Thread-local storage support across dynamic-library boundaries.
Normally, a std::thread::LocalKey refers to a specific memory location in a
single binary. This library adds support for thread-local storage across
binaries by storing thread-local values in a shared map between a host and one
or more plugins.
Use the host feature to enable the host capabilities (in exactly one binary to
be loaded in memory), and use the plugin feature to enable plugin capabilities
(in zero or more binaries to be loaded).
In each plugin binary, call Context::initialize_tls with the host context
prior to any thread-local storage being accessed (typically as part of the
binary startup routine). The host binary will default to the correct state,
however this may be overwritten with initialize_tls as well.
Besides that, the thread_local! macro works exactly like std::thread_local!
for both the host and plugins. Note that thead-local values are indexed by the
given name, type, and module path, so these must not conflict. Also note that
stored values must be abi-stable. Abi-stability is not enforced in LocalKey
with the abi_stable::StableAbi trait for library flexibility, but use of
abi_stable is highly encouraged.
Note that both host and plugin may be enabled together, specifically for use
in workspaces that may contain both the host and plugin(s).
If unloading plugins, one must ensure that any memory set by plugins in
thread-local storage is removed (including any initial values!). In the future
there could be additions to track which initial values are created by a
particular plugin, but for now one solution is to call Context::reset to clear
the thread-local values for the current thread.