rsend: push phone folders to a home SSH host over rsync

A small Android app for one-way folder backup, a KISS alternative to
Syncthing. It bundles rsync (built from pinned source via the NDK) and
a pure-Go SSH transport, both shipped in the APK as lib*.so and run from
the native library directory.

rsend pins the host key, stores the ed25519 identity Keystore-encrypted,
pushes each folder additively or as a mirror, and runs on demand or on a
WiFi-only schedule. The build is self-contained and reproducible: make
setup provisions the toolchain, make builds rsync, rsh, and the APK.

1 files changed, 141 insertions, 0 deletions

diff --git a/rsh/sshserver_test.go b/rsh/sshserver_test.go
new file mode 100644
index 0000000..087bff4
--- /dev/null
+++ b/rsh/sshserver_test.go
@@ -0,0 +1,141 @@
+package main
+
+import (
+	"crypto/ed25519"
+	"crypto/rand"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"os/exec"
+	"testing"
+
+	"golang.org/x/crypto/ssh"
+)
+
+// execHandler runs the remote command for one session and returns its exit
+// code.
+type execHandler func(cmd string, stdin io.Reader, stdout, stderr io.Writer) int
+
+// testServer is a minimal in-process SSH server for tests: one host key, one
+// accepted client public key, and a pluggable exec handler. No sshd needed.
+type testServer struct {
+	ln      net.Listener
+	hostKey ssh.Signer
+	authPub ssh.PublicKey
+	handle  execHandler
+}
+
+func newTestServer(t *testing.T, clientPub ssh.PublicKey, handle execHandler) *testServer {
+	t.Helper()
+	_, hpriv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	signer, err := ssh.NewSignerFromKey(hpriv)
+	if err != nil {
+		t.Fatal(err)
+	}
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	s := &testServer{ln: ln, hostKey: signer, authPub: clientPub, handle: handle}
+	go s.serve()
+	t.Cleanup(func() { ln.Close() })
+	return s
+}
+
+func (s *testServer) port() int { return s.ln.Addr().(*net.TCPAddr).Port }
+
+func (s *testServer) serve() {
+	cfg := &ssh.ServerConfig{
+		PublicKeyCallback: func(_ ssh.ConnMetadata, key ssh.PublicKey) (*ssh.Permissions, error) {
+			if s.authPub != nil && string(key.Marshal()) == string(s.authPub.Marshal()) {
+				return &ssh.Permissions{}, nil
+			}
+			return nil, fmt.Errorf("unknown public key")
+		},
+	}
+	cfg.AddHostKey(s.hostKey)
+	for {
+		nConn, err := s.ln.Accept()
+		if err != nil {
+			return
+		}
+		go s.handleConn(nConn, cfg)
+	}
+}
+
+func (s *testServer) handleConn(nConn net.Conn, cfg *ssh.ServerConfig) {
+	conn, chans, reqs, err := ssh.NewServerConn(nConn, cfg)
+	if err != nil {
+		return
+	}
+	defer conn.Close()
+	go ssh.DiscardRequests(reqs)
+	for newCh := range chans {
+		if newCh.ChannelType() != "session" {
+			newCh.Reject(ssh.UnknownChannelType, "only session")
+			continue
+		}
+		ch, requests, err := newCh.Accept()
+		if err != nil {
+			return
+		}
+		go s.handleSession(ch, requests)
+	}
+}
+
+func (s *testServer) handleSession(ch ssh.Channel, requests <-chan *ssh.Request) {
+	for req := range requests {
+		if req.Type != "exec" {
+			if req.WantReply {
+				req.Reply(false, nil)
+			}
+			continue
+		}
+		var payload struct{ Command string }
+		ssh.Unmarshal(req.Payload, &payload)
+		if req.WantReply {
+			req.Reply(true, nil)
+		}
+		code := s.handle(payload.Command, ch, ch, ch.Stderr())
+		ch.SendRequest("exit-status", false, ssh.Marshal(struct{ Status uint32 }{uint32(code)}))
+		ch.Close()
+		return
+	}
+}
+
+// shellExec runs cmd via /bin/sh -c, exactly as sshd would, wiring the channel
+// to the process. Used by the real-rsync end-to-end test.
+//
+// stdin is fed through a detached goroutine rather than c.Stdin so that Wait
+// does not block on the stdin copy: rsync's remote receiver exits while the
+// client still holds the channel open, and a c.Run with a non-file Stdin would
+// deadlock waiting for that copy to finish.
+func shellExec(cmd string, stdin io.Reader, stdout, stderr io.Writer) int {
+	c := exec.Command("/bin/sh", "-c", cmd)
+	c.Stdout = stdout
+	c.Stderr = stderr
+	inPipe, err := c.StdinPipe()
+	if err != nil {
+		return 1
+	}
+	if err := c.Start(); err != nil {
+		return 1
+	}
+	go func() {
+		io.Copy(inPipe, stdin)
+		inPipe.Close()
+	}()
+	err = c.Wait()
+	var ee *exec.ExitError
+	if errors.As(err, &ee) {
+		return ee.ExitCode()
+	}
+	if err != nil {
+		return 1
+	}
+	return 0
+}