Shared Structs

Shared structs live in one SharedArrayBuffer that every agent attaches to, and each agent sees them through a Proxy whose traps are the spec's.

Every agent attaches to one SharedArrayBuffer of a fixed size (256 MB, which is reserved rather than used up front). A Shared Struct is a block in it with a slot per field. Each agent sees the block through a Proxy whose traps are the spec's internal methods (§2.1.5 to §2.1.10). There is one Proxy per block per agent, so === holds.

a shared object  [kind][shape: its keys, interned][..][..] then 16 bytes per field
a slot           [a tagged word: the value][a Float64, for a Number]

A field holds undefined, null, a boolean, a Number, a string (interned in the heap), a BigInt, a registered or well-known symbol, or another shared object. Anything else is a TypeError (CanBeSharedAcrossAgents, §2.2.4). A read never tears: a Number's Float64 is written before its word, and the word is stored atomically.

Mutex and Condition are Shared Structs with no fields. Their internal slots are heap words that Atomics.wait and Atomics.notify work on. Blocking where an agent cannot (a browser's main thread) is a TypeError, as the spec says (AgentCanSuspend).

The code is in src/runtime/heap.ts (the heap) and src/runtime/shared.ts (slots, shapes and the Proxies).