Allow PATCH method to apply partial updates.

Gated behind the `patch` feature.

test/stresspatch/main.go

@@ -0,0 +1,112 @@
+package main
+
+import (
+	"archive/tar"
+	"bytes"
+	"flag"
+	"fmt"
+	"io"
+	"net/http"
+	"sync"
+	"time"
+)
+
+func makeInit() []byte {
+	writer := bytes.NewBuffer(nil)
+	archive := tar.NewWriter(writer)
+	archive.WriteHeader(&tar.Header{
+		Typeflag: tar.TypeReg,
+		Name:     "index.html",
+	})
+	archive.Write([]byte{})
+	archive.Flush()
+	return writer.Bytes()
+}
+
+func initSite() {
+	req, err := http.NewRequest(http.MethodPut, "http://localhost:3000",
+		bytes.NewReader(makeInit()))
+	if err != nil {
+		panic(err)
+	}
+
+	req.Header.Add("Content-Type", "application/x-tar")
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		panic(err)
+	}
+	defer resp.Body.Close()
+}
+
+func makePatch(n int) []byte {
+	writer := bytes.NewBuffer(nil)
+	archive := tar.NewWriter(writer)
+	archive.WriteHeader(&tar.Header{
+		Typeflag: tar.TypeReg,
+		Name:     fmt.Sprintf("%d.txt", n),
+	})
+	archive.Write([]byte{})
+	archive.Flush()
+	return writer.Bytes()
+}
+
+func patchRequest(n int) int {
+	req, err := http.NewRequest(http.MethodPatch, "http://localhost:3000",
+		bytes.NewReader(makePatch(n)))
+	if err != nil {
+		panic(err)
+	}
+
+	req.Header.Add("Race-Free", "no")
+	req.Header.Add("Content-Type", "application/x-tar")
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		panic(err)
+	}
+	defer resp.Body.Close()
+
+	data, err := io.ReadAll(resp.Body)
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Printf("%d: %s %q\n", n, resp.Status, string(data))
+	return resp.StatusCode
+}
+
+func concurrentWriter(wg *sync.WaitGroup, n int) {
+	for {
+		if patchRequest(n) == 200 {
+			break
+		}
+	}
+	wg.Done()
+}
+
+var count = flag.Int("count", 10, "request count")
+
+func main() {
+	flag.Parse()
+
+	initSite()
+	time.Sleep(time.Second)
+
+	wg := &sync.WaitGroup{}
+	for n := range *count {
+		wg.Add(1)
+		go concurrentWriter(wg, n)
+	}
+	wg.Wait()
+
+	success := 0
+	for n := range *count {
+		resp, err := http.Get(fmt.Sprintf("http://localhost:3000/%d.txt", n))
+		if err != nil {
+			panic(err)
+		}
+		if resp.StatusCode == 200 {
+			success++
+		}
+	}
+	fmt.Printf("written: %d of %d\n", success, *count)
+}