rpctest: scope shared state to the current process

Two pieces of rpctest's global state silently aliased across concurrent
test processes (which is what `go test ./...` does by default, so any
`make unit` that exercises -tags=rpctest hit this):

- btcdExecutablePath compiled to a fixed path /tmp/btcd/rpctest/btcd.
  Two `go build` invocations would race on the same file, occasionally
  yielding a truncated or stale binary and downstream "tls: certificate
  signed by unknown authority" failures when the harness tried to talk
  to the resulting node.

- lastPort started at the same defaultNodePort in every process. The
  bind-test in NextAvailablePort closes the listener before returning,
  so two processes climbing from the same base would frequently hand
  out the same port and one harness would die with "connection refused"
  when btcd failed to bind.

Suffix the executable with a random uint32 and seed lastPort with a
random offset into a 50k-port window so each process climbs through
its own range.

Author: kcalvinalvin

integration/rpctest/btcd.go

@@ -6,6 +6,7 @@ package rpctest
 
 import (
 	"fmt"
+	"math/rand/v2"
 	"os/exec"
 	"path/filepath"
 	"runtime"
@@ -43,8 +44,12 @@ func btcdExecutablePath() (string, error) {
 		return "", err
 	}
 
-	// Build btcd and output an executable in a static temp path.
-	outputPath := filepath.Join(testDir, "btcd")
+	// Build btcd to a random path so concurrent `go test` processes
+	// (e.g. when test packages run in parallel under `make unit`) do
+	// not race on the same output file. Each test process pays a
+	// one-time compile cost; within a process the compileMtx-guarded
+	// cache keeps it to one build.
+	outputPath := filepath.Join(testDir, fmt.Sprintf("btcd-%d", rand.Uint32()))
 	if runtime.GOOS == "windows" {
 		outputPath += ".exe"
 	}

integration/rpctest/rpc_harness.go

@@ -6,6 +6,7 @@ package rpctest
 
 import (
 	"fmt"
+	"math/rand/v2"
 	"net"
 	"os"
 	"path/filepath"
@@ -76,7 +77,15 @@ var (
 
 	// lastPort is the last port determined to be free for use by a new
 	// node. It should be used atomically.
-	lastPort uint32 = defaultNodePort
+	//
+	// Seed with a random offset so concurrent `go test` processes
+	// (e.g. when integration/ and integration/rpctest/ run in parallel
+	// under `make unit`) do not race on the same port range. The
+	// bind-test in NextAvailablePort closes the listener before
+	// returning, leaving a window where another process could grab the
+	// same port; staggering each process's starting point avoids the
+	// collision. The 50k-port window leaves headroom below 65535.
+	lastPort uint32 = defaultNodePort + rand.Uint32N(50000)
 )
 
 // HarnessTestCase represents a test-case which utilizes an instance of the