[nicolas-maman]

Main's Linux / GCC leg has failed the cross-version ABI matrix since the 0.861.0 release (#3481), so every PR is red there. This fixes it with the model the owner chose: a library that carries its own runtime keeps it on Linux and FreeBSD, as it already does on macOS and Windows.

What was wrong

The first commit makes the matrix print an Aether consumer's stderr on failure, which showed:

aether: the library was made for Aether runtime 0.860.0/abi1 and runs on 0.863.0/abi1: rebuild it with the Aether of the runtime it runs on
aether: panic outside any try/catch or actor: ...

The chain:

• ae build links a program -rdynamic, because plugins loaded through std.dl and script_gateway resolve runtime symbols against it. On ELF the program therefore exports its runtime.
• A library's calls into the runtime it carries then bound to the program's runtime instead.
• The library's runtime ABI check (Discussion: compiler/runtime ABI compatibility policy for shipped native libraries #2957) saw a runtime it was not made for, and refused.
• The importer ended on the panic.

It surfaced only when two runtimes that both have the check met with different versions:

• equal versions passed (0.860.0 against 0.860.0);
• a pair where one side predates the check passed (0.860.0 against 0.859.0);
• macOS (two-level namespace) and Windows (PE) never bind across, and pass.

The fix

• The link. On ELF, ae links a library with its own runtime -Bsymbolic, so its references bind to its own definitions. It still exports them, for a C host. A --shared-runtime library is unchanged.
• The same behaviour everywhere. Two things ae did only on macOS and Windows now apply everywhere, both naming --shared-runtime: refusing an import that needs one runtime (a struct holding strings, a builder that uses its config, A string an --emit=lib export returns is never released (C hosts and binary imports) #2931), and warning that a library's actors are not drained at exit.
• Libraries linked before this fix. On ELF, a library of another runtime ABI linked without -Bsymbolic is refused at import, naming the rebuild, instead of the program ending on its first call. The flag is read from the file's dynamic section (DT_SYMBOLIC, or DF_SYMBOLIC in DT_FLAGS). The reader was checked on x86_64, aarch64 and 32-bit ARM libraries built with and without -Bsymbolic.

What changes for Linux and FreeBSD users:

• A static-runtime library's panics no longer reach the program's catch.
• Its actors' in-flight messages are no longer drained at exit.
• An import that needs one runtime is refused.

--shared-runtime gives one runtime, as it already does on macOS and Windows.

Tests

• The ABI matrix:
  • checks that this ae links SYMBOLIC on ELF (it fails on main);
  • runs the 0.860.0 importer with this library (it aborts on main);
  • expects the refusal for the 0.860.0 library under this importer.
• Four tests drop their Linux-only branch: binary_import_actors, binary_import_ownership, binary_import_structs and sandbox_nested_libraries now expect on Linux and FreeBSD what they already expected on macOS and Windows.
• Docs: emit-lib.md, abi-policy.md and ownership-model.md.

Checked locally on Windows (there is no ELF machine here):

• ae builds with -Werror;
• it compiles for x86_64 Linux with zig (-Wall -Wextra -Werror);
• the scripts parse.

CI shows Linux and FreeBSD.

Closes #3481

[nicolas-maman]

Folded into #3477, which was meant to be the one PR: the same two commits are there (012c9a4, 682c917), and it closes #3481.