Global Weak Cache

Cache big expensive objects (that consume a lot of OS resources).

get_through_global_weak_cache(getter::Function, path::AbstractString, key::Any)::Any

Get some object associated with a path and identified by the key from the global cache (if it is there) or invoke the getter to obtain it (and cache it for future reference). As a convenience, the getter is passed the path and key . If purge is set, then any existing value is ignored and is replaced by the result of the getter . The file's inode is always compared, so a cached entry is invalidated if the file at path has been replaced (for example, by rm followed by re-creation). By default, the mtime of the file is **not** checked: callers are assumed to be the sole writers of the file, and their own writes (which bump the mtime ) must not throw away a cached object that is still in use. If the caller instead wants to treat external mtime changes as invalidation (for example, because the file may be rewritten by an unrelated process), set check_mtime = true`.

There's a singleton GLOBAL_CACHE and all accesses are protected by a single GLOBAL_LOCK , so while this is thread-safe, it does not support parallelism. It is meant for "expensive" OS operations such as mmap , not for memoizing functions results for performance.

The cache holds weak references to the objects, so they are let go if there remain no active references to them (outside the cache).

```jldoctest first = get through global weak cache("fake path", :test) do _ return [1] end @assert first == [1] @assert first === get through global weak cache("fake path", :test) do _ @assert false; end

second = get through global weak cache("fake path", :test; purge = true) do _ return [2] end @assert second == [2];

first = second = nothing gc() third = get through global weak cache("fake path", :test) do _ return [3] end @assert third == [3];

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