[libc] Accommodate Fuchsia's death test framework in fenv tests.

[sivachandra]

Summary

Fuchsia's death test framework runs the closure which can die in a
different thread. Hence, the FP exceptions which cause the closure to
die should be enalbed in the closure.

[michaelrj]

LGTM with nit

libc/test/src/fenv/feholdexcept_test.cpp

37

You need an "it" between "closure" and "sets".

libc/test/src/fenv/enabled_exceptions_test.cpp

for (int e : excepts) {
     // can raise FE_INEXACT as well, we don't verify the other
     // exception flags when FE_INEXACT is enabled.
     if (e != FE_INEXACT) {
       int others = allExcepts & ~e;
       ASSERT_EQ(__llvm_libc::feraiseexcept(others), 0);
       ASSERT_EQ(__llvm_libc::fetestexcept(others), others);
     }
 
-    ASSERT_RAISES_FP_EXCEPT([=] { __llvm_libc::feraiseexcept(e); });
+    ASSERT_RAISES_FP_EXCEPT([=] {
+      // In test frameworks like Fuchsia's zxtest, this translates to
+      // a death test which run in a different thread. So, we enable
+      // the exception again so that the exception gets enabled in
+      // the child thread.
+      __llvm_libc::fputil::enableExcept(e);
+      __llvm_libc::feraiseexcept(e);
+    });
+
+    // Cleanup.
+    __llvm_libc::fputil::disableExcept(FE_ALL_EXCEPT);
+    ASSERT_EQ(__llvm_libc::feclearexcept(FE_ALL_EXCEPT), 0);
   }
 }

libc/test/src/fenv/feholdexcept_test.cpp

for (int e : excepts) {
     __llvm_libc::fputil::disableExcept(FE_ALL_EXCEPT);
     __llvm_libc::fputil::enableExcept(e);
     ASSERT_EQ(__llvm_libc::fputil::clearExcept(FE_ALL_EXCEPT), 0);
     ASSERT_EQ(__llvm_libc::feholdexcept(&env), 0);
     // feholdexcept should disable all excepts so raising an exception
     // should not crash/invoke the exception handler.
     ASSERT_EQ(__llvm_libc::fputil::raiseExcept(e), 0);
 
+    ASSERT_RAISES_FP_EXCEPT([=] {
       // When we put back the saved env which has the exception enabled, it
-    // should crash with SIGFPE.
+      // should crash with SIGFPE. Note that we set the old environment
+      // back in this closure because in some test frameworks like Fuchsia's
+      // zxtest, this translates to a death test in which this closure is
+      // run in a different thread. By setting the old environment inside
+      // the closure sets the environment for that different thread as well.
       __llvm_libc::fputil::setEnv(&env);
-    ASSERT_RAISES_FP_EXCEPT([=] { __llvm_libc::fputil::raiseExcept(e); });
+      __llvm_libc::fputil::raiseExcept(e);
+    });
+
+    // Cleanup
+    __llvm_libc::fputil::disableExcept(FE_ALL_EXCEPT);
+    ASSERT_EQ(__llvm_libc::fputil::clearExcept(FE_ALL_EXCEPT), 0);
   }
 }