Add waiting on hosts in docker entrypoint for distributed setups. (#4244)

Thanks to Remco Verhoef <remco@dutchcoders.io> for the script.

Fixes #4225

buildscripts/docker-entrypoint.sh

@@ -15,15 +15,50 @@
 # limitations under the License.
 #
 
-# if command starts with an option, prepend minio
+# If command starts with an option, prepend minio.
 if [ "${1}" != "minio" ]; then
     if [ -n "${1}" ]; then
         set -- minio "$@"
     fi
 fi
 
-## Look for swarm secrets in default location.
-file_env() {
+# Wait for all the hosts to come online and have
+# their DNS entries populated properly.
+docker_wait_hosts() {
+    hosts="$@"
+    num_hosts=0
+    # Count number of hosts in arguments.
+    for host in $hosts; do
+        [ $(echo "$host" | grep -E "^http") ] || continue
+        num_hosts=$((num_hosts+1))
+    done
+    if [ $num_hosts -gt 0 ]; then
+        echo -n "Waiting for all hosts to resolve..."
+        while true; do
+            x=0
+            for host in $hosts; do
+                [ $(echo "$host" | grep -E "^http") ] || continue
+                # Extract the domain.
+                host=$(echo $host | sed -e 's/^http[s]\?:\/\/\([^\/]\+\).*/\1/')
+                echo -n .
+                val=$(ping -c 1 $host 2>/dev/null)
+                if [ $? != 0 ]; then
+                    echo "Failed to lookup $host"
+                    continue
+                fi
+                x=$((x+1))
+            done
+            # Provided hosts same as successful hosts, should break out.
+            test $x -eq $num_hosts && break
+            echo "Failed to resolve hosts.. retrying after 1 second."
+            sleep 1
+        done
+        echo "All hosts are resolving proceeding to initialize Minio."
+    fi
+}
+
+## Look for docker secrets in default documented location.
+docker_secrets_env() {
     local MINIO_ACCESS_KEY_FILE="/run/secrets/access_key"
     local MINIO_SECRET_KEY_FILE="/run/secrets/secret_key"
 
@@ -37,7 +72,10 @@ file_env() {
     fi
 }
 
-## Set env if necessary.
-file_env
+## Set access env from secrets if necessary.
+docker_secrets_env
+
+## Wait for all the hosts to come online.
+docker_wait_hosts "$@"
 
 exec "$@"