Clearly fail when VM scanning isn't available

It appears that in some environments (JRE 15 on Windows, at least) you can
try to scan, but it just always returns an empty list. Since it'll never
normally be an empty list, we need to clearly and loudly fail if that
happens, so we know scanning isn't available.

Author: pimterry

src/main/kotlin/tech/httptoolkit/javaagent/AttachMain.kt

@@ -28,12 +28,20 @@ fun main(args: Array<String>) {
         // This isn't guaranteed to work everywhere, but it should work in most places:
         val (pid) = ManagementFactory.getRuntimeMXBean().name.split("@")
 
+        val vms = VirtualMachine.list()
+        if (vms.isEmpty()) {
+            // VMs should never be empty, because at the very least _we_ should be in there! If it's empty then
+            // scanning isn't working at all, and we should fail clearly.
+            System.err.println("Can't scan for attachable JVMs. Are we running in a JRE instead of a JDK?")
+            exitProcess(4)
+        }
 
-        VirtualMachine.list().forEach { vmd ->
+        vms.forEach { vmd ->
             if (vmd.id() != pid) {
                 println("${vmd.id()}:${vmd.displayName()}")
             }
         }
+
         exitProcess(0)
     } else if (args.size != 4) {
         System.err.println("Usage: java -jar <agent.jar> <target-PID> <proxyHost> <proxyPort> <path-to-certificate>")
@@ -63,4 +71,4 @@ fun main(args: Array<String>) {
             exitProcess(3)
         }
     }
-}
+}

src/test/kotlin/IntegrationTests.kt

@@ -7,6 +7,8 @@ import io.kotest.core.spec.style.StringSpec
 import io.kotest.core.test.TestCase
 import io.kotest.core.test.TestResult
 import io.kotest.matchers.shouldBe
+import io.kotest.matchers.string.shouldNotBeEmpty
+import java.io.BufferedReader
 import java.lang.Thread.sleep
 import java.nio.file.Path
 import java.nio.file.Paths
@@ -40,6 +42,24 @@ val wireMockServer = WireMockServer(options()
 val runningProcs = arrayListOf<Process>()
 
 class IntegrationTests : StringSpec({
+    "Launching with list-targets should return successfully" {
+        val proc = ProcessBuilder(
+            javaPath,
+            "-jar", AGENT_JAR_PATH,
+            "list-targets"
+        ).start()
+        runningProcs.add(proc)
+        val outputReader = proc.inputStream.bufferedReader()
+
+        proc.waitFor(10, TimeUnit.SECONDS)
+
+        val output = outputReader.use(BufferedReader::readText)
+        output.shouldNotBeEmpty()
+
+        proc.isAlive.shouldBe(false)
+        proc.exitValue().shouldBe(0)
+    }
+
     "Launching with -javaagent should intercept all clients" {
 
         val agentArgs = "$proxyHost|${wireMockServer.port()}|$certPath"