Create a portability header for working with environment variables

Summary:Because `extern char** environ` is not the correct way to access environ on Windows.
This also implements setenv and unsetenv for Windows, which means that TestUtils no longer needs to be disabled for the Windows build.

Reviewed By: mzlee

Differential Revision: D2973704

fb-gh-sync-id: 84db7db3494cf183fcbcc25063cb0482ef84ebf4
shipit-source-id: 84db7db3494cf183fcbcc25063cb0482ef84ebf4

folly/Makefile.am

@@ -265,6 +265,7 @@ nobase_follyinclude_HEADERS = \
 	PicoSpinLock.h \
 	Portability.h \
 	portability/Constexpr.h \
+	portability/Environment.h \
 	portability/Syscall.h \
 	portability/SysUio.h \
 	Preprocessor.h \
@@ -390,6 +391,7 @@ libfolly_la_SOURCES = \
 	detail/MemoryIdler.cpp \
 	MacAddress.cpp \
 	MemoryMapping.cpp \
+	portability/Environment.cpp \
 	Random.cpp \
 	SafeAssert.cpp \
 	SharedMutex.cpp \

folly/Subprocess.cpp

@@ -42,8 +42,7 @@
 #include <folly/ScopeGuard.h>
 #include <folly/String.h>
 #include <folly/io/Cursor.h>
-
-extern char** environ;
+#include <folly/portability/Environment.h>
 
 constexpr int kExecFailure = 127;
 constexpr int kChildFailure = 126;

folly/experimental/TestUtil.cpp

@@ -27,10 +27,7 @@
 #include <folly/File.h>
 #include <folly/FileUtil.h>
 #include <folly/String.h>
-
-#ifndef _MSC_VER
-extern char** environ;
-#endif
+#include <folly/portability/Environment.h>
 
 namespace folly {
 namespace test {

folly/portability/Environment.cpp

+/*
+ * Copyright 2016 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <folly/portability/Environment.h>
+
+#ifdef _WIN32
+#include <Windows.h>
+
+extern "C" {
+
+int setenv(const char* name, const char* value, int overwrite) {
+  if (overwrite == 0 && getenv(name) != nullptr) {
+    return 0;
+  }
+
+  // _putenv_s deletes entries if the value is an empty string,
+  // so we have to call the windows API function to safely assign
+  // these.
+  if (SetEnvironmentVariableA(name, value) != 0) {
+    errno = EINVAL;
+    return -1;
+  }
+  return 0;
+}
+
+int unsetenv(const char* name) {
+  if (_putenv_s(name, "") != 0) {
+    return -1;
+  }
+  return 0;
+}
+}
+#endif

folly/portability/Environment.h

+/*
+ * Copyright 2016 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <stdlib.h>
+
+extern "C" {
+#ifndef _WIN32
+extern char** environ;
+#else
+int setenv(const char* name, const char* value, int overwrite);
+int unsetenv(const char* name);
+#endif
+}