wal-g: Add support for incremental backups.

This only defaults to on for local-disk backups, since they are more
disk-size-sensitive, and local accesses are quite cheap compared to
loading multiple incremental backups from S3.

puppet/zulip/files/postgresql/env-wal-g

@@ -5,6 +5,9 @@ fi
 
 export PGHOST=/var/run/postgresql/
 
+WALG_DELTA_MAX_STEPS=$(crudini --get /etc/zulip/zulip.conf postgresql backups_incremental 2>/dev/null)
+export WALG_DELTA_MAX_STEPS
+
 s3_backups_bucket=$(crudini --get "$ZULIP_SECRETS_CONF" secrets s3_backups_bucket 2>/dev/null)
 
 if [ "$s3_backups_bucket" != "" ]; then
@@ -30,6 +33,11 @@ else
     WALG_FILE_PREFIX=$(crudini --get /etc/zulip/zulip.conf postgresql backups_directory 2>/dev/null)
     if [ "$WALG_FILE_PREFIX" != "" ]; then
         export WALG_FILE_PREFIX
+        if [ "$WALG_DELTA_MAX_STEPS" = "" ]; then
+            # Default to only taking a full backup every week
+            export WALG_DELTA_MAX_STEPS=6
+        fi
+
     else
         echo "Could not determine where to back up data to!"
         exit 1