm.mounts[0].Type == "rbind") {
 		ro := false
 		for _, opt := range lm.mounts[0].Options {
diff --git a/snapshot/localmounter_windows.go b/snapshot/localmounter_windows.go
@@ -1,8 +1,6 @@
 package snapshot
 
 import (
-	"os"
-
 	"github.com/containerd/containerd/mount"
 )
 
@@ -14,7 +12,6 @@ func (lm *localMounter) Unmount() error {
 		if err := mount.Unmount(lm.target, 0); err != nil {
 			return err
 		}
-		os.RemoveAll(lm.target)
 		lm.target = ""
 	}
 

Original file line number	Diff line number	Diff line change
`@@ -1,8 +1,6 @@`
`1`	`1`	`package snapshot`
`2`	`2`
`3`	`3`	`import (`
`4`		`- "os"`
`5`		`-`
`6`	`4`	`"github.com/containerd/containerd/mount"`
`7`	`5`	`)`
`8`	`6`
`@@ -14,7 +12,6 @@ func (lm *localMounter) Unmount() error {`
`14`	`12`	`if err := mount.Unmount(lm.target, 0); err != nil {`
`15`	`13`	`return err`
`16`	`14`	`}`
`17`		`- os.RemoveAll(lm.target)`
`18`	`15`	`lm.target = ""`
`19`	`16`	`}`
`20`	`17`