35coreos-live/coreos-livepxe-rootfs: retry fetching rootfs forever

We don't know how long it will take for networking to come up, so let's
match Ignition's semantic of retrying to fetch remote resources forever.

`curl` doesn't natively have support for this, so wrap it in a `while`
loop. This also works around the fact that el8's `curl` doesn't support
`--retry-all-errors`.

This was previously added in coreos#929.

(cherry picked from commit d2776ed)

overlay.d/05core/usr/lib/dracut/modules.d/35coreos-live/coreos-livepxe-rootfs.sh

@@ -34,20 +34,24 @@ elif [[ -n "${rootfs_url}" ]]; then
     # First, reach out to the server to verify connectivity before
     # trying to download and pipe content through other programs.
     # Doing this allows us to retry all errors (including transient
-    # "no route to host" errors during startup), without using the
-    # --retry-all-errors, which is problematic (see curl man page)
-    # when piping the output.
-    curl_common_args="--silent --show-error --insecure --location --retry 5"
-    if ! curl --head --retry-all-errors $curl_common_args "${rootfs_url}" >/dev/null; then
+    # "no route to host" errors during startup). Note we can't use
+    # curl's --retry-all-errors here because it's not in el8's curl yet.
+    # We retry forever, matching Ignition's semantics.
+    curl_common_args="--silent --show-error --insecure --location"
+    while ! curl --head $curl_common_args "${rootfs_url}" >/dev/null; do
         echo "Couldn't establish connectivity with the server specified by coreos.live.rootfs_url=" >&2
-        echo "Check that the URL is correct and can be reached." >&2
-        exit 1
-    fi
+        echo "Retrying in 5s..." >&2
+        sleep 5
+    done
+
     # We don't need to verify TLS certificates because we're checking the
     # image hash.
     # bsdtar can read cpio archives and we already depend on it for
     # coreos-liveiso-persist-osmet.service, so use it instead of cpio.
-    if ! curl $curl_common_args "${rootfs_url}" | \
+    # We shouldn't need a --retry here since we've just successfully HEADed the
+    # file, but let's add one just to be safe (e.g. if the connection just went
+    # online and flickers or something).
+    if ! curl $curl_common_args --retry 5 "${rootfs_url}" | \
             rdcore stream-hash /etc/coreos-live-want-rootfs | \
             bsdtar -xf - -C / ; then
         echo "Couldn't fetch, verify, and unpack image specified by coreos.live.rootfs_url=" >&2