Fix stack scanning for the Julia GC when GAP is used as a library.

[rbehrends]

Fix stack scanning for the Julia GC when GAP is used as a library.

When GAP is being initialized from GAP.jl, we cannot use GapStackBottom to figure out the start of the stack, but have to query Julia for it. Conversely, when GAP is the main program, we cannot rely on Julia knowing the start and end of the stack of the main thread.

This pull request introduces a flag, called UsingLibGap, which allows us to distinguish between the two cases and then calculates the stack extent accordingly.

Checklist for pull request reviewers

• proper formatting

If your code contains kernel C code, run clang-format on it; the
simplest way is to use git clang-format, e.g. like this (don't
forget to commit the resulting changes):

git clang-format $(git merge-base master)

• usage of relevant labels

  1. either release notes: not needed or release notes: to be added
  2. at least one of the labels bug or enhancement or new feature
  3. for changes meant to be backported to stable-4.X add the backport-to-4.X label
  4. consider adding any of the labels build system, documentation, kernel, library, tests

• runnable tests

• adequate pull request title

• well formulated text for release notes

• relevant documentation updates

• sensible comments in the code

[coveralls]

Coverage decreased (-0.0002%) to 84.991% when pulling 52bff6e on rbehrends:julia-gc-fix-stack into dbfbba7 on gap-system:master.

[sebasguts]

Overall, this solution looks good to me, thank you :).

I am actually kind of puzzled that we didn't had such a variable before, but I cannot find one, so I guess it is not there.

[fingolfin]

This seems to be very tightly related to issue #3089, except that you only solve a very special case of that here.

[ThomasBreuer]

The problem with

using GAP, Nemo
GAP.Globals.<TAB>

disappears after the changes from this pull request.

Also in other examples, the effect that Julia freezes at some point disappears.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@dbfbba7). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
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  Coverage          ?   85.17%           
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  Files             ?      699           
  Lines             ?   346082           
  Branches          ?        0           
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  Hits              ?   294785           
  Misses            ?    51297           
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Impacted Files	Coverage Δ
src/gap.c	81.55% <ø> (ø)
src/libgap-api.c	62.8% <100%> (ø)
src/julia_gc.c	77.4% <100%> (ø)