Fix edge case involving round-tripping an empty trie

[mcculls]

What Does This Do

• Translates EMPTY_TRIE to empty builder, and persisted empty builder to EMPTY_TRIE
• Moves EMPTY_TRIE constant from Builder to ClassNameTrie

Motivation

Avoids the following exception when an empty trie is persisted and restored:

ERROR> java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0
ERROR> 	at datadog.instrument.utils.ClassNameTrie.apply(ClassNameTrie.java:172)
ERROR> 	at datadog.instrument.utils.ClassNameTrie.apply(ClassNameTrie.java:152)
ERROR> 	at datadog.instrument.utils.ClassNameTrie.apply(ClassNameTrie.java:129)
ERROR> 	at datadog.trace.agent.tooling.KnownTypesIndex.apply(KnownTypesIndex.java:41)
ERROR> 	at datadog.trace.agent.tooling.CombiningMatcher.matches(CombiningMatcher.java:78)
ERROR> 	at net.bytebuddy.agent.builder.AgentBuilder$RawMatcher$Conjunction.matches(AgentBuilder.java:1810)
ERROR> 	at net.bytebuddy.agent.builder.AgentBuilder$Default$ExecutingTransformer.doTransform(AgentBuilder.java:12801)
ERROR> 	at net.bytebuddy.agent.builder.AgentBuilder$Default$ExecutingTransformer.transform(AgentBuilder.java:12759)
ERROR> 	at net.bytebuddy.agent.builder.AgentBuilder$Default$ExecutingTransformer.access$1800(AgentBuilder.java:12468)
ERROR> 	at net.bytebuddy.agent.builder.AgentBuilder$Default$ExecutingTransformer$Java9CapableVmDispatcher.run(AgentBuilder.java:13250)
ERROR> 	at net.bytebuddy.agent.builder.AgentBuilder$Default$ExecutingTransformer$Java9CapableVmDispatcher.run(AgentBuilder.java:13180)
ERROR> 	at java.base/java.security.AccessController.doPrivileged(AccessController.java:400)
ERROR> 	at net.bytebuddy.agent.builder.AgentBuilder$Default$ExecutingTransformer.doPrivileged(AgentBuilder.java)
ERROR> 	at net.bytebuddy.agent.builder.AgentBuilder$Default$ExecutingTransformer.transform(AgentBuilder.java:12702)
ERROR> 	at net.bytebuddy.agent.builder.AgentBuilder$Default$ExecutingTransformer$ByteBuddy$ModuleSupport.transform(Unknown Source)
ERROR> 	at datadog.trace.agent.tooling.bytebuddy.DDJava9ClassFileTransformer.transform(DDJava9ClassFileTransformer.java:60)
ERROR> 	at java.instrument/sun.instrument.TransformerManager.transform(TransformerManager.java:188)
ERROR> 	at java.instrument/sun.instrument.InstrumentationImpl.transform(InstrumentationImpl.java:610)
ERROR> 	at java.base/java.lang.ClassLoader.findBootstrapClass(Native Method)
ERROR> 	at java.base/java.lang.ClassLoader.findBootstrapClassOrNull(ClassLoader.java:1277)
ERROR> 	at java.base/java.lang.System$2.findBootstrapClassOrNull(System.java:2397)
ERROR> 	at java.base/jdk.internal.loader.ClassLoaders$BootClassLoader.loadClassOrNull(ClassLoaders.java:140)
ERROR> 	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClassOrNull(BuiltinClassLoader.java:700)
ERROR> 	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClassOrNull(BuiltinClassLoader.java:669)
ERROR> 	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:639)
ERROR> 	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:580)
ERROR> 	at datadog.trace.bootstrap.DatadogClassLoader.loadClass(DatadogClassLoader.java:114)
ERROR> 	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526)
ERROR> 	at datadog.communication.ddagent.SharedCommunicationObjects.createRemaining(SharedCommunicationObjects.java:91)
ERROR> 	at datadog.trace.agent.core.CoreTracer.<init>(CoreTracer.java:767)
ERROR> 	at datadog.trace.agent.core.CoreTracer.<init>(CoreTracer.java:137)
ERROR> 	at datadog.trace.agent.core.CoreTracer$CoreTracerBuilder.build(CoreTracer.java:530)
ERROR> 	at datadog.trace.agent.tooling.TracerInstaller.installGlobalTracer(TracerInstaller.java:34)
ERROR> 	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
ERROR> 	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
ERROR> 	at datadog.trace.bootstrap.Agent.installDatadogTracer(Agent.java:815)
ERROR> 	at datadog.trace.bootstrap.Agent.access$300(Agent.java:87)
ERROR> 	at datadog.trace.bootstrap.Agent$InstallDatadogTracerCallback.<init>(Agent.java:666)
ERROR> 	at datadog.trace.bootstrap.Agent.start(Agent.java:391)
ERROR> 	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
ERROR> 	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
ERROR> 	at datadog.trace.bootstrap.AgentBootstrap.agentmainImpl(AgentBootstrap.java:166)
ERROR> 	at datadog.trace.bootstrap.AgentBootstrap.agentmain(AgentBootstrap.java:73)
ERROR> 	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
ERROR> 	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
ERROR> 	at datadog.trace.bootstrap.AgentPreCheck.continueBootstrap(AgentPreCheck.java:169)
ERROR> 	at datadog.trace.bootstrap.AgentPreCheck.agentmain(AgentPreCheck.java:24)
ERROR> 	at datadog.trace.bootstrap.AgentPreCheck.premain(AgentPreCheck.java:17)
ERROR> 	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
ERROR> 	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
ERROR> 	at java.instrument/sun.instrument.InstrumentationImpl.loadClassAndStartAgent(InstrumentationImpl.java:560)
ERROR> 	at java.instrument/sun.instrument.InstrumentationImpl.loadClassAndCallPremain(InstrumentationImpl.java:572)

Contributor Checklist

Jira ticket: [PROJ-IDENT]

[amarziali]

Thanks - the stacktrace reminds me something :)

[PerfectSlayer]

👏 praise: ‏Thanks for fixing my report