[RemoteUtil] Add support for sftp (#64)

In a future CL, we will switch from scp to sftp. This CL adds support
for calling sftp from RemoteUtil.

In order to maintain backwards compatibility where people still set
--scp-command or CDC_SCP_COMMAND instead of the sftp versions, this CL
also adds the helper method RemoteUtil::ScpToSftpCommand, which
attempts to convert an scp command to an sftp command. This is usually
possible since the args are almost the same. For instance, if the scp
command is
  C:\path\to\scp.exe -P 1234 -i <key_file> -oUserKnownHostsFile=known_hosts
then the corresponding sftp command is most likely
  C:\path\to\sftp.exe -P 1234 -i <key_file> -oUserKnownHostsFile=known_hosts
This works for instance for OpenSSH.

common/remote_util.cc

@@ -20,6 +20,8 @@
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_format.h"
 #include "common/path.h"
+#include "common/status_macros.h"
+#include "common/util.h"
 
 namespace cdc_ft {
 namespace {
@@ -47,10 +49,34 @@ void RemoteUtil::SetScpCommand(std::string scp_command) {
   scp_command_ = std::move(scp_command);
 }
 
+void RemoteUtil::SetSftpCommand(std::string sftp_command) {
+  sftp_command_ = std::move(sftp_command);
+}
+
 void RemoteUtil::SetSshCommand(std::string ssh_command) {
   ssh_command_ = std::move(ssh_command);
 }
 
+// static
+std::string RemoteUtil::ScpToSftpCommand(std::string scp_command) {
+  // "scp", "SCP", "winscp.exe", "C:\path\to\scp", "/scppath/scp --foo" etc.
+  std::string lower_scp_command = scp_command;
+  std::transform(lower_scp_command.begin(), lower_scp_command.end(),
+                 lower_scp_command.begin(), ::tolower);
+  size_t pos = 0;
+  while ((pos = lower_scp_command.find("scp", pos)) != std::string::npos) {
+    // This may access the string at scp_command.size(), but that's well defined
+    // in C++11 and returns 0.
+    const char next_ch = lower_scp_command[pos + 3];
+    if ((next_ch == 0 || next_ch == '.' || next_ch == ' ')) {
+      return scp_command.replace(pos, 3, "sftp");
+    }
+    ++pos;
+  }
+
+  return std::string();
+}
+
 absl::Status RemoteUtil::Scp(std::vector<std::string> source_filepaths,
                              const std::string& dest, bool compress) {
   std::string source_args;
@@ -77,6 +103,33 @@ absl::Status RemoteUtil::Scp(std::vector<std::string> source_filepaths,
   return process_factory_->Run(start_info);
 }
 
+absl::Status RemoteUtil::Sftp(const std::string& commands,
+                              const std::string& initial_local_dir,
+                              bool compress) {
+  // sftp doesn't take |commands| as argument, so write it to a temp file.
+  std::string cmd_path =
+      path::Join(path::GetTempDir(), "__sftp_cmd__" + Util::GenerateUniqueId());
+  RETURN_IF_ERROR(path::WriteFile(cmd_path, commands),
+                  "Failed to write sftp commands to '%s'", cmd_path);
+
+  // -p preserves timestamps. This enables timestamp-based up-to-date checks.
+  ProcessStartInfo start_info;
+  start_info.flags = ProcessFlags::kNoWindow;
+  start_info.command = absl::StrFormat(
+      "%s %s %s -p -b %s %s", sftp_command_,
+      quiet_ || verbosity_ < 2 ? "-q" : "", compress ? "-C" : "",
+      QuoteForWindows(cmd_path), QuoteForWindows(user_host_));
+  start_info.name = "sftp";
+  start_info.startup_dir = initial_local_dir;
+  start_info.forward_output_to_log = forward_output_to_log_;
+
+  RETURN_IF_ERROR(process_factory_->Run(start_info));
+
+  // Note: Keep |cmd_path| in case of an error for debugging purposes.
+  path::RemoveFile(cmd_path).IgnoreError();
+  return absl::OkStatus();
+}
+
 absl::Status RemoteUtil::Chmod(const std::string& mode,
                                const std::string& remote_path, bool quiet) {
   std::string remote_command =