[libc] simplify test for getrandom

`getrandom` is implemented as a syscall.
We don't want to test linux implementation of the syscall. We just want to verify that it reacts as expected to sensible values.

Runtime before
```
[ RUN      ] LlvmLibcGetRandomTest.InvalidFlag
[       OK ] LlvmLibcGetRandomTest.InvalidFlag (took 0 ms)
[ RUN      ] LlvmLibcGetRandomTest.InvalidBuffer
[       OK ] LlvmLibcGetRandomTest.InvalidBuffer (took 0 ms)
[ RUN      ] LlvmLibcGetRandomTest.ReturnsSize
[       OK ] LlvmLibcGetRandomTest.ReturnsSize (took 83 ms)
[ RUN      ] LlvmLibcGetRandomTest.PiEstimation
[       OK ] LlvmLibcGetRandomTest.PiEstimation (took 9882 ms)
```

Runtime after
```
[ RUN      ] LlvmLibcGetRandomTest.InvalidFlag
[       OK ] LlvmLibcGetRandomTest.InvalidFlag (took 0 ms)
[ RUN      ] LlvmLibcGetRandomTest.InvalidBuffer
[       OK ] LlvmLibcGetRandomTest.InvalidBuffer (took 0 ms)
[ RUN      ] LlvmLibcGetRandomTest.ReturnsSize
[       OK ] LlvmLibcGetRandomTest.ReturnsSize (took 0 ms)
[ RUN      ] LlvmLibcGetRandomTest.CheckValue
[       OK ] LlvmLibcGetRandomTest.CheckValue (took 0 ms)
```

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D151336

Author: gchatelet

libc/test/src/sys/random/linux/getrandom_test.cpp

@@ -6,60 +6,49 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "src/__support/CPP/array.h"
 #include "src/errno/libc_errno.h"
 #include "src/math/fabs.h"
 #include "src/sys/random/getrandom.h"
 #include "test/ErrnoSetterMatcher.h"
 #include "test/UnitTest/Test.h"
 
 TEST(LlvmLibcGetRandomTest, InvalidFlag) {
-  using __llvm_libc::testing::ErrnoSetterMatcher::Fails;
-  static constexpr size_t SIZE = 256;
-  char data[SIZE];
-  libc_errno = 0;
-  ASSERT_THAT(__llvm_libc::getrandom(data, SIZE, -1), Fails(EINVAL));
+  __llvm_libc::cpp::array<char, 10> buffer;
   libc_errno = 0;
+  ASSERT_THAT(__llvm_libc::getrandom(buffer.data(), buffer.size(), -1),
+              __llvm_libc::testing::ErrnoSetterMatcher::Fails(EINVAL));
 }
 
 TEST(LlvmLibcGetRandomTest, InvalidBuffer) {
-  using __llvm_libc::testing::ErrnoSetterMatcher::Fails;
-
-  libc_errno = 0;
-  ASSERT_THAT(__llvm_libc::getrandom(nullptr, 65536, 0), Fails(EFAULT));
   libc_errno = 0;
+  ASSERT_THAT(__llvm_libc::getrandom(nullptr, 65536, 0),
+              __llvm_libc::testing::ErrnoSetterMatcher::Fails(EFAULT));
 }
 
 TEST(LlvmLibcGetRandomTest, ReturnsSize) {
-  static constexpr size_t SIZE = 8192;
-  uint8_t buf[SIZE];
-  for (size_t i = 0; i < SIZE; ++i) {
+  __llvm_libc::cpp::array<char, 10> buffer;
+  for (size_t i = 0; i < buffer.size(); ++i) {
     // Without GRND_RANDOM set this should never fail.
-    ASSERT_EQ(__llvm_libc::getrandom(buf, i, 0), static_cast<ssize_t>(i));
+    ASSERT_EQ(__llvm_libc::getrandom(buffer.data(), i, 0),
+              static_cast<ssize_t>(i));
   }
 }
 
-TEST(LlvmLibcGetRandomTest, PiEstimation) {
-  static constexpr size_t LIMIT = 10000000;
-  static constexpr double PI = 3.14159265358979;
+TEST(LlvmLibcGetRandomTest, CheckValue) {
+  // Probability of picking one particular value amongst 256 possibilities a
+  // hundred times in a row is (1/256)^100 = 1.49969681e-241.
+  __llvm_libc::cpp::array<char, 100> buffer;
 
-  auto generator = []() {
-    uint16_t data;
-    __llvm_libc::getrandom(&data, sizeof(data), 0);
-    return data;
-  };
+  for (char &c : buffer)
+    c = 0;
 
-  auto sample = [&]() {
-    auto x = static_cast<double>(generator()) / 65536.0;
-    auto y = static_cast<double>(generator()) / 65536.0;
-    return x * x + y * y < 1.0;
-  };
+  __llvm_libc::getrandom(buffer.data(), buffer.size(), 0);
 
-  double counter = 0;
-  for (size_t i = 0; i < LIMIT; ++i) {
-    if (sample()) {
-      counter += 1.0;
-    }
-  }
-  counter = counter / LIMIT * 4.0;
-  ASSERT_TRUE(__llvm_libc::fabs(counter - PI) < 0.1);
+  bool all_zeros = true;
+  for (char c : buffer)
+    if (c != 0)
+      all_zeros = false;
+
+  ASSERT_FALSE(all_zeros);
 }