add(sdk_util.sh): Port basic SDK download logic to a shell library.

The current logic for downloading SDK tarballs is in cros_sdk and
written in python which isn't super convenient for re-using in the rest
of our shell scripts. This is a start of rewriting that logic into a
re-usable library but does not yet replace the functionality in cros_sdk.

sdk_lib/sdk_util.sh

@@ -0,0 +1,60 @@
+#!/bin/bash
+
+# Copyright (c) 2013 The CoreOS Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# common.sh must be properly sourced before this file.
+[[ -n "${COREOS_SDK_VERSION}" ]] || exit 1
+
+COREOS_SDK_ARCH="amd64" # We are unlikely to support anything else.
+COREOS_SDK_TARBALL="coreos-sdk-${COREOS_SDK_ARCH}-${COREOS_SDK_VERSION}.tar.bz2"
+COREOS_SDK_TARBALL_CACHE="${REPO_CACHE_DIR}/sdks"
+COREOS_SDK_TARBALL_PATH="${COREOS_SDK_TARBALL_CACHE}/${COREOS_SDK_TARBALL}"
+COREOS_SDK_URL_PREFIX="http://storage.core-os.net/coreos/sdk"
+COREOS_SDK_URL="${COREOS_SDK_URL_PREFIX}/${COREOS_SDK_ARCH}/${COREOS_SDK_VERSION}/${COREOS_SDK_TARBALL}"
+
+# Download the current SDK tarball (if required) and verify digests/sig
+sdk_download_tarball() {
+    if sdk_verify_digests; then
+        return 0
+    fi
+
+    info "Downloading ${COREOS_SDK_TARBALL}"
+    info "URL: ${COREOS_SDK_URL}"
+    local suffix
+    for suffix in "" ".DIGESTS"; do # TODO(marineam): download .asc
+        wget --tries=3 --timeout=30 --continue \
+            -O  "${COREOS_SDK_TARBALL_PATH}${suffix}" \
+            "${COREOS_SDK_URL}${suffix}" \
+            || die_notrace "SDK download failed!"
+    done
+
+    sdk_verify_digests || die_notrace "SDK digest verification failed!"
+    sdk_clean_cache
+}
+
+sdk_verify_digests() {
+    if [[ ! -f "${COREOS_SDK_TARBALL_PATH}" || \
+          ! -f "${COREOS_SDK_TARBALL_PATH}.DIGESTS" ]]; then
+        return 1
+    fi
+
+    # TODO(marineam): Add gpg signature verification too.
+
+    verify_digests "${COREOS_SDK_TARBALL_PATH}" || return 1
+}
+
+sdk_clean_cache() {
+    pushd "${COREOS_SDK_TARBALL_CACHE}" >/dev/null
+    local filename
+    for filename in *; do
+        if [[ "${filename}" == "${COREOS_SDK_TARBALL}"* ]]; then
+            continue
+        fi
+        info "Cleaning up ${filename}"
+        # Not a big deal if this fails
+        rm -f "${filename}" || true
+    done
+    popd >/dev/null
+}