[ci] Don't update Jenkinsfile timestamp on image updates (apache#12621)

The timestamp in the Jenkinsfile is there to prevent post-merge
conflicts from different PRs that edit the templates merging
non-sequentially. This is not an issue when a line is edited in place
though, which is often the case when Docker image tags are updated. This
PR makes it so the timestamp is not updated in these cases which should
reduce merge conflicts on these types of PRs.

ci/jenkins/generate.py

@@ -18,11 +18,12 @@
 import jinja2
 import argparse
 import difflib
-import re
 import datetime
+import re
 import textwrap
 
 from pathlib import Path
+from typing import List
 
 
 REPO_ROOT = Path(__file__).resolve().parent.parent.parent
@@ -82,16 +83,62 @@ def lines_without_generated_tag(content):
     ]
 
 
+def is_changed_images_only(lines: List[str]) -> bool:
+    """
+    Return True if 'line' only edits an image tag or if 'line' is not a changed
+    line in a diff
+    """
+    added_images = []
+    removed_images = []
+    diff_lines = []
+
+    for line in lines[2:]:
+        if not line.startswith("-") and not line.startswith("+"):
+            # not a diff line, ignore it
+            continue
+
+        diff_lines.append(line)
+
+    if len(diff_lines) == 0:
+        # no changes made
+        return True
+
+    for line in diff_lines:
+        is_add = line.startswith("+")
+        line = line.strip().lstrip("+").lstrip("-")
+        match = re.search(
+            r"^(ci_[a-zA-Z0-9]+) = \'.*\'$",
+            line.strip().lstrip("+").lstrip("-"),
+            flags=re.MULTILINE,
+        )
+        if match is None:
+            # matched a non-image line, quit early
+            return False
+
+        if is_add:
+            added_images.append(match.groups()[0])
+        else:
+            removed_images.append(match.groups()[0])
+
+    # make sure that the added image lines match the removed image lines
+    return len(added_images) > 0 and added_images == removed_images
+
+
 if __name__ == "__main__":
     help = "Regenerate Jenkinsfile from template"
     parser = argparse.ArgumentParser(description=help)
+    parser.add_argument("--force", action="store_true", help="always overwrite timestamp")
     parser.add_argument("--check", action="store_true", help="just verify the output didn't change")
     args = parser.parse_args()
 
     with open(JENKINSFILE) as f:
         content = f.read()
 
     data["generated_time"] = datetime.datetime.now().isoformat()
+    timestamp_match = re.search(r"^// Generated at (.*)$", content, flags=re.MULTILINE)
+    if not timestamp_match:
+        raise RuntimeError("Could not find timestamp in Jenkinsfile")
+    original_timestamp = timestamp_match.groups()[0]
 
     environment = jinja2.Environment(
         loader=jinja2.FileSystemLoader(REPO_ROOT),
@@ -103,11 +150,18 @@ def lines_without_generated_tag(content):
     template = environment.get_template(str(JENKINSFILE_TEMPLATE.relative_to(REPO_ROOT)))
     new_content = template.render(**data)
 
-    diff = "".join(
-        difflib.unified_diff(
+    diff = [
+        line
+        for line in difflib.unified_diff(
             lines_without_generated_tag(content), lines_without_generated_tag(new_content)
         )
-    )
+    ]
+    if not args.force and is_changed_images_only(diff):
+        new_content = new_content.replace(data["generated_time"], original_timestamp)
+        print("Detected only Docker-image name changed, skipping timestamp update")
+
+    diff = "".join(diff)
+
     if args.check:
         if not diff:
             print("Success, the newly generated Jenkinsfile matched the one on disk")