Define _REENTRANT for SUNOS

[AustinWise]

This matches mono. This is required to make errno a thread-local variable. This fixes problems where calls into libSystem.Native return failure but the the errno is 0. This was causing all kinds of odd asserts in System.IO.Tests and other libraries tests.

See the man page for errno(3c):

In the case of multithreaded applications, the -mt option must be specified on the command line at compilation time (see threads(7)). When the -mt option is specified, errno becomes a macro that enables each thread to have its own errno.

Since -mt is presumably specific to an old Sun compiler, the threads(7) man page says:

Users of other compilers such as gcc and clang should manually set -D_REENTRANT on the compilation line. There are no other libraries flags necessary.

Contributes to #34944

[am11]

I hope you won't take it the wrong way, and I am happy this port is moving forward. I think it'd be better to port something sizeable, such as one of the remaining runtime library needed to complete shared framework, rather than many teeny tiny patches. Otherwise, we would be burdening the maintainers which will slow down the port process / interest.

Thanks for pushing it forward.

[AustinWise]

I hope you won't take it the wrong way, and I am happy this port is moving forward. I think it'd be better to port something sizeable, such as one of the remaining runtime library needed to complete shared framework, rather than many teeny tiny patches. Otherwise, we would be burdening the maintainers which will slow down the port process / interest.

Thanks for pushing it forward.

No offense taken. My intention in making these small PRs was to make them easier to review, not more difficult. I can conceive that that per PR overhead for maintainers like evaluating Build Analysis of the flaky CI system can add up. Please let me know if there is a better way to collaborate on this port.

For what it's worth, this PR and #104448 make it possible to run library tests without the test runner dying in asserts or memory corruption. I expect future PRs to implement larger swaths of missing functionality in the libraries.

[am11]

LGTM, it is aligning with mono:

runtime/src/mono/CMakeLists.txt

Line 295 in 9882188

add_definitions(-DGC_SOLARIS_THREADS -DGC_SOLARIS_PTHREADS -D_REENTRANT -D_POSIX_PTHREAD_SEMANTICS -DUSE_MMAP -DUSE_MUNMAP -DHOST_SOLARIS -D__EXTENSIONS__ -D_XPG4_2)

[AustinWise]

Now that .NET 9 has branched, I rebased this PR.

[janvorli]

LGTM, thank you!