process: fix reading zero-length env vars on win32

src/node.cc

@@ -2598,6 +2598,7 @@ static void EnvGetter(Local<Name> property,
 #else  // _WIN32
   node::TwoByteValue key(isolate, property);
   WCHAR buffer[32767];  // The maximum size allowed for environment variables.
+  SetLastError(ERROR_SUCCESS);
   DWORD result = GetEnvironmentVariableW(reinterpret_cast<WCHAR*>(*key),
                                          buffer,
                                          arraysize(buffer));
@@ -2646,6 +2647,7 @@ static void EnvQuery(Local<Name> property,
 #else  // _WIN32
     node::TwoByteValue key(info.GetIsolate(), property);
     WCHAR* key_ptr = reinterpret_cast<WCHAR*>(*key);
+    SetLastError(ERROR_SUCCESS);
     if (GetEnvironmentVariableW(key_ptr, nullptr, 0) > 0 ||
         GetLastError() == ERROR_SUCCESS) {
       rc = 0;

test/parallel/test-process-env-windows-error-reset.js

@@ -0,0 +1,22 @@
+'use strict';
+require('../common');
+const assert = require('assert');
+
+// This checks that after accessing a missing env var, a subsequent
+// env read will succeed even for empty variables.
+
+{
+  process.env.FOO = '';
+  process.env.NONEXISTENT_ENV_VAR;
+  const foo = process.env.FOO;
+
+  assert.strictEqual(foo, '');
+}
+
+{
+  process.env.FOO = '';
+  process.env.NONEXISTENT_ENV_VAR;
+  const hasFoo = 'FOO' in process.env;
+
+  assert.strictEqual(hasFoo, true);
+}