fix(git-node): fix handling of existing trailers

Author: aduh95

lib/landing_session.js

@@ -316,7 +316,7 @@ export default class LandingSession extends Session {
     const BACKPORT_RE = /^BACKPORT-PR-URL\s*:\s*(\S+)$/i;
     const PR_RE = /^PR-URL\s*:\s*(\S+)$/i;
     const REVIEW_RE = /^Reviewed-By\s*:\s*(\S+)$/i;
-    const CVE_RE = /^CVE-ID\s*:\s*(\S+)$/i;
+    const CVE_RE = /^CVE-ID\s*:\s*(\S+)$/im;
 
     this.startAmending();
 
@@ -328,41 +328,31 @@ export default class LandingSession extends Session {
     // git has very specific rules about what is a trailer and what is not.
     // Instead of trying to implement those ourselves, let git parse the
     // original commit message and see if it outputs any trailers.
-    const originalHasTrailers = runSync('git', [
+    const originalTrailers = runSync('git', [
       'interpret-trailers', '--parse', '--no-divider'
     ], {
       input: `${original}\n`
-    }).trim().length !== 0;
+    });
+    const containCVETrailer = CVE_RE.test(originalTrailers);
 
     const metadata = this.metadata.trim().split('\n');
-    // Filtering out metadata that we will add ourselves:
-    const isFiltered = line =>
+    // Only keep existing trailers that we won't add ourselves
+    const shouldKeepTrailer = line =>
+      !!line &&
       (!PR_RE.test(line) || this.backport) &&
       !BACKPORT_RE.test(line) &&
       !REVIEW_RE.test(line);
-    const amended = original.split('\n').filter(isFiltered);
-
-    // If the original commit message already contains trailers (such as
-    // "Co-authored-by"), we simply add our own metadata after those. Otherwise,
-    // we have to add an empty line so that git recognizes our own metadata as
-    // trailers in the amended commit message.
-    if (!originalHasTrailers) {
-      amended.push('');
-    }
+    const amended = original.slice(0, -originalTrailers.length).trim().split('\n');
+
+    amended.push('', ...originalTrailers.split('\n').filter(shouldKeepTrailer));
 
-    let containCVETrailer = false;
     for (const line of metadata) {
-      if (line.length !== 0 && original.includes(line) && !isFiltered(line)) {
-        containCVETrailer ||= CVE_RE.test(line);
-        if (originalHasTrailers) {
-          cli.warn(`Found ${line}, skipping..`);
-          continue;
-        } else {
-          cli.error('Git found no trailers in the original commit message, ' +
+      if (originalTrailers.includes(line)) {
+        cli.error('Git found no trailers in the original commit message, ' +
                     `but '${line}' is present and should be a trailer.`);
-          process.exit(1);  // make it work with git rebase -x
-        }
+        process.exit(1);  // make it work with git rebase -x
       }
+
       if (BACKPORT_RE.test(line)) {
         const prIndex =
           (amended.findIndex(datum => PR_RE.test(datum)) + 1) ||