proposal: runtime: maps: add optional concurrency

[metux]

The current map implementation currently does not support concurrency - just detects concurrent calls and panics. This is very unfortunate for a language desinged for parrallel computing. Concurrent code cannot use maps directly - it needs to do an own implementation, at least by some wrapper. This drastically devalues the whole existance of the builtin map type. Thefore builtin maps should support concurrency.

Proposal: add rwlock to hmap

Simple and straightforward implementation can just take the lock on the entry of the (user-facing) functions (mapaccess et al). A more sophisticated one can do it on indivual pieces (eg per bucket), eg. skipping if just an existing entry is updated, and take the hmap lock only if necessary.

Performance & backwards compat:

Locking has some (little) performance impact. It would mostly impact concurrent reads, since concurrent writes are currently forbidden anyways.

If this really is a concern, we can add a global bit to enable locking - if unset, the locking is skipped entirely (checking a single bit shouldn't introduce any measurable penalty). Any package that wants it, just sets this bit - applications w/o such packages wont have locking at all, thus no performance impact for existing code.
This global bit could be set eg by some (inlinable) runtime.MapsEnableLocking().

[mvdan]

Have you seen https://pkg.go.dev/sync#Map and #47657?

[mvdan]

Also, since this looks like a proposal to change the map type as defined in the spec, please note that you should fill https://github.com/golang/proposal/blob/master/go2-language-changes.md when proposing a language change.

[metux]

Have you seen https://pkg.go.dev/sync#Map and #47657?

yes, of course :)
but unfortunately, this (and all the other similar implementations out there) doesn't solve the actual problem:

• can't be used as a map (eg. index, range, ...)
• complicated to integrate with existing code thats already based on maps (possibly nested ones, ...)

[metux]

Also, since this looks like a proposal to change the map type as defined in the spec, please note that you should fill https://github.com/golang/proposal/blob/master/go2-language-changes.md when proposing a language change.

My goal was doing it w/o changing the language itself, just the map piece of the standard library.
(If I understand it correctly, map is nothing more than a struct having pointer to hmap with some syntactic sugar for convenient access.)

[seankhliao]

Duplicate of #1682

[mvdan]

(If I understand it correctly, map is nothing more than a struct having pointer to hmap with some syntactic sugar for convenient access.)

Maps are defined in the spec, along with all the operations that can be done on them: https://go.dev/ref/spec#Map_types