在 `yield` 后抛出 `HTTPException` 或类似的异常是很诱人的，但是**这不起作用**。

带有`yield`的依赖中的退出代码在响应发送之后执行，因此[异常处理程序](../handling-errors.md#install-custom-exception-handlers){.internal-link target=_blank}已经运行过。没有任何东西可以捕获退出代码（在`yield`之后）中的依赖抛出的异常。

所以，如果在`yield`之后抛出`HTTPException`，默认（或任何自定义）异常处理程序捕获`HTTPException`并返回HTTP 400响应的机制将不再能够捕获该异常。

这就是允许在依赖中设置的任何东西（例如数据库会话(DB session)）可以被后台任务使用的原因。

后台任务在响应发送之后运行。因此，无法触发`HTTPException`，因为甚至没有办法更改*已发送*的响应。

	}
	// (pipe).Field1.Field2 has pipe as .Node, fields as .Field. Eval the pipeline, then the fields.
	pipe := s.evalArg(dot, nil, chain.Node)
	return s.evalFieldChain(dot, pipe, chain, chain.Field, args, final)
}

func (s *state) evalVariableNode(dot reflect.Value, variable *parse.VariableNode, args []parse.Node, final reflect.Value) reflect.Value {

	}
	typ := reflect.Indirect(reflect.ValueOf(&UserMigrateColumn2{})).Type()
	numField := typ.NumField()
	if numField != len(columnTypes) {
		t.Fatalf("column's number not match struct and ddl, %d != %d", numField, len(columnTypes))
	}
	namer := schema.NamingStrategy{}
	for i := 0; i < numField; i++ {
		expectName := namer.ColumnName("", typ.Field(i).Name)
		if columnTypes[i].Name() != expectName {

		var a abbrev
		a.tag = Tag(b.uint())
		a.children = b.uint8() != 0
		a.field = make([]afield, n)
		for i := range a.field {
			a.field[i].attr = Attr(b.uint())
			a.field[i].fmt = format(b.uint())
			a.field[i].class = formToClass(a.field[i].fmt, a.field[i].attr, vers, &b)
			if a.field[i].fmt == formImplicitConst {
				a.field[i].val = b.int()
			}
		}
		b.uint()
		b.uint()

		m[id] = a
	}

class BrokenClass {
    Unknown1 field1
    Unknown2 field2

	// TODO(adonovan): remove workaround when we drop go1.10.
	if analysisutil.IsNamedType(typ, "sync", "noCopy") {
		return []string{typ.String()}
	}

	nfields := styp.NumFields()
	for i := 0; i < nfields; i++ {
		ftyp := styp.Field(i).Type()
		subpath := lockPath(tpkg, ftyp, seen)
		if subpath != nil {
			return append(subpath, typ.String())
		}
	}

	return nil
}

	ok(func() { clear(Field(v, 3)) })                      // .NamedT1
	ok(func() { clear(Field(Field(v, 3), 0)) })            // .NamedT1.Y
	bad(func() { clear(Field(Field(v, 3), 1)) })           // .NamedT1.t0
	ok(func() { clear(Field(Field(Field(v, 3), 1), 0)) })  // .NamedT1.t0.W
	ok(func() { clear(Field(v, 4)) })                      // .NamedT2

      // of field_tags). Merge chunk in, attempt to locate & hash the target
      // field by recursing.
      TF_ASSIGN_OR_RETURN(std::vector<Field> fields,
                          GetFieldTypes(chunked_field_tags));
      for (const auto& field : fields) {
        TF_ASSIGN_OR_RETURN(MutableFieldResult mfr,
                            GetMutableField(merged_message, field));
        merged_message =

            }
        }
    }

    private
    fun KtFile.isDocumentedAsSince(version: String, declaringClass: CtClass, field: CtField): Boolean =
        "${declaringClass.baseQualifiedKotlinName}.${field.name}".let { fqn ->
            collectDescendantsOfType<KtProperty>()
                .firstOrNull { it.fqName?.asString() == fqn }
                ?.isDocumentedAsSince(version) == true

	expand1(t, true)

	// check each method to be uniquely reachable
	var ms []*types.Field
	for i, sl := range slist {
		slist[i].field = nil
		sl.field.Sym.SetUniq(false)

		var f *types.Field
		path, _ := dotpath(sl.field.Sym, t, &f, false)
		if path == nil {
			continue
		}

		// dotpath may have dug out arbitrary fields, we only want methods.
		if !f.IsMethod() {
			continue
		}