Port Devel::Size for PerlOnJava #1880

Summary

Port Devel::Size for PerlOnJava so CPAN modules that use it as a runtime dependency can load and estimate Perl object sizes on the JVM and interpreter backends.

Devel::Size is currently absent from PerlOnJava's bundled modules. A minimal use fails during module lookup:

use Devel::Size qw(size total_size);

The missing module blocks consumers that declare it as a runtime prerequisite. One concrete consumer observed during CPAN testing is DBG 0.4.1.

Upstream behavior and expected API

The current CPAN release is Devel-Size 0.87, released 2026-03-31. Its Perl wrapper exports size and total_size on request (and via the :all tag), and exposes $Devel::Size::warn and $Devel::Size::dangle controls. The implementation currently loads XS and traverses Perl values, including aggregate/reference structures.

On PerlOnJava, size($value) should return a useful estimate for the value's PerlOnJava representation, and total_size($value) should estimate the reachable structure while counting shared referents once and handling cycles. Estimates should be stable enough for comparisons and diagnostics, but need not match Perl's C allocator byte-for-byte. The module should load without XS and work on both execution backends.

Porting scope

• Add a bundled Devel::Size Perl module and Java runtime implementation, following the existing PerlOnJava module-registration pattern.
• Traverse PerlOnJava runtime values and account for scalars, arrays, hashes, references, code/glob-like values where supported, shared references, and cyclic graphs.
• Preserve the documented exports and warning/dangling-reference controls where applicable; document any deliberate limitations of the Java object model.
• Adapt the upstream tests as project-owned tests, including recursion/cycle and shared-reference cases, and run them on system Perl before relying on them for PerlOnJava behavior.
• Document that these values estimate PerlOnJava-managed object footprints, not process RSS or exact native Perl allocation sizes.

Reverse-dependency estimate

MetaCPAN's reverse-dependency endpoint currently reports 24 distinct distributions mentioning Devel::Size across prerequisite phases. Filtering the latest release metadata to direct runtime requires gives an estimated 22 direct runtime dependants; 6 of those latest releases were updated in the two years before 2026-10-24. These are distribution-level metadata counts, not a count of active users or all transitive consumers. The runtime dependants include diagnostic, profiling, benchmark, GUI, and application distributions.

Sources: MetaCPAN reverse dependencies API and MetaCPAN API documentation.

Acceptance criteria

• use Devel::Size qw(size total_size) succeeds without an installed native Devel::Size on both JVM and interpreter backends.
• size and total_size return non-negative numeric estimates for supported values; aggregate size grows as contained data grows.
• Shared referents are not counted repeatedly by total_size, and cyclic structures terminate.
• Export/tag behavior and documented controls have focused regression coverage, with unsupported behavior documented.
• The adapted project-owned tests pass with system Perl as an oracle where the semantics are standard Perl behavior, and on both PerlOnJava backends.

Related: #1873 (a separate FileHandle/IO::Seekable porting issue whose CPAN investigation also encountered this missing prerequisite).