VAULT-40224: pipeline(github): limit comments bodies and pull request messages to 65536 characters (#10771) (#10877)

* pipeline(github): limit comments bodies and pull request messages to 65536 characters

* comment/message cleanup

Co-authored-by: Luis (LT) Carbonell <lt.carbonell@hashicorp.com>

tools/pipeline/internal/pkg/github/copy_pull_request.go

@@ -303,6 +303,7 @@ func (r *CopyPullRequestReq) Run(
 		err = fmt.Errorf("creating copy pull request body %w", err)
 		return res, errors.Join(cherryPickErr, err)
 	}
+	limitedPRBody := limitCharacters(prBody)
 
 	res.PullRequest, _, err = github.PullRequests.Create(
 		ctx, r.ToOwner, r.ToRepo, &libgithub.NewPullRequest{
@@ -310,7 +311,7 @@ func (r *CopyPullRequestReq) Run(
 			Head:     &branchName,
 			HeadRepo: &r.ToRepo,
 			Base:     &baseRef,
-			Body:     &prBody,
+			Body:     &limitedPRBody,
 		},
 	)
 	if err != nil {

tools/pipeline/internal/pkg/github/create_backport.go

@@ -697,13 +697,14 @@ func (r *CreateBackportReq) backportRef(
 		res.Error = fmt.Errorf("creating backport pull request body %w", err)
 		return res
 	}
+	limitedPRBody := limitCharacters(prBody)
 	res.PullRequest, _, err = github.PullRequests.Create(
 		ctx, r.Owner, r.Repo, &libgithub.NewPullRequest{
 			Title:    &prTitle,
 			Head:     &branchName,
 			HeadRepo: &r.Repo,
 			Base:     &ref,
-			Body:     &prBody,
+			Body:     &limitedPRBody,
 		},
 	)
 	if err != nil {

tools/pipeline/internal/pkg/github/pull_request.go

@@ -30,9 +30,10 @@ func createPullRequestComment(
 	slog.Default().DebugContext(ctx, "creating pull request comment")
 
 	// Always try and write a comment on the pull request
+	limitedBody := limitCharacters(body)
 	comment, _, err := github.Issues.CreateComment(
 		ctx, owner, repo, pullNumber, &libgithub.IssueComment{
-			Body: &body,
+			Body: &limitedBody,
 		},
 	)
 	if err != nil {

tools/pipeline/internal/pkg/github/templates.go

@@ -6,6 +6,7 @@ package github
 import (
 	"bytes"
 	"embed"
+	"fmt"
 	"io"
 	"os"
 	"text/template"
@@ -71,3 +72,40 @@ func renderEmbeddedTemplate(name string, data any) (string, error) {
 
 	return buf.String(), nil
 }
+
+// limitCharacters truncates a string to maxGitHubMessageChars while preserving
+// markdown formatting integrity. It adds a truncation notice if truncation occurs.
+const maxGitHubMessageChars = 65536
+
+func limitCharacters(content string) string {
+	if len(content) <= maxGitHubMessageChars {
+		return content
+	}
+
+	// Define the truncation notice to calculate its length
+	truncationNotice := "\n\n---\n\n:scissors: **Message truncated due to GitHub's character limit**\n\n" +
+		fmt.Sprintf("This message was automatically truncated because it exceeded GitHub's %d character limit for "+
+			"comments and pull request descriptions.", maxGitHubMessageChars)
+
+	// Calculate how much space the notice needs
+	noticeLen := len(truncationNotice)
+	maxContentLen := maxGitHubMessageChars - noticeLen
+
+	// Find a good truncation point to avoid breaking markdown
+	truncateAt := maxContentLen
+	for truncateAt > 0 && content[truncateAt] != '\n' {
+		truncateAt--
+	}
+
+	// If we can't find a newline within reasonable bounds, just truncate
+	if truncateAt == 0 || (maxContentLen-truncateAt) > 1000 {
+		truncateAt = maxContentLen
+	}
+
+	// Ensure we don't go out of bounds
+	if truncateAt > len(content) {
+		truncateAt = len(content)
+	}
+
+	return content[:truncateAt] + truncationNotice
+}

tools/pipeline/internal/pkg/github/templates_character_limit_test.go

@@ -0,0 +1,83 @@
+// Copyright IBM Corp. 2016, 2025
+// SPDX-License-Identifier: BUSL-1.1
+
+package github
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_limitCharacters(t *testing.T) {
+	tests := []struct {
+		name     string
+		content  string
+		expected func(string) bool
+	}{
+		{
+			name:    "short content is unchanged",
+			content: "This is a short message",
+			expected: func(result string) bool {
+				return result == "This is a short message"
+			},
+		},
+		{
+			name:    "content at limit is unchanged",
+			content: strings.Repeat("a", maxGitHubMessageChars),
+			expected: func(result string) bool {
+				return len(result) == maxGitHubMessageChars && strings.HasPrefix(result, "aaa")
+			},
+		},
+		{
+			name:    "content over limit is truncated",
+			content: strings.Repeat("a", maxGitHubMessageChars+1000),
+			expected: func(result string) bool {
+				return len(result) <= maxGitHubMessageChars &&
+					strings.Contains(result, "Message truncated due to GitHub's character limit")
+			},
+		},
+		{
+			name:    "markdown content with newlines truncates at newline",
+			content: strings.Repeat("line\n", maxGitHubMessageChars/5+100),
+			expected: func(result string) bool {
+				return len(result) <= maxGitHubMessageChars &&
+					strings.Contains(result, "Message truncated due to GitHub's character limit") &&
+					strings.HasPrefix(result, "line\n")
+			},
+		},
+		{
+			name:    "very long line without newlines gets hard truncated",
+			content: strings.Repeat("a", maxGitHubMessageChars+100),
+			expected: func(result string) bool {
+				return len(result) <= maxGitHubMessageChars &&
+					strings.Contains(result, "Message truncated due to GitHub's character limit")
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := limitCharacters(tt.content)
+			assert.True(t, tt.expected(result), "Result does not match expected criteria")
+			assert.LessOrEqual(t, len(result), maxGitHubMessageChars, "Result exceeds maximum character limit")
+		})
+	}
+}
+
+func Test_limitCharacters_preserves_original_under_limit(t *testing.T) {
+	content := "# PR Title\n\nThis is the body content.\n\n## Details\n\nSome details here."
+	result := limitCharacters(content)
+	assert.Equal(t, content, result, "Short content should be unchanged")
+}
+
+func Test_limitCharacters_adds_truncation_notice(t *testing.T) {
+	longContent := strings.Repeat("This is a very long line that will exceed the GitHub character limit. ", maxGitHubMessageChars/70+10)
+	result := limitCharacters(longContent)
+
+	assert.LessOrEqual(t, len(result), maxGitHubMessageChars, "Result should not exceed max chars")
+	assert.Contains(t, result, ":scissors: **Message truncated due to GitHub's character limit**", "Should contain truncation notice")
+	assert.Contains(t, result, fmt.Sprintf("%d character limit", maxGitHubMessageChars), "Should mention the specific limit")
+}