fix(legacylibrarian): prevent false breaking change positives (#4783)

The separateBodyAndFooters function previously parsed commit lines
top-down to find the footer section. This caused body content containing
text that matches the footer pattern (such as Renovate release notes
documenting breaking changes) to be incorrectly swept into the footers
section, leading to false positives for breaking changes.

The parser now operates bottom-up from the end of the commit message to
identify the true footer section divider line. Additional safety logic
prevents the upwards search from erroneously engulfing nested commit
headers.

Fixes #4779

Author: codyoss

internal/legacylibrarian/legacygitrepo/conventional_commits.go

@@ -320,34 +320,49 @@ func parseHeader(headerLine string) (*parsedHeader, bool) {
 }
 
 // separateBodyAndFooters splits the lines after the header into body and footer sections.
-// It identifies the footer section by looking for a blank line followed by a
-// line that matches the conventional commit footer format.
+// It parses lines from the bottom up to identify the true footer section at the end
+// of the message, preventing body content (like Renovate release notes) from being
+// misidentified as footers. It walks up the last blank line boundary to support
+// correctly identifying footers while avoiding consuming nested commit headers.
 func separateBodyAndFooters(lines []string) (bodyLines, footerLines []string) {
-	inFooterSection := false
-	for i, line := range lines {
-		if inFooterSection {
-			footerLines = append(footerLines, line)
-			continue
+	lastBlankLine := -1
+	for i := len(lines) - 1; i >= 0; i-- {
+		if strings.TrimSpace(lines[i]) == "" {
+			lastBlankLine = i
+			break
 		}
-		if strings.TrimSpace(line) == "" {
-			isSeparator := false
-			// Look ahead at the next non-blank line.
-			for j := i + 1; j < len(lines); j++ {
-				if strings.TrimSpace(lines[j]) != "" {
-					if footerRegex.MatchString(lines[j]) {
-						isSeparator = true
-					}
+	}
+
+	if lastBlankLine == -1 {
+		return lines, nil
+	}
+
+	potentialFooters := lines[lastBlankLine+1:]
+	if len(potentialFooters) == 0 {
+		return lines, nil
+	}
+
+	if !footerRegex.MatchString(potentialFooters[0]) {
+		return lines, nil
+	}
+
+	// Walk up past blank lines to include preceding footers, stopping if we hit a nested commit header.
+	boundaryIndex := lastBlankLine
+	for i := lastBlankLine; i >= 0; i-- {
+		if strings.TrimSpace(lines[i]) == "" {
+			if i > 0 && footerRegex.MatchString(lines[i-1]) {
+				if commitRegex.MatchString(lines[i-1]) {
+					boundaryIndex = i
 					break
 				}
+				continue
 			}
-			if isSeparator {
-				inFooterSection = true
-				continue // Skip the blank separator line.
-			}
+			boundaryIndex = i
+			break
 		}
-		bodyLines = append(bodyLines, line)
 	}
-	return bodyLines, footerLines
+
+	return lines[:boundaryIndex], lines[boundaryIndex+1:]
 }
 
 // parseFooters parses footer lines from a conventional commit message into a map

internal/legacylibrarian/legacygitrepo/conventional_commits_test.go

@@ -640,6 +640,55 @@ END_NESTED_COMMIT`,
 				},
 			},
 		},
+		{
+			name: "renovate_pr_false_positive",
+			message: "chore(deps): update dependency aip to v0.83.0\n\n" +
+				"### Release Notes for aip\n\n" +
+				"BREAKING CHANGE: The HAS operator on timestamp fields now only accepts...\n\n" +
+				"This PR was generated by Renovate.",
+			want: []*ConventionalCommit{
+				{
+					Type:       "chore",
+					Subject:    "update dependency aip to v0.83.0",
+					Body:       "### Release Notes for aip\nBREAKING CHANGE: The HAS operator on timestamp fields now only accepts...\nThis PR was generated by Renovate.",
+					LibraryID:  "example-id",
+					IsNested:   false,
+					IsBreaking: false,
+					Footers:    map[string]string{},
+					CommitHash: sha.String(),
+					When:       now,
+				},
+			},
+		},
+		{
+			name: "nested_commit_with_breaking_change_footer",
+			message: "chore: main commit\n\n" +
+				"BEGIN_NESTED_COMMIT\n" +
+				"feat: nested feature\n\n" +
+				"BREAKING CHANGE: nested breaking change\n" +
+				"END_NESTED_COMMIT",
+			want: []*ConventionalCommit{
+				{
+					Type:       "chore",
+					Subject:    "main commit",
+					LibraryID:  "example-id",
+					IsNested:   false,
+					Footers:    map[string]string{},
+					CommitHash: sha.String(),
+					When:       now,
+				},
+				{
+					Type:       "feat",
+					Subject:    "nested feature",
+					LibraryID:  "example-id",
+					IsNested:   true,
+					IsBreaking: true,
+					Footers:    map[string]string{"BREAKING CHANGE": "nested breaking change"},
+					CommitHash: sha.String(),
+					When:       now,
+				},
+			},
+		},
 	} {
 		t.Run(test.name, func(t *testing.T) {
 			commit := &Commit{