tstest/natlab/vmtest, cmd/tta: add TestTaildrop

Add a vmtest that brings up two Ubuntu nodes, each behind its own
EasyNAT, joined to the tailnet. The sender pushes a small file via
"tailscale file cp" and the receiver fetches it via "tailscale file
get --wait", asserting that the filename and contents round-trip
unchanged.

To make Taildrop work in vmtest, three small pieces were needed:

The Linux/FreeBSD cloud-init now starts tailscaled with --statedir as
well as --state=mem:, so the daemon has a VarRoot to host Taildrop's
incoming-files directory. State itself remains in-memory (so nothing
persists across reboots); only the var-root scratch space is on disk.

vmtest.New grows a variadic EnvOption parameter and a SameTailnetUser
helper. When the option is passed, Start sets AllNodesSameUser=true
on the embedded testcontrol.Server. Cross-node Taildrop requires the
sender and receiver to share a Tailnet user (or have an explicit
PeerCapabilityFileSharingTarget granted between them, which we don't
plumb here), so TestTaildrop opts in. Existing tests don't.

cmd/tta gains /taildrop-send and /taildrop-recv handlers that wrap
"tailscale file cp" and "tailscale file get --wait", plus
Env.SendTaildropFile and Env.RecvTaildropFile helpers in vmtest that
drive them.

Updates #13038

Change-Id: I8f5f70f88106e6e2ee07780dd46fe00f8efcfdf1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/natlab/vmtest/vmtest_test.go

@@ -4,6 +4,7 @@
 package vmtest_test
 
 import (
+	"bytes"
 	"fmt"
 	"net/netip"
 	"strings"
@@ -363,6 +364,58 @@ func TestSubnetRouterAndExitNode(t *testing.T) {
 	}
 }
 
+// TestTaildrop verifies that one Ubuntu node can send a file to another
+// Ubuntu node via Taildrop, and the receiver gets the same content.
+//
+// Topology: two Ubuntu nodes, each behind its own EasyNAT, both joined to the
+// tailnet. The sender runs `tailscale file cp` to push to the receiver's
+// Tailscale IP; the receiver then runs `tailscale file get --wait` to fetch
+// it.
+func TestTaildrop(t *testing.T) {
+	env := vmtest.New(t, vmtest.SameTailnetUser())
+
+	senderNet := env.AddNetwork("1.0.0.1", "192.168.1.1/24", vnet.EasyNAT)
+	receiverNet := env.AddNetwork("2.0.0.1", "192.168.2.1/24", vnet.EasyNAT)
+
+	sender := env.AddNode("sender", senderNet,
+		vmtest.OS(vmtest.Ubuntu2404))
+	receiver := env.AddNode("receiver", receiverNet,
+		vmtest.OS(vmtest.Ubuntu2404))
+
+	// Declare test-specific steps for the web UI.
+	sendStep := env.AddStep("Taildrop send (sender -> receiver)")
+	recvStep := env.AddStep("Taildrop receive (on receiver)")
+	verifyStep := env.AddStep("Verify received name and contents")
+
+	env.Start()
+
+	const filename = "hello.txt"
+	want := []byte("hello world this is a Taildrop test\n")
+
+	sendStep.Begin()
+	env.SendTaildropFile(sender, receiver, filename, want)
+	sendStep.End(nil)
+
+	recvStep.Begin()
+	gotName, gotContent := env.RecvTaildropFile(t.Context(), receiver)
+	recvStep.End(nil)
+
+	verifyStep.Begin()
+	if gotName != filename {
+		err := fmt.Errorf("received name = %q; want %q", gotName, filename)
+		verifyStep.End(err)
+		t.Error(err)
+		return
+	}
+	if !bytes.Equal(gotContent, want) {
+		err := fmt.Errorf("received content = %q; want %q", gotContent, want)
+		verifyStep.End(err)
+		t.Error(err)
+		return
+	}
+	verifyStep.End(nil)
+}
+
 // TestExitNode verifies that switching the client's exit node setting between
 // off, exit1, and exit2 correctly routes the client's internet traffic.
 //