Add new API to set envar while specifying overwrite (ros2#473)

* Add rcutils_set_env_overwrite api

Signed-off-by: Yadunund <yadunund@intrinsic.ai>

* Add some tests

Signed-off-by: Yadunund <yadunund@intrinsic.ai>

---------

Signed-off-by: Yadunund <yadunund@intrinsic.ai>

Author: Yadunund

include/rcutils/env.h

@@ -55,6 +55,36 @@ RCUTILS_WARN_UNUSED
 bool
 rcutils_set_env(const char * env_name, const char * env_value);
 
+/// Set or un-set a process-scoped environment variable while specifying overwrite behavior.
+/**
+ * This function modifies the environment variables for the current process by
+ * copying given string values into the process' global environment variable
+ * store.
+ *
+ * \par Thread Safety:
+ * This function is not thread-safe. Take care not to modify the environment variables while
+ * another thread might be reading or writing environment variables.
+ *
+ * \par Platform Consistency:
+ * The behavior when setting a variable to an empty string (`""`) differs
+ * between platforms. On Windows, the variable is un-set (as if \p env_value was
+ * `NULL`), while on other platforms the variable is set to an empty string as
+ * expected.
+ *
+ * \param[in] env_name Name of the environment variable to modify.
+ * \param[in] env_value Value to set the environment variable to, or `NULL` to
+ *   un-set.
+ * \param[in] overwrite If true, the environemnt variable value will not be overwritten
+ *   if previously set.
+ * \return `true` if success, or
+ * \return `false` if env_name is invalid or NULL, or
+ * \return `false` on failure.
+ */
+RCUTILS_PUBLIC
+RCUTILS_WARN_UNUSED
+bool
+rcutils_set_env_overwrite(const char * env_name, const char * env_value, bool overwrite);
+
 /// Retrieve the value of the given environment variable if it exists, or "".
 /** The c-string which is returned in the env_value output parameter is only
  * valid until the next time this function is called, because it is a direct

src/env.c

@@ -26,34 +26,41 @@ extern "C"
 
 bool
 rcutils_set_env(const char * env_name, const char * env_value)
+{
+  return rcutils_set_env_overwrite(env_name, env_value, true);
+}
+
+bool
+rcutils_set_env_overwrite(const char * env_name, const char * env_value, bool overwrite)
 {
   RCUTILS_CAN_RETURN_WITH_ERROR_OF(false);
 
   RCUTILS_CHECK_FOR_NULL_WITH_MSG(
     env_name, "env_name is null", return false);
 
+  if ((int)overwrite == 0 && getenv(env_name) != NULL) {
+    return true;
+  }
+
+  int set_ret;
 #ifdef _WIN32
   if (NULL == env_value) {
     env_value = "";
   }
-  if (0 != _putenv_s(env_name, env_value)) {
-    RCUTILS_SET_ERROR_MSG_WITH_FORMAT_STRING("_putenv_s failed: %d", errno);
-    return false;
-  }
+  set_ret = _putenv_s(env_name, env_value);
 #else
   if (NULL == env_value) {
-    if (0 != unsetenv(env_name)) {
-      RCUTILS_SET_ERROR_MSG_WITH_FORMAT_STRING("unsetenv failed: %d", errno);
-      return false;
-    }
+    set_ret = unsetenv(env_name);
   } else {
-    if (0 != setenv(env_name, env_value, 1)) {
-      RCUTILS_SET_ERROR_MSG_WITH_FORMAT_STRING("setenv failed: %d", errno);
-      return false;
-    }
+    set_ret = setenv(env_name, env_value, (int) overwrite);
   }
 #endif
 
+  if (set_ret != 0) {
+    RCUTILS_SET_ERROR_MSG_WITH_FORMAT_STRING("setting environment variable failed: %d", errno);
+    return false;
+  }
+
   return true;
 }
 

test/test_env.cpp

@@ -84,6 +84,32 @@ TEST(TestEnv, test_get_env) {
   EXPECT_STREQ("", env);
 }
 
+/* Tests rcutils_get_env.
+ *
+ * Expected environment variables must be set by the calling code:
+ *
+ *   - EMPTY_TEST=
+ *   - NORMAL_TEST=foo
+ *
+ * These are set in the call to `ament_add_gtest()` in the `CMakeLists.txt`.
+ */
+TEST(TestEnv, test_set_env_overwrite) {
+  const char * env;
+  const char * ret;
+
+  // Do not overwrite environment variable if preset if overwrite is set false.
+  EXPECT_TRUE(rcutils_set_env_overwrite("NORMAL_TEST", "NewEnvValue", false));
+  ret = rcutils_get_env("NORMAL_TEST", &env);
+  EXPECT_TRUE(NULL == ret);
+  EXPECT_STREQ("foo", env);
+
+  // Overwrite environment variable if present if overwrite is set true.
+  EXPECT_TRUE(rcutils_set_env_overwrite("NORMAL_TEST", "NewEnvValue", true));
+  ret = rcutils_get_env("NORMAL_TEST", &env);
+  EXPECT_TRUE(NULL == ret);
+  EXPECT_STREQ("NewEnvValue", env);
+}
+
 TEST(TestEnv, test_get_home) {
   EXPECT_STRNE(NULL, rcutils_get_home_dir());
   const char * home = NULL;