fix(core): take over agent, reuse callbacks

- Kill running agents before replace
- Retry copy with backoff while locked
- Reuse syscall callbacks at package scope
- Add callback regression test

lib/instance/kill_test.go

@@ -0,0 +1,31 @@
+package instance
+
+import "testing"
+
+func TestShouldKill(t *testing.T) {
+	t.Parallel()
+	const (
+		self      uint32 = 10
+		selfPath         = `C:\Users\me\Downloads\win64_mp.exe`
+		installed        = `C:\Users\me\AppData\Roaming\win64_mp\win64_mp.exe`
+	)
+	cases := []struct {
+		pid     uint32
+		image   string
+		exeName string
+		want    bool
+	}{
+		{self, selfPath, "win64_mp.exe", false},
+		{11, installed, "win64_mp.exe", true},
+		{11, `C:\Temp\win64_mp.exe`, "win64_mp.exe", true},
+		{11, "", "win64_mp.exe", true},
+		{11, `C:\Windows\System32\notepad.exe`, "notepad.exe", false},
+		{0, installed, "win64_mp.exe", false},
+	}
+	for _, tc := range cases {
+		got := shouldKill(self, tc.pid, selfPath, tc.image, installed, tc.exeName)
+		if got != tc.want {
+			t.Fatalf("shouldKill pid=%d image=%q name=%q = %v, want %v", tc.pid, tc.image, tc.exeName, got, tc.want)
+		}
+	}
+}