SERVER-101171 Make copybara preserve Co-authored-by lines (#32756)

GitOrigin-RevId: 593f9c86851700c5415aaf25a84078d4136f9cbc

Author: devkev

copy.bara.sky

@@ -23,12 +23,40 @@ core.workflow(
     origin_files = glob(["**"], exclude = ["src/mongo/db/modules/**"]),
     authoring = authoring.pass_thru("MongoDB <[email protected]>"),
     mode = "ITERATIVE",
-    # Change the path here to the folder you want to publish publicly
     transformations = [
-        # (^.*?) - matches the first line (without the newline char)
-        # \n - matches the first newline (or nothing at all if there is no newline). If there is no match then nothing happens
-        # ((\n|.)*) - matches everything after
-        # Overall, this copies only the first line of the commit rather than the body
-        metadata.scrubber("(^.*?)\n((\n|.)*)", replacement = "$1"),
+        # The transformation rules below will remove the commit message's body, while preserving
+        # any trailer lines that have acceptable keys (Co-authored-by, etc).  The first line of
+        # the commit message (ie. the commit's subject/summary line) is also always retained.
+        #
+        # Achieving this requires a 4 step process.
+        #
+        # STEP 1: Non-initial lines (ie. those which appear after a newline char) that start with
+        # an acceptable trailer key are preserved, but any other lines have their content removed
+        # (leaving a blank line).
+        #
+        # This works because the first .* (inside capture group 1) only matches if the line starts
+        # with a trailer key.  If it doesn't, then the line is fully consumed by the second .*
+        # (which is outside the capture group, and therefore $1 is empty).
+        metadata.scrubber(
+            "\n((?:Co-authored-by|Signed-off-by|Reviewed-by): .*)?.*",
+            replacement = "\n$1",
+        ),
+        #
+        # STEP 2: Remove blank lines, ie. sequences of newlines get condensed down to one newline.
+        metadata.scrubber("\n+", replacement = "\n"),
+        #
+        # STEP 3: Remove any trailing newline.
+        metadata.scrubber("\n$", replacement = ""),
+        #
+        # STEP 4: If there are trailer lines (ie. if the first line has a newline followed by more
+        # text), then add an extra blank line after the first line, ie. to separate the commit's
+        # trailers from the subject line.
+        #
+        # The first capture group (^.*?\n) is the first line (non-greedily consuming chars until a
+        # newline), while the second capture group ((\n|.)*) is the rest of the message (greedily
+        # consume all chars, including newlines).
+        # If there are no trailers, then there will only be a single line of text, with no newline
+        # chars, and so the pattern will fail to match.
+        metadata.scrubber("(^.*?\n)((?:\n|.)*)", replacement = "$1\n$2"),
     ],
 )