archive/zip: reduce CPU usage in index construction

Constructing the zip index (which is done once when first opening
a file in an archive) can consume large amounts of CPU when
processing deeply-nested directory paths.

Switch to a less inefficient algorithm.

Thanks to Jakub Ciolek for reporting this issue.

	goos: darwin
	goarch: arm64
	pkg: archive/zip
	cpu: Apple M4 Pro
	                          │  /tmp/bench.0  │            /tmp/bench.1            │
	                          │     sec/op     │   sec/op     vs base               │
	ReaderOneDeepDir-14         25983.62m ± 2%   46.01m ± 2%  -99.82% (p=0.000 n=8)
	ReaderManyDeepDirs-14          16.221 ± 1%    2.763 ± 6%  -82.96% (p=0.000 n=8)
	ReaderManyShallowFiles-14      130.3m ± 1%   128.8m ± 2%   -1.20% (p=0.003 n=8)
	geomean                         3.801        253.9m       -93.32%

Fixes #77102
Fixes CVE-2025-61728

Change-Id: I2c9c864be01b2a2769eb67fbab1b250aeb8f6c42
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/3060
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Neal Patel <nealpatel@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/736713
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>

src/archive/zip/reader.go

@@ -834,7 +834,16 @@ func (r *Reader) initFileList() {
 				continue
 			}
 
-			for dir := path.Dir(name); dir != "."; dir = path.Dir(dir) {
+			dir := name
+			for {
+				if idx := strings.LastIndex(dir, "/"); idx < 0 {
+					break
+				} else {
+					dir = dir[:idx]
+				}
+				if dirs[dir] {
+					break
+				}
 				dirs[dir] = true
 			}
 

src/archive/zip/reader_test.go

@@ -9,6 +9,7 @@ import (
 	"encoding/binary"
 	"encoding/hex"
 	"errors"
+	"fmt"
 	"internal/obscuretestdata"
 	"io"
 	"io/fs"
@@ -1874,3 +1875,83 @@ func TestBaseOffsetPlusOverflow(t *testing.T) {
 	// as the section reader offset & size were < 0.
 	NewReader(bytes.NewReader(data), int64(len(data))+1875)
 }
+
+func BenchmarkReaderOneDeepDir(b *testing.B) {
+	var buf bytes.Buffer
+	zw := NewWriter(&buf)
+
+	for i := range 4000 {
+		name := strings.Repeat("a/", i) + "data"
+		zw.CreateHeader(&FileHeader{
+			Name:   name,
+			Method: Store,
+		})
+	}
+
+	if err := zw.Close(); err != nil {
+		b.Fatal(err)
+	}
+	data := buf.Bytes()
+
+	for b.Loop() {
+		zr, err := NewReader(bytes.NewReader(data), int64(len(data)))
+		if err != nil {
+			b.Fatal(err)
+		}
+		zr.Open("does-not-exist")
+	}
+}
+
+func BenchmarkReaderManyDeepDirs(b *testing.B) {
+	var buf bytes.Buffer
+	zw := NewWriter(&buf)
+
+	for i := range 2850 {
+		name := fmt.Sprintf("%x", i)
+		name = strings.Repeat("/"+name, i+1)[1:]
+
+		zw.CreateHeader(&FileHeader{
+			Name:   name,
+			Method: Store,
+		})
+	}
+
+	if err := zw.Close(); err != nil {
+		b.Fatal(err)
+	}
+	data := buf.Bytes()
+
+	for b.Loop() {
+		zr, err := NewReader(bytes.NewReader(data), int64(len(data)))
+		if err != nil {
+			b.Fatal(err)
+		}
+		zr.Open("does-not-exist")
+	}
+}
+
+func BenchmarkReaderManyShallowFiles(b *testing.B) {
+	var buf bytes.Buffer
+	zw := NewWriter(&buf)
+
+	for i := range 310000 {
+		name := fmt.Sprintf("%v", i)
+		zw.CreateHeader(&FileHeader{
+			Name:   name,
+			Method: Store,
+		})
+	}
+
+	if err := zw.Close(); err != nil {
+		b.Fatal(err)
+	}
+	data := buf.Bytes()
+
+	for b.Loop() {
+		zr, err := NewReader(bytes.NewReader(data), int64(len(data)))
+		if err != nil {
+			b.Fatal(err)
+		}
+		zr.Open("does-not-exist")
+	}
+}