syscontainers: environment variable detection

Add functionality to detect environment variables in template files.
The variables that can be set by the user through --set is displayed
on 'atomic images info'. The variables with overridable default
values will either have their default value displayed, or shown as
{SET_BY_OS} if atomic sets them. Variables without default value
must be set by the user and is listed separately.

Currently install --system already checks for variables that have no
value after install, and will error out.

Signed-off-by: Yu Qi Zhang <jerzhang@redhat.com>

Closes: #752
Approved by: giuseppe

Atomic/info.py

@@ -120,6 +120,16 @@ class Info(Atomic):
                     labels = manifest["Labels"]
                     for label in labels:
                         buf += ('\n{0}: {1}'.format(label, labels[label]))
+
+                    template_variables, template_variables_to_set = self.syscontainers.get_template_variables(self.image)
+                    buf += ("\n\nEnvironment variables with default value, but overridable with --set:")
+                    for variable in template_variables.keys():
+                        buf += ('\n{}: {}'.format(variable, template_variables.get(variable)))
+
+                    if template_variables_to_set:
+                        buf += ("\n\nEnvironment variables that has no default value, and must be set with --set:")
+                        for variable in template_variables_to_set.keys():
+                            buf += ('\n{}: {}'.format(variable, template_variables_to_set.get(variable)))
                     return buf
             # Check if the input is an image id associated with more than one
             # repotag.  If so, error out.
@@ -141,6 +151,16 @@ class Info(Atomic):
                     labels = manifest["Labels"]
                     for label in labels:
                         buf += ('\n{0}: {1}'.format(label, labels[label]))
+
+                    template_variables, template_variables_to_set = self.syscontainers.get_template_variables(self.image)
+                    buf += ("\n\nEnvironment variables with default value, but overridable with --set:")
+                    for variable in template_variables.keys():
+                        buf += ('\n{}: {}'.format(variable, template_variables.get(variable)))
+
+                    if template_variables_to_set:
+                        buf += ("\n\nEnvironment variables that has no default value, and must be set with --set:")
+                        for variable in template_variables_to_set.keys():
+                            buf += ('\n{}: {}'.format(variable, template_variables_to_set.get(variable)))
                     return buf
             else:
                 self._no_such_image()

Atomic/syscontainers.py

@@ -52,6 +52,9 @@ WorkingDirectory=$DESTDIR
 [Install]
 WantedBy=multi-user.target
 """
+TEMPLATE_FORCED_VARIABLES = ["DESTDIR", "NAME", "EXEC_START", "EXEC_STOP",
+                             "HOST_UID", "HOST_GID"]
+TEMPLATE_OVERRIDABLE_VARIABLES = ["RUN_DIRECTORY", "STATE_DIRECTORY"]
 
 class SystemContainers(object):
 
@@ -624,6 +627,66 @@ class SystemContainers(object):
             ret.append(container)
         return ret
 
+    def get_template_variables(self, image):
+        repo = self._get_ostree_repo()
+        _, commit_rev = self._resolve_image(repo, image)
+        if not commit_rev:
+            return
+
+        manifest = self._image_manifest(repo, commit_rev)
+        layers = SystemContainers.get_layers_from_manifest(json.loads(manifest))
+        templates = {}
+        manifest_template = None
+        for i in layers:
+            layer = i.replace("sha256:", "")
+            commit = repo.read_commit(repo.resolve_rev("%s%s" % (OSTREE_OCIIMAGE_PREFIX, layer), True)[1])[1]
+            exports = commit.get_root().get_child("exports")
+            if not exports.query_exists():
+                continue
+
+            children = exports.enumerate_children("", Gio.FileQueryInfoFlags.NONE, None)
+            for child in reversed(list(children)):
+                name = child.get_name()
+                if name == "manifest.json":
+                    manifest_template = exports.get_child(name).read()
+
+                if name.endswith(".template"):
+                    if name.startswith(".wh"):
+                        name = name[4:]
+                        templates.pop(name, None)
+                    else:
+                        templates[name] = exports.get_child(name).read()
+
+        variables = {}
+        for v in templates.values():
+            fd = v.get_fd()
+            with os.fdopen(fd) as f:
+                data = f.read()
+                template = Template(data)
+                for variable in ["".join(x) for x in template.pattern.findall(data)]: # pylint: disable=no-member
+                    if variable not in TEMPLATE_FORCED_VARIABLES:
+                        variables[variable] = variable
+
+        variables_with_default = {}
+        if manifest_template:
+            fd = manifest_template.get_fd()
+            with os.fdopen(fd) as f:
+                data = json.loads(f.read())
+                for variable in data['defaultValues']:
+                    variables_with_default[variable] = data['defaultValues'][variable]
+
+        # Also include variables that are set by the OS
+        # but can be overriden by --set
+        for variable in TEMPLATE_OVERRIDABLE_VARIABLES:
+            variables_with_default[variable] = "{SET_BY_OS}"
+
+        variables_to_set = {}
+        for variable in variables:
+            if variable not in variables_with_default:
+                variables_to_set[variable] = "{DEF_VALUE}"
+
+        return variables_with_default, variables_to_set
+
     def delete_image(self, image):
         repo = self._get_ostree_repo()
         if not repo:

docs/atomic-images.1.md

@@ -26,6 +26,8 @@ atomic images allows the user to view and operate on container images in a docke
 
   Displays the LABEL fields within an image. By default, it will check first for a local image and then all configured registries.
 
+  For a system container image, this will also display the environment variables a user can set.
+
 **help**
 
   Displays a help file associated with a container or image.