Add reusing of existing steps.

Fockerfile

@@ -1,6 +1,8 @@
-base: freebsd-12.1
+base: freebsd-latest
 
 steps:
   - run: |
       pkg -y install -y python3 && \
       pkg -y install -y py37-pip
+  - run: |
+      pkg install -y py37-yaml

image.py

@@ -38,6 +38,11 @@ def build(spec):
     for st in steps:
         st = create_step(st)
         st_sha256 = st.hash(base_sha256)
+        if zfs_exists_snapshot_sha256(st_sha256):
+            base = zfs_snapshot_by_sha256(st_sha256)
+            base_sha256 = st_sha256
+            print('Reusing:', base)
+            continue
         for pre in range(7, 64):
             name = root + '/' + st_sha256[:pre]
             if not zfs_exists(name):

zfs.py

@@ -54,6 +54,24 @@ def zfs_mountpoint(name):
     return lst[0][0]
 
 
+def zfs_exists_snapshot_sha256(sha256):
+    lst = zfs_parse_output(['zfs', 'list', '-o', 'focker:sha256', '-t', 'snap'])
+    lst = list(filter(lambda a: a[0] == sha256, lst))
+    if len(lst) == 0:
+        return False
+    return True
+
+
+def zfs_snapshot_by_sha256(sha256):
+    lst = zfs_parse_output(['zfs', 'list', '-o', 'focker:sha256,name', '-t', 'snap', '-H'])
+    lst = list(filter(lambda a: a[0] == sha256, lst))
+    if len(lst) == 0:
+        raise ValueError('Snapshot with given sha256 does not exist: ' + sha256)
+    if len(lst) > 1:
+        raise ValueError('Ambiguous snapshot sha256: ' + sha256)
+    return lst[0][1]
+
+
 def zfs_init():
     poolname = zfs_parse_output(['zfs', 'list', '-H', '/'])
     if len(poolname) == 0: