Fixing precompilation hangs due to open tasks or IO

On Julia 1.10 or higher, you might see the following message:

This may repeat. If it continues to repeat with no hints that it will resolve itself, you may have a “precompilation hang” that requires fixing. Even if it’s transient, you might prefer to resolve it so that users will not be bothered by this warning. This page walks you through how to analyze and fix such issues.

If you follow the advice and hit Ctrl-C, you might see

^C Interrupted: Exiting precompilation...
  1 dependency had warnings during precompilation:
┌ Test1 [ac89d554-e2ba-40bc-bc5c-de68b658c982]
│  [pid 2745] waiting for IO to finish:
│   Handle type        uv_handle_t->data
│   timer              0x55580decd1e0->0x7f94c3a4c340

This message conveys two key pieces of information:

• the hang is occurring during precompilation of Test1, a dependency of Test2 (the package we were trying to load with using Test2)
• during precompilation of Test1, Julia created a Timer object (use ?Timer if you’re unfamiliar with Timers) which is still open; until that closes, the process is hung

If this is enough of a hint for you to figure out how timer = Timer(args...) is being created, one good solution is to add wait(timer) if timer eventually finishes on its own, or close(timer) if you need to force-close it, before the final end of the module.

However, there are cases that may not be that straightforward. Usually the best option is to start by determining whether the hang is due to code in Test1 or whether it is due to one of Test1’s dependencies:

• Option 1: Pkg.add("Aqua") and use Aqua.test_persistent_tasks. This should help you identify which package is causing the problem, after which the instructions below should be followed. If needed, you can create a PkgId as Base.PkgId(UUID("..."), "Test1"), where ... comes from the uuid entry in Test1/Project.toml.
• Option 2: manually diagnose the source of the hang.

To manually diagnose:

1. Pkg.develop("Test1")
2. Comment out all the code included or defined in Test1, except the using/import statements.
3. Try using Test2 (or even using Test1 assuming that hangs too) again

Now we arrive at a fork in the road: either

• the hang persists, indicating it is due to one of your dependencies
• the hang disappears, indicating that it is due to something in your code.

Diagnosing and fixing hangs due to a package dependency

Use a binary search to identify the problematic dependency: start by commenting out half your dependencies, then when you isolate which half is responsible comment out half of that half, etc. (You don’t have to remove them from the project, just comment out the using/import statements.)

Once you’ve identified a suspect (here we’ll call it ThePackageYouThinkIsCausingTheProblem), first try precompiling that package. If it also hangs during precompilation, continue chasing the problem backwards.

However, most likely ThePackageYouThinkIsCausingTheProblem will precompile fine. This suggests it’s in the function ThePackageYouThinkIsCausingTheProblem.__init__, which does not run during precompilation of ThePackageYouThinkIsCausingTheProblem but does in any package that loads ThePackageYouThinkIsCausingTheProblem. To test this theory, set up a minimal working example (MWE), something like

(@v1.10) pkg> generate MWE
  Generating  project MWE:
    MWE\Project.toml
    MWE\src\MWE.jl

where the source code of MWE.jl is

module MWE
using ThePackageYouThinkIsCausingTheProblem
end

and you’ve added ThePackageYouThinkIsCausingTheProblem to MWE’s dependencies.

If that MWE reproduces the hang, you’ve found your culprit: ThePackageYouThinkIsCausingTheProblem.__init__ must be creating the Timer object. If the timer object can be safely closed, that’s a good option. Otherwise, the most common solution is to avoid creating the timer while any package is being precompiled: add

ccall(:jl_generating_output, Cint, ()) == 1 && return nothing

as the first line of ThePackageYouThinkIsCausingTheProblem.__init__, and it will avoid doing any initialization in any Julia process whose purpose is to precompile packages.

Fixing package code to avoid hangs

Search your package for suggestive words (here like “Timer”) and see if you can identify where the problem is being created. Note that a method definition like

maketimer() = Timer(timer -> println("hi"), 0; interval=1)

is not problematic in and of itself: it can cause this problem only if maketimer gets called while the module is being defined. This might be happening from a top-level statement such as

const GLOBAL_TIMER = maketimer()

or it might conceivably occur in a precompile workload.

If you struggle to identify the causative lines, then consider doing a binary search: comment out sections of your package (or include lines to omit entire files) until you’ve reduced the problem in scope.