fix: fork-open exec emulation and pipe open %ENV passing

[fglock]

Summary

This PR contains multiple fixes for fork/pipe/exec emulation issues and related bugs discovered while testing Module::Build.

Fix 1: Handle indirect object syntax in fork-open exec emulation

Problem: Module::Build's _backticks() method uses this pattern:

my $pid = open *FH, "-|";
if ($pid) {
    return join "", <FH>;
} else {
    exec { $cmd[0] } @cmd;
}

The fork-open emulation was returning empty output because completeForkOpen() was not handling the indirect object syntax correctly.

Fix: Mirror the logic from regular exec() to extract the correct arguments.

Fix 2: Pipe open not passing %ENV to child processes

Problem: Pipe open (open($fh, "$command|")) was NOT passing the Perl %ENV hash to child processes. This caused:

• Environment variables set in the parent (like $ENV{PERL5LIB}) to be invisible to children
• Test::Harness and Module::Build tests to fail because PERL5LIB was not propagating

Fix: Added copyPerlEnvToProcessBuilder() method to both PipeInputChannel.java and PipeOutputChannel.java.

Fix 3: Test/Harness.pm relative path handling

Fixes Test/Harness.pm to convert relative paths to absolute paths so they work correctly in child processes that may run from a different directory.

Fix 4: blib.pm not setting PERL5LIB

Problem: The -Mblib switch only modified @INC in the current process but did not set PERL5LIB. Since PerlOnJava cannot use fork() to share address space, child processes did not inherit the blib paths.

Fix: Modified blib.pm to also set PERL5LIB so child processes can find modules in blib/lib and blib/arch.

Fix 5: File::Spec->canonpath not removing leading ./

Problem: The Java implementation of File::Spec->canonpath() did not strip leading ./ prefixes from paths, unlike the Perl version.

Example: canonpath("./lib/Simple.pm") returned "./lib/Simple.pm" instead of "lib/Simple.pm"

Impact: Caused Module::Build's DistGen->clean() to fail matching files, leading to incorrect file deletion.

Fix: Updated FileSpec.java to remove leading ./ while preserving ./ when it is the entire path.

Fix 6: Regex capture groups for optional groups

Problem: When a regex with optional capture groups (like m{^(.*/)?(.*)}s) matched, groups that did not participate were skipped in the return list instead of returning undef.

Example: my ($a, $b) = ("MANIFEST" =~ m{^(.*/)?(.*)}s) was yielding ($a = "MANIFEST", $b = undef) instead of ($a = undef, $b = "MANIFEST")

Impact: Broke File::Basename::dirname() which relies on this pattern, causing dirname("MANIFEST") to return "MANIFEST" instead of ".". This in turn caused Module::Build to incorrectly delete all files during clean().

Fix: Updated RuntimeRegex.java to include undef values for unmatched groups.

Fix 7: Regex capture groups for branch reset patterns

Problem: Fix 6 caused a regression (-8 tests) in re/pat.t for branch reset patterns ((?|...)). These patterns were returning undef values for groups in non-matching alternatives.

Root cause: Java's regex engine creates separate capture groups for each alternative in a branch reset pattern, while Perl reuses the same group numbers. When alternative 3 of (?|(p)(q)|(x)(y)|(a)(b)) matches "ab":

• Java groups 1-4: null (from non-matching alternatives)
• Java groups 5-6: "a" and "b"

With Fix 6, all 6 groups were returned as (undef, undef, undef, undef, "a", "b") instead of ("a", "b").

Fix: Track whether a pattern uses branch reset syntax and:

• For branch reset patterns: skip null groups (original behavior)
• For non-branch-reset: include undef for unmatched optional groups

This restores re/pat.t to 1065/1298 while preserving the File::Basename fix.

Fix 8: Document test environment variables

Added documentation to AGENTS.md about environment variables needed when running tests directly (JPERL_UNIMPLEMENTED, JPERL_OPTS, PERL_SKIP_BIG_MEM_TESTS).

Test Plan

• Unit tests pass (make)
• Manual test: exec { $cmd[0] } @cmd with fork-open now returns correct output
• Environment variables now pass through pipe open to child processes
• -Mblib now properly propagates blib paths to child processes
• File::Spec->canonpath("./foo") correctly returns "foo"
• File::Basename::dirname("MANIFEST") correctly returns "."
• re/pat.t: 1065/1298 tests pass (no regression from Fix 6)
• Branch reset: "ab" =~ /(?|(p)(q)|(a)(b))/ returns ("a", "b") correctly
• Module::Build basic.t: Runs all 58 tests (previously stopped at test 20)
• Module::Build compat.t: 138/165 tests pass

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