ui: Improve display for result diffs

Fixes: https://github.com/ansible-community/ara/issues/29

Change-Id: Id4442f0f9c4dcb5520f63970bb9fb17a03d5ec0d

ara/ui/templates/result.html

@@ -3,6 +3,7 @@
 {% include "partials/playbook_card.html" with playbook=result.playbook %}
 {% load pygments_highlights %}
 {% load datetime_formatting %}
+{% load diff_result %}
 
 <div class="pf-c-card" style="margin: 1em 0;">
     <div class="pf-c-card__header pf-c-title pf-m-md">
@@ -35,7 +36,11 @@
                         <a href="#{{ field }}">{{ field }}</a>
                     </td>
                     <td role="cell" data-label="Value" class="pf-m-width-80">
-                        {{ value | format_data | safe }}
+                        {% if field == "diff" %}
+                            {{ value | diff_result | format_data | safe }}
+                        {% else %}
+                            {{ value | format_data | safe }}
+                        {% endif %}
                     </td>
                 </tr>
                 {% endfor %}

ara/ui/templatetags/diff_result.py

@@ -0,0 +1,61 @@
+# Copyright (c) 2020 The ARA Records Ansible authors
+# GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
+
+import difflib
+import json
+
+from django import template
+
+register = template.Library()
+
+
+def render_diff(before="", after="", before_header="before", after_header="after"):
+    """
+    Renders a diff provided by Ansible task results
+    """
+    # fmt: off
+    # Some modules, such as file, might provide a diff in a dict format
+    if isinstance(before, dict) and isinstance(after, dict):
+        return difflib.unified_diff(
+            json.dumps(before, indent=4).splitlines(),
+            json.dumps(after, indent=4).splitlines(),
+            fromfile=before_header,
+            tofile=after_header
+        )
+    else:
+        return difflib.unified_diff(
+            before.splitlines(),
+            after.splitlines(),
+            fromfile=before_header,
+            tofile=after_header
+        )
+    # fmt: on
+
+
+@register.filter(name="diff_result")
+def diff_result(diff):
+    """
+    Renders a diff (or a list of diffs) provided by Ansible task results
+    """
+    diffs = []
+
+    # Modules are free to provide their own diff key which might not respect the convention set by modules such
+    # as "file", "ini_file", "lineinfile, "template" or "copy" causing a parsing failure.
+    # If that happens, give up and return value as it was provided so we don't raise an
+    # exception/internal server error.
+    try:
+        if isinstance(diff, list):
+            diffs = [render_diff(**result) for result in diff]
+        elif isinstance(diff, dict):
+            diffs = [render_diff(**diff)]
+    except (TypeError, AttributeError):
+        return diff
+
+    # The unified diff is a generator, we need to iterate through it to return
+    # the entire diff.
+    lines = []
+    for d in diffs:
+        for line in d:
+            lines.append(line)
+
+    return "\n".join(lines)