https://searchfox.org/mozilla-central/rev/cd742d763809089925a38178dd2ba5a9069fa855/xpcom/threads/MozPromise.h#337 MozPromise::All will create a AllPromiseHolder and call Then() on each of the promises contained in the array. it then return holder->Promise() which returns holder::mPromise However, if any of the promises handling is prior the loop completing, either calling AllPromiseHolder::Resolve or AllPromiseHolder::Reject then AllPromiseHolder::mPromise is set to nullptr: Resolve: https://searchfox.org/mozilla-central/rev/cd742d763809089925a38178dd2ba5a9069fa855/xpcom/threads/MozPromise.h#311 Reject: https://searchfox.org/mozilla-central/rev/cd742d763809089925a38178dd2ba5a9069fa855/xpcom/threads/MozPromise.h#324 this causes MozPromise::All to incorrectly return nullptr.

Comment on attachment 8936179 [details] Bug 1424647: Prevent race on AllPromiseHolder::mPromise. https://reviewboard.mozilla.org/r/206938/#review212764 Good catch! Is this bug causing you any trouble?

Comment on attachment 8936179 [details] Bug 1424647: Prevent race on AllPromiseHolder::mPromise. https://reviewboard.mozilla.org/r/206938/#review212764 yes... was causing my new AwaitAll to crash with a null deref. I guess All wasnt used enough previously to expose the problem. it is used however by GMP/Widevine, it would likely cause crashes there too.

Pushed by jyavenard@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/6d926a50dcf5 Prevent race on AllPromiseHolder::mPromise. r=jwwang