chore: establish local platform baseline

internal/concurrency/etag.go

@@ -0,0 +1,39 @@
+package concurrency
+
+import (
+	"errors"
+	"fmt"
+	"net/http"
+	"strconv"
+	"strings"
+)
+
+var (
+	ErrPreconditionRequired = errors.New("If-Match header is required")
+	ErrInvalidETag          = errors.New("invalid ETag")
+	ErrVersionConflict      = errors.New("resource version conflict")
+)
+
+func FormatETag(version int64) string { return fmt.Sprintf("\"v%d\"", version) }
+
+func ParseIfMatch(request *http.Request) (int64, error) {
+	value := strings.TrimSpace(request.Header.Get("If-Match"))
+	if value == "" {
+		return 0, ErrPreconditionRequired
+	}
+	if value == "*" || strings.HasPrefix(value, "W/") || !strings.HasPrefix(value, "\"v") || !strings.HasSuffix(value, "\"") {
+		return 0, ErrInvalidETag
+	}
+	version, err := strconv.ParseInt(strings.TrimSuffix(strings.TrimPrefix(value, "\"v"), "\""), 10, 64)
+	if err != nil || version < 1 {
+		return 0, ErrInvalidETag
+	}
+	return version, nil
+}
+
+func RequireVersion(expected, current int64) error {
+	if expected != current {
+		return ErrVersionConflict
+	}
+	return nil
+}

internal/concurrency/etag_test.go

@@ -0,0 +1,33 @@
+package concurrency
+
+import (
+	"errors"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestParseIfMatch(t *testing.T) {
+	request := httptest.NewRequest("PATCH", "/resource", nil)
+	if _, err := ParseIfMatch(request); !errors.Is(err, ErrPreconditionRequired) {
+		t.Fatalf("missing header error = %v", err)
+	}
+	request.Header.Set("If-Match", FormatETag(3))
+	if version, err := ParseIfMatch(request); err != nil || version != 3 {
+		t.Fatalf("version = %d, err = %v", version, err)
+	}
+	for _, invalid := range []string{"*", "W/\"v3\"", "\"3\"", "\"v0\""} {
+		request.Header.Set("If-Match", invalid)
+		if _, err := ParseIfMatch(request); !errors.Is(err, ErrInvalidETag) {
+			t.Fatalf("expected %q to be invalid, got %v", invalid, err)
+		}
+	}
+}
+
+func TestRequireVersion(t *testing.T) {
+	if err := RequireVersion(2, 2); err != nil {
+		t.Fatal(err)
+	}
+	if err := RequireVersion(1, 2); !errors.Is(err, ErrVersionConflict) {
+		t.Fatalf("error = %v", err)
+	}
+}