sanity: add libc/stdlib sanity checks

These are meant to test our back-compat stubs, but they are enabled for all
builds for the sake of consistency.

src/compat/glibc_sanity.cpp

@@ -0,0 +1,61 @@
+#include "bitcoin-config.h"
+
+#include <cstddef>
+#if defined(HAVE_SYS_SELECT_H)
+#include <sys/select.h>
+#endif
+
+extern "C" void* memcpy(void* a, const void* b, size_t c);
+void* memcpy_int(void* a, const void* b, size_t c)
+{
+  return memcpy(a,b,c);
+}
+
+namespace {
+// trigger: Use the memcpy_int wrapper which calls our internal memcpy.
+//   A direct call to memcpy may be optimized away by the compiler.
+// test: Fill an array with a sequence of integers. memcpy to a new empty array.
+//   Verify that the arrays are equal. Use an odd size to decrease the odds of
+//   the call being optimized away.
+template <unsigned int T>
+bool sanity_test_memcpy()
+{
+    unsigned int memcpy_test[T];
+    unsigned int memcpy_verify[T] = {};
+    for (unsigned int i = 0; i != T; ++i)
+        memcpy_test[i] = i;
+
+    memcpy_int(memcpy_verify,memcpy_test,sizeof(memcpy_test));
+
+    for (unsigned int i = 0; i != T; ++i)
+    {
+        if(memcpy_verify[i] != i)
+            return false;
+    }
+    return true;
+}
+
+#if defined(HAVE_SYS_SELECT_H)
+// trigger: Call FD_SET to trigger __fdelt_chk. FORTIFY_SOURCE must be defined
+//   as >0 and optimizations must be set to at least -O2.
+// test: Add a file descriptor to an empty fd_set. Verify that it has been
+//   correctly added.
+bool sanity_test_fdelt()
+{
+    fd_set fds;
+    FD_ZERO(&fds);
+    FD_SET(0, &fds);
+    return FD_ISSET(0,&fds);
+}
+#endif
+
+} // anon namespace
+
+bool glibc_sanity_test()
+{
+#if defined(HAVE_SYS_SELECT_H)
+    if (!sanity_test_fdelt())
+        return false;
+#endif
+    return sanity_test_memcpy<1025>();
+}