Fix potential panic on invalid xml

internal/format/xml.go

@@ -51,6 +51,14 @@ func FormatXML(buf []byte, w *core.Printer) error {
 			}
 			stack = append(stack, false)
 		case xml.EndElement:
+			if len(stack) == 0 {
+				// Malformed XML: more closing tags than opening tags.
+				// Skip indent adjustment, just write the closing tag.
+				w.WriteString("</")
+				writeXMLTagName(w, t.Name.Local)
+				w.WriteString(">\n")
+				continue
+			}
 			last := stack[len(stack)-1]
 			stack = stack[:len(stack)-1]
 
@@ -162,6 +170,21 @@ func writeXMLProcInst(p *core.Printer, inst []byte) {
 // Mostly taken from the Go encoding/xml package in the standard library:
 // https://cs.opensource.google/go/go/+/refs/tags/go1.24.0:src/encoding/xml/xml.go;l=1964-1999
 func escapeXMLString(p *core.Printer, s string) {
+	// Fast path: check if string needs escaping.
+	needsEscape := false
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c == '"' || c == '\'' || c == '&' || c == '<' || c == '>' ||
+			c == '\t' || c == '\n' || c == '\r' || c >= 0x80 {
+			needsEscape = true
+			break
+		}
+	}
+	if !needsEscape {
+		p.WriteString(s)
+		return
+	}
+
 	var esc string
 	var last int
 	for i := 0; i < len(s); {

internal/format/xml_test.go

@@ -0,0 +1,147 @@
+package format
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/ryanfowler/fetch/internal/core"
+)
+
+func TestFormatXML(t *testing.T) {
+	tests := []struct {
+		name    string
+		input   string
+		wantErr bool
+	}{
+		{
+			name:    "valid simple xml",
+			input:   "<root><child>text</child></root>",
+			wantErr: false,
+		},
+		{
+			name:    "valid nested xml",
+			input:   "<a><b><c>text</c></b></a>",
+			wantErr: false,
+		},
+		{
+			name:    "valid xml with attributes",
+			input:   `<root attr="value"><child id="1">text</child></root>`,
+			wantErr: false,
+		},
+		{
+			name:    "malformed xml extra closing tag at start",
+			input:   "</foo><bar></bar>",
+			wantErr: true, // XML decoder catches this
+		},
+		{
+			name:    "malformed xml extra closing tag at end",
+			input:   "<a></a></a>",
+			wantErr: true, // XML decoder catches this
+		},
+		{
+			name:    "malformed xml multiple extra closing tags",
+			input:   "</x></y></z>",
+			wantErr: true, // XML decoder catches this
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			p := core.NewHandle(core.ColorOff).Stderr()
+			err := FormatXML([]byte(tt.input), p)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("FormatXML() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}
+
+func TestFormatXMLOutput(t *testing.T) {
+	input := "<root><child>text</child></root>"
+	p := core.NewHandle(core.ColorOff).Stderr()
+	err := FormatXML([]byte(input), p)
+	if err != nil {
+		t.Fatalf("FormatXML() error = %v", err)
+	}
+
+	output := string(p.Bytes())
+	if !strings.Contains(output, "<root>") {
+		t.Errorf("output should contain <root>, got: %s", output)
+	}
+	if !strings.Contains(output, "</root>") {
+		t.Errorf("output should contain </root>, got: %s", output)
+	}
+	if !strings.Contains(output, "text") {
+		t.Errorf("output should contain text, got: %s", output)
+	}
+}
+
+func TestEscapeXMLString(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  string
+	}{
+		{
+			name:  "ascii no escape needed",
+			input: "hello world",
+			want:  "hello world",
+		},
+		{
+			name:  "with ampersand",
+			input: "foo & bar",
+			want:  "foo &amp; bar",
+		},
+		{
+			name:  "with less than",
+			input: "a < b",
+			want:  "a &lt; b",
+		},
+		{
+			name:  "with greater than",
+			input: "a > b",
+			want:  "a &gt; b",
+		},
+		{
+			name:  "with quotes",
+			input: `"quoted"`,
+			want:  "&quot;quoted&quot;",
+		},
+		{
+			name:  "with single quotes",
+			input: "'quoted'",
+			want:  "&apos;quoted&apos;",
+		},
+		{
+			name:  "with tab",
+			input: "a\tb",
+			want:  "a&#x9;b",
+		},
+		{
+			name:  "with newline",
+			input: "a\nb",
+			want:  "a&#xA;b",
+		},
+		{
+			name:  "with carriage return",
+			input: "a\rb",
+			want:  "a&#xD;b",
+		},
+		{
+			name:  "unicode chars",
+			input: "日本語",
+			want:  "日本語",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			p := core.NewHandle(core.ColorOff).Stderr()
+			escapeXMLString(p, tt.input)
+			got := string(p.Bytes())
+			if got != tt.want {
+				t.Errorf("escapeXMLString() = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}