Check and split multistage dockerfile

Implemented two functions:
1. check_mutistage_dockerfile(): Given a dockerfile object, return if
it is a multistage dockerfile.
2. split_multistage_dockerfile(): Given a multistage dockerfile object,
return the splited dockerfile object list.

Works towards tern-tools#767.
Super issue tern-tools#612.

Signed-off-by: WangJL <hazard15020@gmail.com>

tern/analyze/docker/dockerfile.py

@@ -10,6 +10,7 @@
 from dockerfile_parse import DockerfileParser
 import re
 import logging
+import copy
 
 from tern.utils import general
 from tern.utils import constants
@@ -333,3 +334,32 @@ def write_locked_dockerfile(dfile, destination=None):
         file_name = constants.locked_dockerfile
     with open(file_name, 'w') as f:
         f.write(dfile)
+
+
+def check_mutistage_dockerfile(dfobj):
+    """Given a dockerfile object, return if it is a multistage dockerfile."""
+    from_line = [1 for c in dfobj.structure if c['instruction'] == 'FROM']
+    return len(from_line) > 1
+
+
+def split_multistage_dockerfile(dfobj):
+    """Given a multistage dockerfile object, return the splited dockerfile
+    object list."""
+    dfobj_list = []
+    # Dockerfile object to record.
+    dfobj_temp = Dockerfile()
+    dfobj_temp.envs = dfobj.envs
+    dfobj_temp.prev_env = dfobj.prev_env
+    dfobj_temp.file_path = "Split_dockerfile"
+    dfobj_temp.structure = []
+    for command_dict in dfobj.structure:
+        if command_dict['instruction'] == 'FROM':
+            if dfobj_temp.structure:
+                # dfobj_temp not empty, append to the list
+                dfobj_list.append(copy.copy(dfobj_temp))
+                dfobj_temp.structure = []
+        dfobj_temp.structure.append(command_dict)
+    if dfobj_temp.structure:
+        # dfobj_temp not empty, append to the list
+        dfobj_list.append(copy.copy(dfobj_temp))
+    return dfobj_list