Add support for TMP override on toml

When running containerd as a service it may be hard to
override the TMP location of the process. This is especially
true on Windows when running containerd in SCM. This change
allows you to set the 'temp' location in the config.toml when
the service starts up that overrides its TEMP/TMP/TMPDIR usage.

This is helpful on Linux as well but it primarily solves the
performance issue on Windows when running containerd across
volumes. IE: If you configure your data/root paths on a volume
other than the SystemDrive the snapshotter does a temporary unpack
on the SystemDrive and then has to copy contents of that data
to the snapshot folder on the destination volume. By alinging the
tmp with the destination it is a simple move operation instead of
a copy operation.

Signed-off-by: Justin Terry <jlterry@amazon.com>

services/server/config/config.go

@@ -39,6 +39,8 @@ type Config struct {
 	Root string `toml:"root"`
 	// State is the path to a directory where containerd will store transient data
 	State string `toml:"state"`
+	// TempDir is the path to a directory where to place containerd temporary files
+	TempDir string `toml:"temp"`
 	// PluginDir is the directory for dynamic plugins to be stored
 	PluginDir string `toml:"plugin_dir"`
 	// GRPC configuration settings

services/server/server.go

@@ -27,6 +27,7 @@ import (
 	"net/http/pprof"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"sync"
 	"time"
@@ -75,7 +76,28 @@ func CreateTopLevelDirectories(config *srvconfig.Config) error {
 		return err
 	}
 
-	return sys.MkdirAllWithACL(config.State, 0711)
+	if err := sys.MkdirAllWithACL(config.State, 0711); err != nil {
+		return err
+	}
+
+	if config.TempDir != "" {
+		if err := sys.MkdirAllWithACL(config.TempDir, 0711); err != nil {
+			return err
+		}
+		if runtime.GOOS == "windows" {
+			// On Windows, the Host Compute Service (vmcompute) will read the
+			// TEMP/TMP setting from the calling process when creating the
+			// tempdir to extract an image layer to. This allows the
+			// administrator to align the tempdir location with the same volume
+			// as the snapshot dir to avoid a copy operation when moving the
+			// extracted layer to the snapshot dir location.
+			os.Setenv("TEMP", config.TempDir)
+			os.Setenv("TMP", config.TempDir)
+		} else {
+			os.Setenv("TMPDIR", config.TempDir)
+		}
+	}
+	return nil
 }
 
 // New creates and initializes a new containerd server