sftp: add --sftp-set-env option to set environment variables

Fixes #6094

backend/sftp/sftp.go

@@ -302,6 +302,27 @@ cost of using more memory.
 `,
 			Default:  64,
 			Advanced: true,
+		}, {
+			Name:    "set_env",
+			Default: fs.SpaceSepList{},
+			Help: `Environment variables to pass to sftp and commands
+
+Set environment variables in the form:
+
+    VAR=value
+
+to be passed to the sftp client and to any commands run (eg md5sum).
+
+Pass multiple variables space separated, eg
+
+    VAR1=value VAR2=value
+
+and pass variables with spaces in in quotes, eg
+
+    "VAR3=value with space" "VAR4=value with space" VAR5=nospacehere
+
+`,
+			Advanced: true,
 		}},
 	}
 	fs.Register(fsi)
@@ -336,6 +357,7 @@ type Options struct {
 	IdleTimeout             fs.Duration     `config:"idle_timeout"`
 	ChunkSize               fs.SizeSuffix   `config:"chunk_size"`
 	Concurrency             int             `config:"concurrency"`
+	SetEnv                  fs.SpaceSepList `config:"set_env"`
 }
 
 // Fs stores the interface to the remote SFTP files
@@ -483,6 +505,22 @@ func (f *Fs) sftpConnection(ctx context.Context) (c *conn, err error) {
 	return c, nil
 }
 
+// Set any environment variables on the ssh.Session
+func (f *Fs) setEnv(s *ssh.Session) error {
+	for _, env := range f.opt.SetEnv {
+		equal := strings.IndexRune(env, '=')
+		if equal < 0 {
+			return fmt.Errorf("no = found in env var %q", env)
+		}
+		// fs.Debugf(f, "Setting env %q = %q", env[:equal], env[equal+1:])
+		err := s.Setenv(env[:equal], env[equal+1:])
+		if err != nil {
+			return fmt.Errorf("Failed to set env var %q: %w", env[:equal], err)
+		}
+	}
+	return nil
+}
+
 // Creates a new SFTP client on conn, using the specified subsystem
 // or sftp server, and zero or more option functions
 func (f *Fs) newSftpClient(conn *ssh.Client, opts ...sftp.ClientOption) (*sftp.Client, error) {
@@ -490,6 +528,10 @@ func (f *Fs) newSftpClient(conn *ssh.Client, opts ...sftp.ClientOption) (*sftp.C
 	if err != nil {
 		return nil, err
 	}
+	err = f.setEnv(s)
+	if err != nil {
+		return nil, err
+	}
 	pw, err := s.StdinPipe()
 	if err != nil {
 		return nil, err
@@ -1243,6 +1285,10 @@ func (f *Fs) run(ctx context.Context, cmd string) ([]byte, error) {
 	if err != nil {
 		return nil, fmt.Errorf("run: get SFTP session: %w", err)
 	}
+	err = f.setEnv(session)
+	if err != nil {
+		return nil, err
+	}
 	defer func() {
 		_ = session.Close()
 	}()