Added --squeeze argument to focker image build.

example/squeeze/Fockerfile

@@ -0,0 +1,14 @@
+base: freebsd-latest
+
+steps:
+  - run:
+      - touch /a
+      - touch /b
+      - touch /c
+  - run:
+      - touch /d
+      - touch /e
+      - touch /f
+  - copy:
+      - [ files/g, /g ]
+

example/squeeze/files/g

@@ -0,0 +1,2 @@
+foo bar
+

focker/focker.py

@@ -83,6 +83,7 @@ def create_parser():
     parser.set_defaults(func=command_image_build)
     parser.add_argument('focker_dir', type=str)
     parser.add_argument('--tags', '-t', type=str, nargs='+', default=[])
+    parser.add_argument('--squeeze', '-s', action='store_true')
 
     parser = ListForwarder([subparsers.add_parser(cmd) for cmd in ['tag', 't']])
     parser.set_defaults(func=command_image_tag)

focker/image.py

@@ -12,15 +12,55 @@ from .steps import create_step
 from .snapshot import new_snapshot
 from tabulate import tabulate
 import subprocess
+from .misc import find_prefix
 
 
-def build(spec, args):
+def validate_spec(spec):
     if 'base' not in spec:
         raise ValueError('Missing base in specification')
 
     if 'steps' not in spec:
         raise ValueError('Missing steps in specification')
 
+
+def build_squeeze(spec, args):
+    validate_spec(spec)
+
+    base = spec['base']
+    base, sha256 = zfs_find(base, focker_type='image', zfs_type='snapshot')
+
+    root = '/'.join(base.split('/')[:-1])
+    print('base:', base, 'root:', root)
+
+    steps = spec['steps']
+    if not isinstance(steps, list):
+        steps = [ steps ]
+
+    for st in steps:
+        st = create_step(st)
+        sha256 = st.hash(sha256, args=args)
+
+    if zfs_exists_snapshot_sha256(sha256):
+        name = zfs_snapshot_by_sha256(sha256)
+        print('Reusing:', name)
+        return (name, sha256)
+
+    name = find_prefix(root + '/', sha256)
+
+    def atomic():
+        for st in steps:
+            st = create_step(st)
+            st.execute(zfs_mountpoint(name), args=args)
+        zfs_set_props(name,
+            { 'focker:sha256': sha256 })
+
+    name = new_snapshot(base, atomic, name)
+    return (name, sha256)
+
+
+def build(spec, args):
+    validate_spec(spec)
+
     base = spec['base']
     base, base_sha256 = zfs_find(base, focker_type='image', zfs_type='snapshot')
 
@@ -67,7 +107,8 @@ def command_image_build(args):
     with open(fname, 'r') as f:
         spec = yaml.safe_load(f)
     print('spec:', spec)
-    image, image_sha256 = build(spec, args)
+    image, image_sha256 = build_squeeze(spec, args) \
+        if args.squeeze else build(spec, args)
     zfs_untag(args.tags)
     zfs_tag(image.split('@')[0], args.tags)