JSObjectOps.dropProperty

Syntax

typedef void (*JSPropertyRefOp)(JSContext *cx, JSObject *obj, JSProperty *prop);

Description

The following contract governs JSObjectOps callers and implementations:

• Whenever JSObjectOps.defineProperty or JSObjectOps.lookupProperty returns a JSProperty pointer, the property is locked. If defineProperty is called with a null pprop out parameter, the new property is not locked.
• While the property is locked, the caller may access it using other JSObjectOps callbacks. It may not otherwise call into the JSAPI in ways that might trigger other property accesses. (In a JS_THREADSAFE build, that would risk deadlock.)
• When finished with a locked property, the caller must release it by calling the dropProperty callback.
• The lifetime of the JSProperty * extends from the successful defineProperty or lookupProperty call to the dropProperty call.

As a SpiderMonkey implementation detail, what is actually locked here (under the native implementation of JSObjectOps) is not the property but some collection of objects including the object on which the property is defined. The granularity of the locking is up to the JSObjectOps implementation; deadlock does not happen because each thread accesses only one property at a time.

A single, built-in JSObjectOps implementation is used for most SpiderMonkey objects that are exposed to scripts. Custom JSObjectOps implementations can either retain the SpiderMonkey property storage and locking scheme (by copying all or most of the built-in JSObjectOps) or replace it entirely.

Threads. In a JS_THREADSAFE build, any consistency observed by multiple threads operating on the same data is provided solely by the property locking scheme described above. Currently, the main built-in JSObjectOps implementation serializes all accesses to a given object's properties. (That is, for each object, all property accesses happen in some order, and what each thread observes is consistent with that order: no stale reads, for example.) However, SpiderMonkey does not guarantee this high degree of serialization.