workflows/check-cherry-picks: truncate long diffs after 10k characters

GitHub comments have a length limit, so we can't just dump everything.
The 10k limit is arbitrary, but the assumption is that reviewing the
range-diff is not the sensible thing to do once it becomes a certain
size - reviewing the regular diff and treating the commit as "new" is
easier to do in that case. Thus, truncating should work out fine,
especially when the full range-diff is still available in the runner
log.

This could still end up in with an error, if a PR has multiple commits,
which all hit the limit. Let's get there first, before we try to fix
that hypothetical case, too.

.github/workflows/check-cherry-picks.yml

@@ -49,7 +49,7 @@ jobs:
             const header = await readFile('trusted/ci/check-cherry-picks.md')
             const body = await readFile('checked-cherry-picks.md')
             const footer =
-              `\n_Hint: The diffs are also available in the [runner logs](${job_url}) with slightly better highlighting._`
+              `\n_Hint: The full diffs are also available in the [runner logs](${job_url}) with slightly better highlighting._`
 
             const review = header + body + footer
             await writeFile('review.md', review)
@@ -79,6 +79,9 @@ jobs:
               )
             )
 
+            // Either of those two requests could fail for very long comments. This can only happen
+            // with multiple commits all hitting the truncation limit for the diff. If you ever hit
+            // this case, consider just splitting up those commits into multiple PRs.
             if (pendingReview) {
               await github.rest.pulls.updateReview({
                 owner: context.repo.owner,

ci/check-cherry-picks.sh

@@ -112,7 +112,18 @@ while read -r new_commit_sha ; do
           echo '> ```diff' >> $markdown_file
           # The output of `git range-diff` is indented with 4 spaces, which we need to match with the
           # code blocks indent to get proper syntax highlighting on GitHub.
-          $range_diff_common | tail -n +2 | sed -Ee 's/^ {4}/> /g' >> $markdown_file
+          diff="$($range_diff_common | tail -n +2 | sed -Ee 's/^ {4}/> /g')"
+          # Also limit the output to 10k bytes (and remove the last, potentially incomplete line), because
+          # GitHub comments are limited in length. The value of 10k is arbitrary with the assumption, that
+          # after the range-diff becomes a certain size, a reviewer is better off reviewing the regular diff
+          # in GitHub's UI anyway, thus treating the commit as "new" and not cherry-picked.
+          # Note: This could still lead to a too lengthy comment with multiple commits touching the limit. We
+          # consider this too unlikely to happen, to deal with explicitly.
+          max_length=10000
+          if [ "${#diff}" -gt $max_length ]; then
+            printf -v diff "%s\n\n[...truncated...]" "$(echo "$diff" | head -c $max_length | head -n-1)"
+          fi
+          echo "$diff" >> $markdown_file
           echo '> ```' >> $markdown_file
           echo "> </details>" >> $markdown_file