Regression: sysopen rejects O_CREAT|O_EXCL without explicit access mode

首先运行 t/01-basic.t 中聚焦的 tempfile_named 子测试，并跟踪 PerlOnJava 的 sysopen 实现，了解其在 JVM 和解释器后端上对 O_CREAT 与 O_EXCL 标志的转换。为省略访问模式、重复尝试同一文件名以及已存在的 O_EXCL 目标添加项目自有的回归测试覆盖；当聚焦测试和完整的 File::Temp::MoreUtils 测试套件在两个后端上都通过时，即视为完成。

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.