[ CLI ] Add support for modifying the environment from package:dartdev

This change makes it possible to set environment variables for the
current process from package:dartdev.

As a proof of concept, package:dartdev now sets `DART_ROOT` to the path
of the Dart SDK in the environment.

Related to https://github.com/dart-lang/sdk/issues/63210 and https://github.com/dart-lang/sdk/issues/62876

TEST=pkg/dartdev/test/environment_test.dart
Change-Id: If3a90279e99dadaba435ae3e43a752dcfda69227
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/499300
Reviewed-by: Phil Quitslund <pquitslund@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

runtime/bin/dartdev.cc

@@ -429,6 +429,7 @@ class DartDev {
     DartDev_Result_Run = 1,
     DartDev_Result_RunExec = 2,
     DartDev_Result_Exit = 3,
+    DartDev_Result_SetEnvironmentVariable = 4,
   } DartDev_Result;
 
   static CStringUniquePtr ResolvedDartVmPath() {
@@ -734,6 +735,16 @@ class DartDev {
     }
   }
 
+  static void SetEnvironmentVariableCallback(Dart_CObject* message) {
+    ASSERT(GetArrayItem(message, 1)->type == Dart_CObject_kString);
+    const char* name = GetArrayItem(message, 1)->value.as_string;
+    const char* value = nullptr;
+    if (GetArrayItem(message, 2)->type == Dart_CObject_kString) {
+      value = GetArrayItem(message, 2)->value.as_string;
+    }
+    Platform::SetEnvironmentVariable(name, value);
+  }
+
   // Callback that processes the result from execution of dartdev
   //
   static void ResultCallback(Dart_Port dest_port_id, Dart_CObject* message) {
@@ -754,6 +765,10 @@ class DartDev {
         ExitResultCallback(message);
         break;
       }
+      case DartDev_Result_SetEnvironmentVariable: {
+        SetEnvironmentVariableCallback(message);
+        break;
+      }
       default:
         UNREACHABLE();
     }

runtime/bin/platform.h

@@ -110,6 +110,8 @@ class Platform {
 
   static void SetCoreDumpResourceLimit(int value);
 
+  static bool SetEnvironmentVariable(const char* name, const char* value);
+
  private:
   // The path to the executable.
   static const char* executable_name_;

runtime/bin/platform_linux.cc

@@ -215,6 +215,13 @@ void Platform::SetCoreDumpResourceLimit(int value) {
   setrlimit(RLIMIT_CORE, &limit);
 }
 
+bool Platform::SetEnvironmentVariable(const char* name, const char* value) {
+  if (value == nullptr) {
+    return unsetenv(name) == 0;
+  }
+  return setenv(name, value, 0) == 0;
+}
+
 }  // namespace bin
 }  // namespace dart
 

runtime/bin/platform_macos.cc

@@ -408,6 +408,13 @@ void Platform::SetCoreDumpResourceLimit(int value) {
   setrlimit(RLIMIT_CORE, &limit);
 }
 
+bool Platform::SetEnvironmentVariable(const char* name, const char* value) {
+  if (value == nullptr) {
+    return unsetenv(name) == 0;
+  }
+  return setenv(name, value, 0) == 0;
+}
+
 }  // namespace bin
 }  // namespace dart
 

runtime/bin/platform_win.cc

@@ -430,6 +430,19 @@ void Platform::SetCoreDumpResourceLimit(int value) {
   // Not supported.
 }
 
+bool Platform::SetEnvironmentVariable(const char* name, const char* value) {
+  std::unique_ptr<wchar_t[]> name_w = Utf8ToWideChar(name);
+  if (value == nullptr) {
+    return ::SetEnvironmentVariableW(name_w.get(), nullptr) != 0;
+  }
+  DWORD ret = ::GetEnvironmentVariableW(name_w.get(), nullptr, 0);
+  if (ret == 0 && ::GetLastError() == ERROR_ENVVAR_NOT_FOUND) {
+    std::unique_ptr<wchar_t[]> value_w = Utf8ToWideChar(value);
+    return ::SetEnvironmentVariableW(name_w.get(), value_w.get()) != 0;
+  }
+  return true;
+}
+
 }  // namespace bin
 }  // namespace dart