Make the Parser independent from the global C-locale (#5028)

* Make the Parser independent from the global C-locale

* Set a specific test locale using the environment variable FLATBUFFERS_TEST_LOCALE

* Remove redundant static qualifiers

tests/fuzzer/test_init.h

@@ -0,0 +1,56 @@
+
+#ifndef FUZZER_TEST_INIT_H_
+#define FUZZER_TEST_INIT_H_
+
+#include "fuzzer_assert.h"
+#include "test_assert.h"
+
+static_assert(__has_feature(memory_sanitizer) ||
+                  __has_feature(address_sanitizer),
+              "sanitizer disabled");
+
+// Utility for test run.
+struct OneTimeTestInit {
+  // Declare trap for the Flatbuffers test engine.
+  // This hook terminate program both in Debug and Release.
+  static bool TestFailListener(const char *expval, const char *val,
+                               const char *exp, const char *file, int line,
+                               const char *func = 0) {
+    (void)expval;
+    (void)val;
+    (void)exp;
+    (void)file;
+    (void)line;
+    (void)func;
+    // FLATBUFFERS_ASSERT redefined to be fully independent of the Flatbuffers
+    // library implementation (see test_assert.h for details).
+    fuzzer_assert_impl(false);  // terminate
+    return false;
+  }
+
+  OneTimeTestInit() : has_locale_(false) {
+    // Fuzzer test should be independent of the test engine implementation.
+    // This hook will terminate test if TEST_EQ/TEST_ASSERT asserted.
+    InitTestEngine(OneTimeTestInit::TestFailListener);
+
+    // Read a locale for the test.
+    if (flatbuffers::ReadEnvironmentVariable("FLATBUFFERS_TEST_LOCALE",
+                                             &test_locale_)) {
+      TEST_OUTPUT_LINE("The environment variable FLATBUFFERS_TEST_LOCALE=%s",
+                       test_locale_.c_str());
+      test_locale_ = flatbuffers::RemoveStringQuotes(test_locale_);
+      has_locale_ = true;
+    }
+  }
+
+  static const char *test_locale() {
+    return one_time_init_.has_locale_ ? nullptr
+                                      : one_time_init_.test_locale_.c_str();
+  }
+
+  bool has_locale_;
+  std::string test_locale_;
+  static OneTimeTestInit one_time_init_;
+};
+
+#endif  // !FUZZER_TEST_INIT_H_