🏗️🔧：read a wrapped trailer the way git does

build/shared/commit-message.mts

@@ -221,7 +221,25 @@ const trailerBlockOf = (paragraphs: string[][]) => {
       (line) => TRAILER_LINE.test(line) || CONTINUATION_LINE.test(line)
     );
 
-  return isBlock ? last.filter((line) => !CONTINUATION_LINE.test(line)) : [];
+  // A folded trailer is one trailer, not a trailer and a stray line. git
+  // joins the indented line onto the value, and `git interpret-trailers
+  // --parse` prints the two back as one, so this does the same. Dropping the
+  // continuation instead would put whatever got wrapped out of reach of every
+  // check below -- an address most of all, which is the half that says who a
+  // trailer names.
+  return isBlock
+    ? last.reduce<string[]>((folded, line) => {
+        const previous = folded.at(-1);
+
+        if (previous !== undefined && CONTINUATION_LINE.test(line)) {
+          folded[folded.length - 1] = `${previous} ${line.trim()}`;
+        } else {
+          folded.push(line);
+        }
+
+        return folded;
+      }, [])
+    : [];
 };
 
 /**
@@ -454,12 +472,12 @@ export function validateCommitMessage(message: string) {
     problems.push('the line after the subject has to be blank');
   }
 
-  // Where the trailer block starts, or past the end when there is none. A
-  // trailer is one line by construction here: git would read a folded value,
-  // but `readTrailers` keeps only the token line, so wrapping a long
-  // `Signed-off-by:` to fit would put the author out of reach of the sign-off
-  // check. The block is exempt from the width limit rather than made to fit
-  // inside it, which is also what the limit is for -- prose that is read.
+  // Where the trailer block starts, or past the end when there is none. The
+  // block is exempt from the width limit rather than made to fit inside it,
+  // which is what the limit is for -- prose that is read. A trailer long
+  // enough to need wrapping is wrapped by nobody here, and one that arrives
+  // wrapped anyway is read whole, since `trailerBlockOf` folds it back the
+  // way git does.
   const paragraphs = paragraphsOf(rest);
   const blockStart =
     trailerBlockOf(paragraphs).length > 0

build/shared/commit-message.test.mts

@@ -13,6 +13,7 @@ import {
   ACTIONS,
   BODY_MAX,
   CATEGORIES,
+  checkSignOff,
   readTrailers,
   validateCommitMessage,
 } from '@openinf/portal/build/commit-message';
@@ -308,6 +309,31 @@ describe('validateCommitMessage: the trailers', () => {
     );
   });
 
+  test('rejects an assistant whose address is on a folded line', () => {
+    // git joins the indented line onto the value and reads one trailer, so a
+    // wrapped address is still the address. Keeping only the token line would
+    // have made folding a way around the check.
+    match(
+      soleProblem(
+        '🏗️🔧：fix it\n\nCo-authored-by: Some Person\n <noreply@anthropic.com>'
+      ),
+      /credits a tool with authorship/
+    );
+  });
+
+  test('reads a folded sign-off as the whole address', () => {
+    // The same fold, on the trailer whose value the sign-off check compares
+    // against the author. Dropping the continuation put the author out of
+    // reach and the sign-off passed for nobody.
+    deepStrictEqual(
+      checkSignOff(
+        '🏗️🔧：fix it\n\nSigned-off-by: Ada Lovelace\n <ada@example.com>',
+        'Ada Lovelace <ada@example.com>'
+      ),
+      []
+    );
+  });
+
   test('rejects a token this project does not use', () => {
     match(
       soleProblem('🏗️🔧：fix it\n\nCloses: https://x/1'),
@@ -430,8 +456,9 @@ describe('the vocabulary', () => {
 
 describe('the width limit', () => {
   test('leaves a trailer that cannot be wrapped alone', () => {
-    // Folding this to fit would put the author on a continuation line, where
-    // `readTrailers` does not look and so `checkSignOff` could not match it.
+    // Trailers are metadata rather than prose, and the limit is for prose.
+    // Folding this one to fit would be read correctly either way, but there
+    // is no reason to make somebody wrap an address to please a linter.
     const long =
       'Signed-off-by: Christopher Alexander Montgomery ' +
       '<christopher.montgomery@example.org>';