Ensure any partial ReadFile response is passed through.

This makes sure any partial buffer is consumed, in particular the last write that gives us ERROR_BROKEN_PIPE, and ensures we get any debugging output from msiexec by replacing nulls with ? instead.

src/vcpkg/base/downloads.cpp

@@ -659,11 +659,8 @@ namespace vcpkg
                                    }
                                }).value_or_exit(VCPKG_LINE_INFO);
 
-                    auto res = cmd_execute_and_capture_output(Command{}.raw_arg(cmd),
-                                                              default_working_directory,
-                                                              get_clean_environment(),
-                                                              Encoding::Utf8,
-                                                              EchoInDebug::Show);
+                    auto res = cmd_execute_and_capture_output(
+                        Command{}.raw_arg(cmd), default_working_directory, get_clean_environment(), EchoInDebug::Show);
                     if (res.exit_code == 0)
                     {
                         auto maybe_error =

src/vcpkg/base/system.process.cpp

@@ -594,15 +594,28 @@ namespace vcpkg
         {
             CloseHandle(child_stdin);
 
-            DWORD bytes_read = 0;
             static constexpr DWORD buffer_size = 1024 * 32;
             char buf[buffer_size];
-            while (ReadFile(child_stdout, static_cast<void*>(buf), buffer_size, &bytes_read, nullptr))
+            DWORD bytes_read;
+            BOOL read_file_result;
+            do
             {
+                bytes_read = 0; // in case ReadFile doesn't update this on failure
+                Debug::print("wait_and_stream_output() attempt\n");
+                read_file_result = ReadFile(child_stdout, static_cast<void*>(buf), buffer_size, &bytes_read, nullptr);
+                std::replace(buf, buf + bytes_read, '\0', '?');
                 f(StringView{buf, static_cast<size_t>(bytes_read)});
+            } while (read_file_result);
+
+            const auto last_error = GetLastError();
+            switch (last_error)
+            {
+                case ERROR_BROKEN_PIPE:
+                    Debug::print("ReadFile() finished with ERROR_BROKEN_PIPE (probably normal exit)\n");
+                    break;
+                default: Debug::print("ReadFile() finished with GetLastError(): ", GetLastError(), '\n'); break;
             }
 
-            Debug::print("ReadFile() finished with GetLastError(): ", GetLastError(), '\n');
             CloseHandle(child_stdout);
             return proc_info.wait();
         }