Parallel builds with similar apps and deps can cause crashes

[workmanw]

With @rwjblue's help, I discovered that building two of my apps in parallel can cause a crash related to broccoli-persistent-filter. Here is an example stack:

Build failed.
The Broccoli Plugin: [BroccoliMergeTrees: TreeMerger (appAndDependencies)] failed with:
Error: unexpected end of file
    at Zlib._handle.onerror (zlib.js:370:17)

The broccoli plugin was instantiated at:
    at BroccoliMergeTrees.Plugin (/mnt/jenkins-workspaces/batterii-horizons-deploy/src/buttercup-epcot/apps/bc-admin/node_modules/broccoli-plugin/index.js:7:31)
    at new BroccoliMergeTrees (/mnt/jenkins-workspaces/batterii-horizons-deploy/src/buttercup-epcot/apps/bc-admin/node_modules/ember-cli/node_modules/broccoli-merge-trees/index.js:16:10)
    at Function.BroccoliMergeTrees [as _upstreamMergeTrees] (/mnt/jenkins-workspaces/batterii-horizons-deploy/src/buttercup-epcot/apps/bc-admin/node_modules/ember-cli/node_modules/broccoli-merge-trees/index.js:10:53)
    at mergeTrees (/mnt/jenkins-workspaces/batterii-horizons-deploy/src/buttercup-epcot/apps/bc-admin/node_modules/ember-cli/lib/broccoli/merge-trees.js:85:33)
    at EmberApp._mergeTrees (/mnt/jenkins-workspaces/batterii-horizons-deploy/src/buttercup-epcot/apps/bc-admin/node_modules/ember-cli/lib/broccoli/ember-app.js:1815:12)
    at EmberApp.appAndDependencies (/mnt/jenkins-workspaces/batterii-horizons-deploy/src/buttercup-epcot/apps/bc-admin/node_modules/ember-cli/lib/broccoli/ember-app.js:1135:17)
    at EmberApp.javascript (/mnt/jenkins-workspaces/batterii-horizons-deploy/src/buttercup-epcot/apps/bc-admin/node_modules/ember-cli/lib/broccoli/ember-app.js:1235:30)
    at EmberApp.toArray (/mnt/jenkins-workspaces/batterii-horizons-deploy/src/buttercup-epcot/apps/bc-admin/node_modules/ember-cli/lib/broccoli/ember-app.js:1672:12)
    at EmberApp.toTree (/mnt/jenkins-workspaces/batterii-horizons-deploy/src/buttercup-epcot/apps/bc-admin/node_modules/ember-cli/lib/broccoli/ember-app.js:1694:38)
    at module.exports (/mnt/jenkins-workspaces/batterii-horizons-deploy/src/buttercup-epcot/apps/bc-admin/ember-cli-build.js:97:14)

We determined the cause was related to stefanpenner/async-disk-caches use of the system temp directory. The two applications are separate apps (in separate directories), but they are similar in size and almost identical in dependencies. The builds kick off at the same time in parallel. Both builds end up using the same system tmp cache and one clobbers the other.

A workaround is to set an env variable that causes it to not use the system cache:

BROCCOLI_PERSISTENT_FILTER_CACHE_ROOT=./cache ember build -prod

[rwjblue]

Thank you for reporting! I believe we were aware (generally speaking) that a race could occur here. We should try add in some more resiliency to ensure that cross-process caching does not fail the build. I believe that in the worst case scenario, we should just opt out of caching and recalculate.

@stefanpenner - Thoughts?

[workmanw]

One other relevant note is that I realized that in my case it's also possible for the exact same app [w/ same deps] to be building multiple times in parallel in different folders. Basically if the same commit ends up on multiple branches that are being tracked by CI, it could end up building them both at the same time.

[stefanpenner]

building the same project (exact some on-disk files/deps etc) more then once concurrently isn't really something we support. At-least not with a persistent cache... It is possible to support this, but It would come at either a complexity and/or performance cost to the regular case.

It might be a good idea to have your CI box limit its concurrency somewhat, or concurrent tasks (like building) will starve each other for resources...

---

I don't think this is an issue with this library, but we should make clear reference to it in the readme.

If we want to mitigate this issue, we may want to an a locking mechanism to ember-cli itself, essentially disallowing the same project to be actively build concurrently.

---

@workmanw your above "Work-around" actually seems like the exact "configuration" I would expect for supporting concurrent builds. Alternatively, we could also enable an "opt-out" of persistent cache altogether... as you are then essentially opting out of persistent-cache entirely on your CI box.

[workmanw]

@stefanpenner I'm 👍 for making it an option. I know most people run their builds in a containerized service (Travis, etc) and that makes us in the minority here.

---

building the same project (exact some on-disk files/deps etc)

To be clear, it's the same project/deps but on a different branch in a different cloned instance of the repo (and thus in a different directory). A single GitHub post hook can trigger both builds, so they end up running simultaneously and thus fight over the system temp cache.

---

I do think we would get some value out of having a configuration for the cache path because in our case, we build both dev and prod versions of the app back to back. So without really knowing too many of the details, it sounds like there would be some cache benefit for that.

I'm fine to continue using BROCCOLI_PERSISTENT_FILTER_CACHE_ROOT. But it would be nice to have it documented and marked public so I don't have to worry about regressions. And I wouldn't mind a slightly shorter ENV variable 😃 .

[stefanpenner]

To be clear, it's the same project/deps but on a different branch in a different cloned instance of the repo (and thus in a different directory). A single GitHub post hook can trigger both builds, so they end up running simultaneously and thus fight over the system temp cache.

Something seems fishy then, because the cached files should have different mtime/atimes and then ultimately not collide...

I'm assuming you come to this issue via BroccoliPersistentFilter (or one of its many subclasses?) If so, then maybe we are missing including something important in the cache key..

[workmanw]

@stefanpenner One point of interest is that it only ever seemed to happen on prod builds (w/ fingerprinting). I don't know if that's relevant or not.

I'm happy to help in any way I can. We could do a screen hero if you want. But honestly I don't want to suck up your time if I'm the only one experiencing this. And the debugging is not super pleasant because it's all over SSH. That said, I'm always happy to help.

[xbmono]

when I run unit tests in my Ember app in parallel, even if they are in separate workspace so no shared folder, I get this error, I wonder if this is related:

[emberTests2] cleaning up...
[emberTests2] Build failed.
[emberTests2] Build Canceled: Broccoli Builder ran into an error with `Babel` plugin. 💥
[emberTests2] unexpected end of file
[emberTests2] Error: unexpected end of file
[emberTests2]     at Zlib._handle.onerror (zlib.js:370:17)

[stefanpenner]

@xbmono yes, we can fix this by changing the cache key to consider the full path of the workspace.

[xbmono]

@stefanpenner thanks it worked :)