fix(git-node): handle multi-line trailers

lib/landing_session.js

@@ -316,35 +316,43 @@ 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 originalTrailers = runSync('git', [
+    // commit message and see if it outputs any trailers.
+    const interpretTrailers = commitMessage => runSync('git', [
       'interpret-trailers', '--parse', '--no-divider'
     ], {
-      input: `${original}\n`
-    }).split('\n').map(trailer => {
-      const separatorIndex = trailer.indexOf(':');
-      return [trailer.slice(0, separatorIndex), trailer.slice(separatorIndex + 2)];
-    }).slice(0, -1); // Remove the last line return entry
+      input: `${commitMessage}\n`
+    });
 
+    const originalTrailers = interpretTrailers(original);
     const containCVETrailer = CVE_RE.test(originalTrailers);
 
     const filterTrailer = (line) => ([key]) =>
       line.startsWith(key) &&
           new RegExp(`^${RegExp.escape?.(key) ?? key}\\s*:`).test(line);
-    const amended = original.trim().split('\n')
-      .filter(line =>
-        !line.includes(':') ||
-        !originalTrailers.some(filterTrailer(line)));
-    for (let i = amended.length - 1; amended[i] === ''; i--) {
+    const amended = original.trim().split('\n');
+    const stillInTrailers = () => {
+      const result = interpretTrailers(amended.join('\n'));
+      return result.length && originalTrailers.startsWith(result.trim());
+    };
+    for (let i = amended.length - 1; amended[i] === '' || stillInTrailers(); i--) {
+      // Remove last line until git no longer detects any trailers
       amended.pop();
     }
 
     // Only keep existing trailers that we won't add ourselves
     const trailersToFilterOut = ['BACKPORT-PR-URL', 'REVIEWED-BY'];
     if (!this.backport) trailersToFilterOut.push('PR-URL');
-    const keptTrailers = originalTrailers.filter(
-      ([key]) => !trailersToFilterOut.includes(key.toUpperCase())
-    );
+    const keptTrailers =
+      originalTrailers
+        .split('\n')
+        .map(trailer => {
+          const separatorIndex = trailer.indexOf(':');
+          return [trailer.slice(0, separatorIndex), trailer.slice(separatorIndex + 2)];
+        })
+        .slice(0, -1) // Remove the last line return entry
+        .filter(
+          ([key]) => !trailersToFilterOut.includes(key.toUpperCase())
+        );
     amended.push('', ...keptTrailers.map(([key, value]) => `${key}: ${value}`));
 
     for (const line of metadata.trim().split('\n')) {

test/unit/amend.test.js

@@ -112,6 +112,26 @@ describe('LandingSession.prototype.generateAmendedMessage', () => {
     t.assert.strictEqual(result, '\nPR-URL: http://example.com/123\nReviewed-By: user1 <collab@mail.net>');
   });
 
+  it('should handle multi-line trailers', async(t) => {
+    const session = createSession();
+    const result = await session.generateAmendedMessage(
+      'subsystem: foobar\n\nSigned-off-by: user1\n  <foobar@users.noreply.github.com>\n'
+    );
+
+    t.assert.strictEqual(result,
+      'subsystem: foobar\n\nSigned-off-by: user1 <foobar@users.noreply.github.com>\nPR-URL: http://example.com/123\nReviewed-By: user1 <collab@mail.net>');
+  });
+
+  it('should not remove lines that look like trailers in the commit body', async(t) => {
+    const session = createSession();
+    const result = await session.generateAmendedMessage(
+      'subsystem: foobar\n\nNot-A-Trailer: http://example.com/\n\nSigned-off-by: user1\n  <foobar@users.noreply.github.com>\n'
+    );
+
+    t.assert.strictEqual(result,
+      'subsystem: foobar\n\nNot-A-Trailer: http://example.com/\n\nSigned-off-by: user1 <foobar@users.noreply.github.com>\nPR-URL: http://example.com/123\nReviewed-By: user1 <collab@mail.net>');
+  });
+
   it('should handle cherry-pick from upstream', async(t) => {
     const session = createSession({ metadata: 'Refs: https://github.com/v8/v8/commit/cf1bce40a5ef4c7c1da351754f5bf526c0c96463\n' });
     const result = await session.generateAmendedMessage(`deps: V8: cherry-pick cf1bce40a5ef