Regression: sysopen rejects O_CREAT|O_EXCL without explicit access mode

Start by running the focused tempfile_named subtest in t/01-basic.t and tracing PerlOnJava's sysopen implementation for O_CREAT and O_EXCL flag translation on both JVM and interpreter backends. Add project-owned regression coverage for omitted access mode, repeated filename attempts, and existing O_EXCL targets; done means the focused test and complete File::Temp::MoreUtils suite pass on both backends.

Summary

File::Temp::MoreUtils 0.005 regressed on PerlOnJava because sysopen rejects a valid Perl flag combination when no explicit read/write access flag is supplied.

Regression metadata

• CPAN run: 20260918-141920-96054
• Distribution: File-Temp-MoreUtils-0.005
• Previous pass: 2026-08-29 at commit 19c285d26
• Failing test: t/01-basic.t, subtest tempfile_named
• Reproduces on both JVM and interpreter backends

Reproduction

The upstream module calls:

sysopen $fh, $name, O_CREAT | O_CREAT | O_EXCL

The duplicate O_CREAT is harmless. The important compatibility point is that no explicit O_RDONLY, O_WRONLY, or O_RDWR flag is present. Perl treats the absent access bits as the default read-only mode while still allowing creation with O_CREAT | O_EXCL.

Minimal reproducer:

use Fcntl ':DEFAULT';
my $path = '/tmp/perlonjava-sysopen-check';
my $ok = sysopen(my $fh, $path, O_CREAT | O_CREAT | O_EXCL);
print defined($ok) ? "created\\n" : "failed: $!\\n";

For comparison, adding O_WRONLY makes the call succeed on PerlOnJava:

sysopen $fh, $path, O_WRONLY | O_CREAT | O_EXCL;

Expected result

System Perl passes the complete upstream suite. The tempfile_named test creates a.1, then continues through the filename-suffix and directory cases.

Actual result

PerlOnJava fails at the first creation attempt:

tempfile_named(): Can't create temporary file 'a.1'

The CPAN regression record reports one failed test program and one failed subtest. The focused test fails identically on both JVM and interpreter backends.

The minimal reproducer confirms that O_CREAT | O_EXCL fails under PerlOnJava, while O_WRONLY | O_CREAT | O_EXCL succeeds. This is independent of the temporary-directory location and is not a permissions, service, or platform-prerequisite failure.

Likely root cause

PerlOnJava's sysopen implementation appears to require an explicit access mode when translating Perl open flags to Java file-opening options. Standard Perl permits the access bits to be omitted, interpreting them as read-only. PerlOnJava should preserve that default while still honoring O_CREAT, O_EXCL, and the other creation flags.

Acceptance criteria

• sysopen with O_CREAT | O_EXCL and no explicit access flag succeeds on both backends, matching system Perl.
• Existing O_RDONLY, O_WRONLY, and O_RDWR behavior remains correct.
• O_EXCL continues to report an existing target as an expected collision rather than a generic creation failure.
• File::Temp::MoreUtils 0.005 passes its complete upstream test suite on both backends.
• Add focused project-owned regression tests for omitted access mode with O_CREAT, O_EXCL, and repeated filename attempts.
• Re-run the CPAN regression to confirm recovery from the previous pass boundary.

Evidence

• System Perl: PASS for File::Temp::MoreUtils 0.005.
• PerlOnJava: FAIL in tempfile_named on JVM and interpreter.
• The package is pure Perl and uses core Fcntl, File::Temp, and sysopen; no native extension or external service is involved.
• No exact existing GitHub issue was found for this sysopen flag combination.