Simplify/fix MkdirAll usage

This subtle bug keeps lurking in because error checking for `Mkdir()`
and `MkdirAll()` is slightly different wrt to `EEXIST`/`IsExist`:

 - for `Mkdir()`, `IsExist` error should (usually) be ignored
   (unless you want to make sure directory was not there before)
   as it means "the destination directory was already there"

 - for `MkdirAll()`, `IsExist` error should NEVER be ignored.

Mostly, this commit just removes ignoring the IsExist error, as it
should not be ignored.

Also, there are a couple of cases then IsExist is handled as
"directory already exist" which is wrong. As a result, some code
that never worked as intended is now removed.

NOTE that `idtools.MkdirAndChown()` behaves like `os.MkdirAll()`
rather than `os.Mkdir()` -- so its description is amended accordingly,
and its usage is handled as such (i.e. IsExist error is not ignored).

For more details, a quote from my runc commit 6f82d4b (July 2015):

    TL;DR: check for IsExist(err) after a failed MkdirAll() is both
    redundant and wrong -- so two reasons to remove it.

    Quoting MkdirAll documentation:

    > MkdirAll creates a directory named path, along with any necessary
    > parents, and returns nil, or else returns an error. If path
    > is already a directory, MkdirAll does nothing and returns nil.

    This means two things:

    1. If a directory to be created already exists, no error is
    returned.

    2. If the error returned is IsExist (EEXIST), it means there exists
    a non-directory with the same name as MkdirAll need to use for
    directory. Example: we want to MkdirAll("a/b"), but file "a"
    (or "a/b") already exists, so MkdirAll fails.

    The above is a theory, based on quoted documentation and my UNIX
    knowledge.

    3. In practice, though, current MkdirAll implementation [1] returns
    ENOTDIR in most of cases described in #2, with the exception when
    there is a race between MkdirAll and someone else creating the
    last component of MkdirAll argument as a file. In this very case
    MkdirAll() will indeed return EEXIST.

    Because of #1, IsExist check after MkdirAll is not needed.

    Because of #2 and #3, ignoring IsExist error is just plain wrong,
    as directory we require is not created. It's cleaner to report
    the error now.

    Note this error is all over the tree, I guess due to copy-paste,
    or trying to follow the same usage pattern as for Mkdir(),
    or some not quite correct examples on the Internet.

    [1] https://github.com/golang/go/blob/f9ed2f75/src/os/path.go

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Upstream-commit: 516010e92d56cfcd6d1e343bdc02b6f04bc43039
Component: engine

components/engine/daemon/graphdriver/devmapper/deviceset.go

@@ -268,7 +268,7 @@ func (devices *DeviceSet) ensureImage(name string, size int64) (string, error) {
 	if err != nil {
 		return "", err
 	}
-	if err := idtools.MkdirAllAndChown(dirname, 0700, idtools.IDPair{UID: uid, GID: gid}); err != nil && !os.IsExist(err) {
+	if err := idtools.MkdirAllAndChown(dirname, 0700, idtools.IDPair{UID: uid, GID: gid}); err != nil {
 		return "", err
 	}
 
@@ -1697,10 +1697,10 @@ func (devices *DeviceSet) initDevmapper(doInit bool) (retErr error) {
 	if err != nil {
 		return err
 	}
-	if err := idtools.MkdirAndChown(devices.root, 0700, idtools.IDPair{UID: uid, GID: gid}); err != nil && !os.IsExist(err) {
+	if err := idtools.MkdirAndChown(devices.root, 0700, idtools.IDPair{UID: uid, GID: gid}); err != nil {
 		return err
 	}
-	if err := os.MkdirAll(devices.metadataDir(), 0700); err != nil && !os.IsExist(err) {
+	if err := os.MkdirAll(devices.metadataDir(), 0700); err != nil {
 		return err
 	}
 

components/engine/daemon/graphdriver/devmapper/driver.go

@@ -189,7 +189,7 @@ func (d *Driver) Get(id, mountLabel string) (containerfs.ContainerFS, error) {
 	}
 
 	// Create the target directories if they don't exist
-	if err := idtools.MkdirAllAndChown(path.Join(d.home, "mnt"), 0755, idtools.IDPair{UID: uid, GID: gid}); err != nil && !os.IsExist(err) {
+	if err := idtools.MkdirAllAndChown(path.Join(d.home, "mnt"), 0755, idtools.IDPair{UID: uid, GID: gid}); err != nil {
 		d.ctr.Decrement(mp)
 		return nil, err
 	}
@@ -204,7 +204,7 @@ func (d *Driver) Get(id, mountLabel string) (containerfs.ContainerFS, error) {
 		return nil, err
 	}
 
-	if err := idtools.MkdirAllAndChown(rootFs, 0755, idtools.IDPair{UID: uid, GID: gid}); err != nil && !os.IsExist(err) {
+	if err := idtools.MkdirAllAndChown(rootFs, 0755, idtools.IDPair{UID: uid, GID: gid}); err != nil {
 		d.ctr.Decrement(mp)
 		d.DeviceSet.UnmountDevice(id, mp)
 		return nil, err