client/web: add support for zst precomppressed assets

This will enable us to reduce the size of these embedded assets.

Updates tailscale/corp#20099

Signed-off-by: James Tucker <james@tailscale.com>

tsweb/tswebutil/tswebutil.go

@@ -0,0 +1,35 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Package tswebutil contains helper code used in various Tailscale webservers, without the tsweb kitchen sink.
+package tswebutil
+
+import (
+	"net/http"
+	"strings"
+
+	"go4.org/mem"
+)
+
+// AcceptsEncoding reports whether r accepts the named encoding
+// ("gzip", "br", etc).
+func AcceptsEncoding(r *http.Request, enc string) bool {
+	h := r.Header.Get("Accept-Encoding")
+	if h == "" {
+		return false
+	}
+	if !strings.Contains(h, enc) && !mem.ContainsFold(mem.S(h), mem.S(enc)) {
+		return false
+	}
+	remain := h
+	for len(remain) > 0 {
+		var part string
+		part, remain, _ = strings.Cut(remain, ",")
+		part = strings.TrimSpace(part)
+		part, _, _ = strings.Cut(part, ";")
+		if part == enc {
+			return true
+		}
+	}
+	return false
+}

tsweb/tswebutil/tswebutil_test.go

@@ -0,0 +1,37 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package tswebutil
+
+import (
+	"net/http"
+	"testing"
+)
+
+func TestAcceptsEncoding(t *testing.T) {
+	tests := []struct {
+		in, enc string
+		want    bool
+	}{
+		{"", "gzip", false},
+		{"gzip", "gzip", true},
+		{"foo,gzip", "gzip", true},
+		{"foo, gzip", "gzip", true},
+		{"foo, gzip ", "gzip", true},
+		{"gzip, foo ", "gzip", true},
+		{"gzip, foo ", "br", false},
+		{"gzip, foo ", "fo", false},
+		{"gzip;q=1.2, foo ", "gzip", true},
+		{" gzip;q=1.2, foo ", "gzip", true},
+	}
+	for i, tt := range tests {
+		h := make(http.Header)
+		if tt.in != "" {
+			h.Set("Accept-Encoding", tt.in)
+		}
+		got := AcceptsEncoding(&http.Request{Header: h}, tt.enc)
+		if got != tt.want {
+			t.Errorf("%d. got %v; want %v", i, got, tt.want)
+		}
+	}
+}