Add check_internet_connection function

Make sure that the system is online, by checking system's domain name
resolvability, with the help of nslookup tool. It waits up to 30
seconds (by default), while probing known worl-wide resilient internet
domains, to try to make up for loss of connectivity - especially for
systems with suspension enabled where after waking up they don't come
back online quick enough - before doing any operation that involves
downloading data with Zypper.

This feature also acts as a short-circuit for Zypper transactions bound
to fail due to working connection unavailability.

sbin/transactional-update.in

@@ -99,11 +99,76 @@ declare -A ROLES=()
 REPOS=()
 declare -A BUILDTIME=()
 
+# This should make TU more resilient in systems that have sleep/suspension ON,
+# as some systems with deep sleep capable network chipsets won't have their
+# internet connection back soon enough and the system update will fail while
+# trying to refresh repositories metadata. Also, this should short-circuit
+# Zypper transactions bound to fail, because Zypper will only know there's no
+# working internet connection after trying... and trying, for the most cases.
+check_internet_connection() {
+    # check_internet_connection() accepts up to two arguments:
+    # ARG1 NUMBER_OF_TIMES_TO_TRY (6, by default)
+    # ARG2 TIME_TO_WAIT_BETWEEN_TRIES (in seconds and 5, by default)
+
+    log_info "Trying to establish whether system can resolve domain names..."
+
+    if ! command -v nslookup > /dev/null; then
+        log_info "nslookup tool is needed, but wasn't found in PATH." \
+                 "Skipping internet connection check!"
+    else
+        # Use domains known to be resilient worldwide
+        lookup_sites=(www.google.com www.bing.com duckduckgo.com)
+        nslookup_status="failure"
+        # Retrying 6 times while waiting 5 secs, in between, means a total of
+        # 30 secs of waiting (total sleep time).
+        retries="${1:-6}"
+        default_retries="$retries"
+        sleep_time="${2:-5}"
+
+        run_nslookup() {
+            # We start with nslookup_status=failure, settting it to "succes" in
+            # case any of the domain lookup is successful to break out of the
+            # loop.
+            for domain in "${lookup_sites[@]}"; do
+                if nslookup $domain > /dev/null; then
+                    nslookup_status="success"
+                    return
+                fi
+            done
+            return 1
+        }
+
+        while [[ $nslookup_status == failure ]]; do
+            # On nslookup succeess, break out of the loop
+            if run_nslookup; then
+                log_info "System can resolve domain names. Proceeding!"
+                break
+            fi
+
+            # After running out of retries, bail out
+            if ! [[ $retries -gt 0 ]]; then
+                log_error "ERROR: System is either offline or cannot resolve" \
+                          "domain names. Aborting transaction!"
+                quit 1
+            fi
+
+            if [[ $retries -eq $default_retries ]]; then
+                log_info "Retrying up to $retries more times, waiting" \
+                         "$sleep_time secs in between."
+            fi
+
+            retries=$((retries-1))
+            sleep $sleep_time
+        done
+    fi
+}
+
 self_update() {
     if [ ${DO_SELF_UPDATE} == 0 ]; then
 	return
     fi
 
+    check_internet_connection
     log_info "Checking for newer version."
     if zypper --non-interactive info transactional-update | grep -q '^Status *: out-of-date'; then
 	log_info "New version found - updating..."
@@ -1194,6 +1259,7 @@ if [ -z "${TA_UPDATE_TMPFILE}" ]; then
 else # Set exit handler to clean up artifacts of the self-update
     trap 'rm -f "$LOCKFILE" && rm -rf "${TA_UPDATE_TMPFILE}" && unset TA_UPDATE_TMPFILE' EXIT
     pushd "${TA_UPDATE_TMPFILE}" >/dev/null
+    check_internet_connection
     zypper --non-interactive --pkg-cache-dir "${TA_UPDATE_TMPFILE}" download libtukit4 tukit
     find . -name *.rpm -exec sh -c 'rpm2cpio {} | cpio -idmv 2>/dev/null' \;
     popd >/dev/null
@@ -1458,6 +1524,7 @@ if [ -n "${ZYPPER_ARG}" -o ${REWRITE_GRUB_CFG} -eq 1 \
 	if [ ${DO_MIGRATION} -eq 1 ]; then
 	    # transactional-update migration
 	    export DISABLE_RESTART_ON_UPDATE=yes
+        check_internet_connection
 	    tukit ${TUKIT_OPTS} call "${SNAPSHOT_ID}" zypper ${ZYPPER_ARG} ${ZYPPER_NONINTERACTIVE} "${ZYPPER_ARG_PKGS[@]}" |& tee -a ${LOGFILE} 1>&${origstdout}
 	    RETVAL=${PIPESTATUS[0]}
 	else
@@ -1467,6 +1534,7 @@ if [ -n "${ZYPPER_ARG}" -o ${REWRITE_GRUB_CFG} -eq 1 \
 		zypper_cmd="tukit ${TUKIT_OPTS} call ${SNAPSHOT_ID} zypper"
 	    fi
 	    # Check if there are updates at all.
+        check_internet_connection
 	    TMPFILE=`mktemp ${TMPDIR}/transactional-update.XXXXXXXXXX`
 	    ${zypper_cmd} --xmlout ${ZYPPER_ARG} -y --auto-agree-with-product-licenses --dry-run "${ZYPPER_ARG_PKGS[@]}" > ${TMPFILE}
 	    PACKAGE_UPDATES=`grep "install-summary download-size" ${TMPFILE} | sed -e 's|.*install-summary download-size=\"\(.*\)\" space-usage-diff.*|\1|g'`