Add convenient 'GetEnv' and 'SetEnv' implementations back (#91)

It's almost a copy of 'util/system/env.h' (removed now by PR #117). Only now we're using std::getenv with some nullptr-check instead of the old GetEnv. Since this testing library actively uses these functions (the rest of the whole library hardly uses them), it seems reasonable to have them as a part of 'cpp-testing-common' library.

Author: tsayukov

library/cpp/testing/common/CMakeLists.darwin-arm64.txt

@@ -15,6 +15,7 @@ target_link_libraries(cpp-testing-common PUBLIC
 )
 target_sources(cpp-testing-common PRIVATE
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/env.cpp
+  ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/env_var.cpp
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/network.cpp
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/probe.cpp
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/scope.cpp

library/cpp/testing/common/CMakeLists.darwin-x86_64.txt

@@ -15,6 +15,7 @@ target_link_libraries(cpp-testing-common PUBLIC
 )
 target_sources(cpp-testing-common PRIVATE
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/env.cpp
+  ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/env_var.cpp
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/network.cpp
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/probe.cpp
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/scope.cpp

library/cpp/testing/common/CMakeLists.linux-aarch64.txt

@@ -15,6 +15,7 @@ target_link_libraries(cpp-testing-common PUBLIC
 )
 target_sources(cpp-testing-common PRIVATE
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/env.cpp
+  ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/env_var.cpp
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/network.cpp
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/probe.cpp
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/scope.cpp

library/cpp/testing/common/CMakeLists.linux-x86_64.txt

@@ -15,6 +15,7 @@ target_link_libraries(cpp-testing-common PUBLIC
 )
 target_sources(cpp-testing-common PRIVATE
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/env.cpp
+  ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/env_var.cpp
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/network.cpp
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/probe.cpp
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/scope.cpp

library/cpp/testing/common/CMakeLists.windows-x86_64.txt

@@ -15,6 +15,7 @@ target_link_libraries(cpp-testing-common PUBLIC
 )
 target_sources(cpp-testing-common PRIVATE
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/env.cpp
+  ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/env_var.cpp
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/network.cpp
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/probe.cpp
   ${CMAKE_SOURCE_DIR}/library/cpp/testing/common/scope.cpp

library/cpp/testing/common/env_var.cpp

@@ -0,0 +1,42 @@
+#include "env_var.h"
+
+#include <util/generic/yexception.h>
+
+#include <util/system/platform.h>
+
+#if defined(_win_)
+    #include <util/system/wininit.h>
+#else
+    #include <cerrno>
+#endif
+
+#include <cstdlib>
+
+namespace NUtils {
+
+    std::string GetEnv(std::string_view key, std::string_view def) {
+        const char* str_ptr = std::getenv(key.data());
+        return std::string(str_ptr == nullptr ? def : str_ptr);
+    }
+
+    void SetEnv(std::string_view key, std::string_view new_value) {
+        bool is_ok = false;
+        int error_code = 0;
+#if defined(_win_)
+        is_ok = SetEnvironmentVariable(key.data(), new_value.data()) == 0;
+        if (!is_ok) {
+            error_code = GetLastError();
+        }
+#else
+        is_ok = setenv(key.data(), new_value.data(), /* overwrite */ true) == 0;
+        if (!is_ok) {
+            error_code = errno;
+        }
+#endif
+        Y_ENSURE_EX(
+            is_ok, 
+            TSystemError() << "failed to SetEnv with error-code " << error_code
+        );
+    }
+
+} // namespace NUtils

library/cpp/testing/common/env_var.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <string>
+#include <string_view>
+
+namespace NUtils {
+
+    /**
+      * Search the environment list provided by the host environment for associated variable.
+      *
+      * @param key   String identifying the name of the environmental variable to look for
+      * @param def   String that returns if environmental variable not found by key
+      *
+      * @return      String that is associated with the matched environment variable or empty string
+      *              if such variable is missing.
+      *
+      * @note        Use it only in pair with `SetEnv` as there may be inconsistency in their
+      *              behaviour otherwise.
+      * @note        Calls to `GetEnv` and `SetEnv` from different threads must be synchronized.
+      * @see         SetEnv
+      */
+    std::string GetEnv(std::string_view key, std::string_view def = "");
+
+    /**
+      * Add or change environment variable provided by the host environment.
+      *
+      * @param key         String identifying the name of the environment variable to set or change
+      * @param new_value   Value to assign
+      * @note              Use it only in pair with `GetEnv` as there may be inconsistency in their
+      *                    behaviour otherwise.
+      * @note              Calls to `GetEnv` and `SetEnv` from different threads must be synchronized.
+      * @see               GetEnv
+      */
+    void SetEnv(std::string_view key, std::string_view new_value);
+
+} // namespace NUtils