1482828 - Introduce C++ analogs of Rust's nsA[C]StringBulkWriteHandle

Status: RESOLVED FIXED

(Core :: XPCOM, enhancement)

Description

[Henri Sivonen (:hsivonen)]

Starting a bulk write on an nsA[C]String in Rust gives you a RAII handle that puts the string in a valid state in case an error causes an early return before the bulk write is successfully finished. Rust's borrow system also prevents access to the string by other means while the bulk write remains unfinished.

C++ can't replicate the latter point, but it can replicate the former point about handling early returns before successful bulk write finish. We could introduce C++ RAII types (for nsAString and nsACString) that take care of this.

Comment 1

[Henri Sivonen (:hsivonen)]

I failed to figure out how mozilla::BulkWriteHandle, which is supposed to have move semantics, can be used as the success type wrapped in mozilla::Result. Is it possible? How?

https://phabricator.services.mozilla.com/D3405#inline-12152

Comment 2

[Nathan Froyd [:froydnj]]

I guess you encountered some sort of compilation error?  It's hard to say what the problem is without further details about the error.  I did file bug 1418624 about some replacement/addition for Result::unwrap which is friendlier to move-only types, which I think might address your concern?

Comment 3

[Henri Sivonen (:hsivonen)]

(In reply to Nathan Froyd [:froydnj] from comment #2)
> I guess you encountered some sort of compilation error?

Yes. mozilla::Result wants a copy constructor and the compiler tells me I've deleted it.

> I did file bug
> 1418624 about some replacement/addition for Result::unwrap which is
> friendlier to move-only types, which I think might address your concern?

Thanks. It looks like I can't use mozilla::Result for this until that bug is fixed. :-(

I'll use an outparam for error.

Comment 4

[Henri Sivonen (:hsivonen)]

Attachment: Bug 1482828 - Introduce C++ analogs of Rust's nsA[C]StringBulkWriteHandle.

MozReview-Commit-ID: BZyHd4VzNur

Comment 5

[Henri Sivonen (:hsivonen)]

Let's see that it actually compiles on all platforms:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=c6aef12e0030b153224631562a08f3998499f196

Comment 6

[Nathan Froyd [:froydnj]]

Comment on attachment 9001235 [details]
Bug 1482828 - Introduce C++ analogs of Rust's nsA[C]StringBulkWriteHandle.

Nathan Froyd [:froydnj] has approved the revision.

Comment 7

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/400d4a90fe04
Introduce C++ analogs of Rust's nsA[C]StringBulkWriteHandle. r=froydnj

Comment 8

[Andrei Ciure[:aciure]]

https://hg.mozilla.org/mozilla-central/rev/400d4a90fe04