docs: explain what world-age issue is (#362)

Co-authored-by: Tim Holy <[email protected]>

docs/make.jl

@@ -12,6 +12,7 @@ makedocs(
         "registry.md",
         "registering.md",
         "implementing.md",
+        "world_age_issue.md",
         "reference.md",
     ],
 )

docs/src/world_age_issue.md

@@ -0,0 +1,164 @@
+# World age issue
+
+## Motivation: lazy loading
+
+The goal of FileIO is to provide a unified IO frontend so that users can easily deal with file IO
+with the simple `load`/`save` functions. The actual IO work will be dispatched to various IO
+backends. For instance, [PNGFiles.jl](https://github.com/JuliaIO/PNGFiles.jl) is used to load PNG
+format images. If `using FileIO` were to load all registered IO backends, then it would be very slow
+to load, hurting all users of FileIO. For any given user, most of those backends would also be
+unnecessary -- for example, people who don't do image processing probably don't want to load any
+thing related to image IO.
+
+To avoid such unnecessary loading latency, FileIO defers package loading until it's actually used.
+For instance, when you use FileIO, you'll probably observe something like this:
+
+```julia
+julia> using TestImages, FileIO
+
+julia> path = testimage("cameraman"; download_only=true)
+"/home/jc/.julia/artifacts/27a4c26bcdd47eb717bee089ec231a899cb8ef69/cameraman.tif"
+
+julia> load(path) # actual backend loading happens here
+[ Info: Precompiling ImageIO [82e4d734-157c-48bb-816b-45c225c6df19]
+[ Info: Precompiling TiffImages [731e570b-9d59-4bfa-96dc-6df516fadf69]
+...
+```
+
+ImageIO and TiffImages were loaded because the file in `path` was detected to be a TIFF image, well
+after FileIO was loaded into the session.
+
+## The hidden issue
+
+Although this lazy-loading trick reduces the time needed for `using FileIO`, it isn't normal
+practice in Julia because it introduces a so-called _world age issue_ or _world age problem_. The
+world age issue happens when you call methods that get compiled in a newer "world" (get compiled
+after initial compilation finishes) than the one you called them from.
+
+Let's demonstrate the problem concretely. In case you don't have a suitable file to play with, let's
+first create one:
+
+```julia
+julia> using IndirectArrays, ImageCore
+
+julia> img = IndirectArray(rand(1:5, 4, 4), rand(RGB, 5))
+4×4 IndirectArray{RGB{Float64}, 2, Int64, Matrix{Int64}, Vector{RGB{Float64}}}:
+...
+
+julia> save("indexed_image.png", img)
+```
+
+Now, **reopen a new julia REPL** (this is crucial for demonstrating the problem) and call `load`
+from **within a function** (this is also crucial):
+
+```julia
+julia> using FileIO
+
+julia> f() = size(load("indexed_image.png"))
+f (generic function with 1 method)
+
+julia> f()
+ERROR: MethodError: no method matching size(::IndirectArrays.IndirectArray{ColorTypes.RGB{FixedPointNumbers.N0f8}, 2, UInt8, Matrix{UInt8}, OffsetArrays.OffsetVector{ColorTypes.RGB{FixedPointNumbers.N0f8}, Vector{ColorTypes.RGB{FixedPointNumbers.N0f8}}}})
+The applicable method may be too new: running in world age 32382, while current world is 32416.
+Closest candidates are:
+  size(::IndirectArrays.IndirectArray) at ~/.julia/packages/IndirectArrays/BUQO3/src/IndirectArrays.jl:52 (method too new to be called from this world context.)
+  size(::AbstractArray{T, N}, ::Any) where {T, N} at abstractarray.jl:42
+  size(::Union{LinearAlgebra.Adjoint{T, var"#s880"}, LinearAlgebra.Transpose{T, var"#s880"}} where {T, var"#s880"<:(AbstractVector)}) at /Applications/Julia-1.8.app/Contents/Resources/julia/share/julia/stdlib/v1.8/LinearAlgebra/src/adjtrans.jl:173
+  ...
+Stacktrace:
+ [1] f()
+   @ Main ./REPL[2]:1
+ [2] top-level scope
+   @ REPL[3]:1
+```
+
+To understand why this happened, you have to understand the order of events:
+
+1. When calling `f()` from the REPL, Julia first compiled `f`. Importantly, when compiling, Julia
+   didn't know what type of object was going to be returned by `load`, so in the compiled code it
+   waits to see what object actually gets returned before figuring out which method of `size` to
+   call. (This is called _runtime dispatch_.)
+2. It queried the file, recognized a PNG file, and _loaded the ImageIO and PNGFiles packages_. (It's
+   for the loading of these packages that you needed to start a fresh Julia session.)
+3. FileIO calls the appropriate PNG-specific `load` function in PNGFiles. (We'll have more to say
+   about this step further below.) This causes an image to be returned, which is an array of a type
+   defined by the IndirectArrays package (a dependency of PNGFiles).
+4. `f` calls `size` on the returned image. However, this fails, because at the time you called `f`,
+   the IndirectArrays package wasn't loaded.
+
+In other words, `size` method for `IndirectArray` lives in a world that's newer than the one from
+which you called `f()`. This leads to the observed error.
+
+!!! note
+    World age is crucial to Julia's ability to allow you to _redefine_ methods interactively, but
+    the error we're illustrating is an unfortunate side-effect.
+
+The good news is it's easy to fix, just try calling `f()` again:
+
+```julia
+julia> f()
+(4, 4)
+```
+
+The second `f()` works because this time you're calling `f()` in the latest world age with the
+necessary `size(::IndirectArray)` already defined. In essence, you fast-forward to the latest world
+with each statement you type at the REPL.
+
+## Solutions
+
+### `Base.invokelatest`
+
+One solution is to make the call to `size` via `Base.invokelatest`, which exists explicitly to work
+around this world-age dispatch problem. Literally, `invokelatest` dispatches the supplied call using
+the latest world age (which may be newer than when you typed `f()` at the REPL). **In a fresh Julia
+session**,
+
+```julia
+julia> using FileIO
+
+julia> f() = Base.invokelatest(size, load("indexed_image.png"))
+f (generic function with 1 method)
+
+julia> f()
+(4, 4)
+```
+
+!!! note
+    In step 3 above ("FileIO calls the appropriate PNG-specific `load` function in PNGFiles"),
+    the call to the `load` function defined in PNGFiles is made via `invokelatest`.
+    Otherwise, even ordinary interactive usage of FileIO (without burying `load` inside a function)
+    would cause world-age errors.
+
+!!! warning
+    Using `invokelatest` slows your code considerably. Use it only when absolutely necessary.
+
+### Eagerly load the required packages first
+
+Another solution to the world age issue is simple and doesn't have long-term downsides: **eagerly
+load the needed packages**. For instance, if you're seeing world age issue complaining methods
+related to `IndirectArray`, then load IndirectArrays eagerly:
+
+```julia
+julia> using FileIO, IndirectArrays # try this on a new Julia REPL
+
+julia> f() = size(load("indexed_image.png"))
+f (generic function with 1 method)
+
+julia> f()
+(4, 4)
+```
+
+Thus if you want to build a package, it could be something like this:
+
+```julia
+module MyFancyPackage
+
+# This ensures that whoever loads `MyFancyPackage`, he has IndirectArrays loaded and
+# thus avoid the world age issue.
+using IndirectArrays, FileIO
+
+f(file) = length(load(file))
+end
+```
+
+Enjoy the FileIO and its lazy loading, but be aware that its speedy loading comes with some caveats.