[common] Add a way to set the process startup directory (#63)

This will be needed later for switching to sftp, since calling lcd in
sftp is tricky to get right (e.g. may or may not require /cygwin/c on
Windows, depending on whether sftp is native or not).

common/process.h

@@ -53,6 +53,10 @@ struct ProcessStartInfo {
   // Command line, UTF-8 encoded.
   std::string command;
 
+  // Full path to the process startup working directory.
+  // If empty, uses parent process working dir.
+  std::string startup_dir;
+
   // If set, the process stdin is redirected to a pipe.
   // It not set, the input is connected to the stdin of the calling process.
   bool redirect_stdin = false;

common/process_test.cc

@@ -342,5 +342,20 @@ TEST_F(ProcessTest, TerminateAlreadyExited) {
   EXPECT_OK(process->Terminate());
 }
 
+TEST_F(ProcessTest, StartupDir) {
+  ProcessStartInfo start_info;
+  start_info.command = "cmd /C cd";
+  start_info.startup_dir = "C:\\";
+
+  std::string std_out;
+  start_info.stdout_handler = [&std_out](const char* data, size_t) {
+    std_out += data;
+    return absl::OkStatus();
+  };
+
+  EXPECT_OK(process_factory_.Run(start_info));
+  EXPECT_EQ(std_out, "C:\\\r\n");
+}
+
 }  // namespace
 }  // namespace cdc_ft

common/process_win.cc

@@ -719,6 +719,10 @@ absl::Status WinProcess::Start() {
                       Util::GetLastWin32Error());
   }
 
+  std::wstring startup_dir = Util::Utf8ToWideStr(start_info_.startup_dir);
+  const wchar_t* startup_dir_cstr =
+      !startup_dir.empty() ? startup_dir.c_str() : nullptr;
+
   // Start the child process.
   success = CreateProcess(NULL,  // No module name (use command line)
                           command_cstr,
@@ -727,8 +731,7 @@ absl::Status WinProcess::Start() {
                           TRUE,  // Inherit handles
                           ToCreationFlags(start_info_.flags),
                           NULL,  // Use parent's environment block
-                          NULL,  // Use parent's starting directory
-                          &si, &process_info_->pi);
+                          startup_dir_cstr, &si, &process_info_->pi);
 
   if (!success) {
     return MakeStatus("CreateProcess() failed: %s", Util::GetLastWin32Error());