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Rollback to recursively check dependency status
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we moved from `recursive checking` of dependency status to check only the
_current module_'s dependency status and its descendant dependency status and
also circular dependency status.

However, it will cause an issue.

For example, if the module dependency is like following

a -> b -> c -> d -> e
f -> g -> h -> c -> d -> e

In this example, if the d module is still under fetching but g is trying
to advance to finish. Then, it will cause a panic because module d is
g's grand-grand-grand-descendant which means it's still under fetching
and we can't instantiate module g.

Ideally, we should get rid of the checking in #26903 so, before #26903
fixed, we can just move back to the recursive checking way which will
ensure all descendants are not fetching.
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CYBAI committed Jun 23, 2020
1 parent 3f999ce commit a7221fd
Showing 1 changed file with 42 additions and 105 deletions.
147 changes: 42 additions & 105 deletions components/script/
Original file line number Diff line number Diff line change
Expand Up @@ -37,7 +37,7 @@ use crate::task::TaskBox;
use crate::task_source::TaskSourceName;
use encoding_rs::UTF_8;
use hyper_serde::Serde;
use indexmap::{IndexMap, IndexSet};
use indexmap::IndexSet;
use ipc_channel::ipc;
use ipc_channel::router::ROUTER;
use js::jsapi::Handle as RawHandle;
Expand Down Expand Up @@ -65,7 +65,7 @@ use net_traits::{FetchMetadata, Metadata};
use net_traits::{FetchResponseListener, NetworkError};
use net_traits::{ResourceFetchTiming, ResourceTimingType};
use servo_url::ServoUrl;
use std::collections::HashSet;
use std::collections::{HashMap, HashSet};
use std::ffi;
use std::rc::Rc;
use std::str::FromStr;
Expand Down Expand Up @@ -257,64 +257,51 @@ impl ModuleTree {

/// Find circular dependencies in non-recursive way
/// This function is basically referred to
/// [this blog post](
/// The only difference is, in that blog post, its algorithm will throw errors while finding circular
/// dependencies; however, in our use case, we'd like to find circular dependencies so we will just
/// return it.
pub fn find_circular_dependencies(&self, global: &GlobalScope) -> IndexSet<ServoUrl> {
let module_map = global.get_module_map().borrow();
/// recursively checks if all of the transitive descendants are
/// in the FetchingDescendants or later status
fn recursive_check_descendants(
module_tree: &ModuleTree,
module_map: &HashMap<ServoUrl, Rc<ModuleTree>>,
discovered_urls: &mut HashSet<ServoUrl>,
) -> bool {

// A map for checking dependencies and using the module url as key
let mut module_deps: IndexMap<ServoUrl, IndexSet<ServoUrl>> = module_map
.map(|(module_url, module)| {
(module_url.clone(), module.descendant_urls.borrow().clone())

while module_deps.len() != 0 {
// Get all dependencies with no dependencies
let ready: IndexSet<ServoUrl> = module_deps
.filter_map(|(module_url, descendant_urls)| {
if descendant_urls.len() == 0 {
} else {
let descendant_urls = module_tree.descendant_urls.borrow();

for descendant_url in descendant_urls.iter() {
match module_map.get(&descendant_url.clone()) {
None => return false,
Some(descendant_module) => {
if discovered_urls.contains(&descendant_module.url) {

// If there's no ready module but we're still in the loop,
// it means we find circular modules, then we can return them.
if ready.len() == 0 {
return module_deps
.map(|(url, _)| url.clone())

// Remove ready modules from the dependency map
for module_url in ready.iter() {
let descendant_status = descendant_module.get_status();
if descendant_status < ModuleStatus::FetchingDescendants {
return false;

// Also make sure to remove the ready modules from the
// remaining module dependencies as well
for (_, deps) in module_deps.iter_mut() {
*deps = deps
let all_ready_descendants = ModuleTree::recursive_check_descendants(

if !all_ready_descendants {
return false;

return true;

fn has_all_ready_descendants(&self, global: &GlobalScope) -> bool {
let module_map = global.get_module_map().borrow();
let mut discovered_urls = HashSet::new();

return ModuleTree::recursive_check_descendants(&self, &module_map, &mut discovered_urls);

// We just leverage the power of Promise to run the task for `finish` the owner.
Expand Down Expand Up @@ -751,35 +738,8 @@ impl ModuleTree {
/// step 4-7.
fn advance_finished_and_link(&self, global: &GlobalScope) {
let descendant_urls = self.descendant_urls.borrow();

// Check if there's any dependencies under fetching.
// We can't only check `incomplete fetches` here because...
// For example, module `A` has descendants `B`, `C`
// while `A` has added them to incomplete fetches, it's possible
// `B` has finished but `C` is not yet fired its fetch; in this case,
// `incomplete fetches` will be `zero` but the module is actually not ready
// to finish. Thus, we need to check dependencies directly instead of
// incomplete fetches here.
if !is_all_dependencies_ready(&descendant_urls, &global) {
// When we found the `incomplete fetches` is bigger than zero,
// we will need to check if there's any circular dependency.
// If there's no circular dependencies but there are incomplete fetches,
// it means it needs to wait for finish.
// Or, if there are circular dependencies, then we need to confirm
// no circular dependencies are fetching.
// if there's any circular dependencies and they all proceeds to status
// higher than `FetchingDescendants`, then it means we can proceed to finish.
let circular_deps = self.find_circular_dependencies(&global);

if circular_deps.len() == 0 || !is_all_dependencies_ready(&circular_deps, &global) {
if !self.has_all_ready_descendants(&global) {

Expand Down Expand Up @@ -838,29 +798,6 @@ impl ModuleTree {

// Iterate the given dependency urls to see if it and its descendants are fetching or not.
// When a module status is `FetchingDescendants`, it's possible that the module is a circular
// module so we will also check its descendants.
fn is_all_dependencies_ready(dependencies: &IndexSet<ServoUrl>, global: &GlobalScope) -> bool {
dependencies.iter().all(|dep| {
let module_map = global.get_module_map().borrow();
match module_map.get(&dep) {
Some(module) => {
let module_descendants = module.get_descendant_urls().borrow();

module.get_status() >= ModuleStatus::FetchingDescendants &&
module_descendants.iter().all(|descendant_url| {
match module_map.get(&descendant_url) {
Some(m) => m.get_status() >= ModuleStatus::FetchingDescendants,
None => false,
None => false,

#[derive(JSTraceable, MallocSizeOf)]
struct ModuleHandler {
#[ignore_malloc_size_of = "Measuring trait objects is hard"]
Expand Down

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