RFR: 8287812: Cleanup JDWP agent GetEnv initialization [v2]

[Chris Plummer]

On Fri, 9 Dec 2022 19:39:16 GMT, Daniel D. Daugherty <dcubed at openjdk.org> wrote:

> if you asked for JVMTI_VERSION_1, could you get back JVMTI_VERSION_1_2 or JVMTI_VERSION_9 or
JVMTI_VERSION_11 if the JVM in question supported each of those versions (and no higher)? 

Yes, but the debug agent does a version check to make sure the runtime version matches the compile time version.

Right now if I ask for JVMTI_VERSION_1 I will get back version 21, so it seems that GetEnv() always returns the version that was current when JVMTI was built, except of course it will fail if the requested version is newer.

It seems we update the major version with every JDK release. Here's what currently is produced in jvmti.h:

enum {
    JVMTI_VERSION_1   = 0x30010000,
    JVMTI_VERSION_1_0 = 0x30010000,
    JVMTI_VERSION_1_1 = 0x30010100,
    JVMTI_VERSION_1_2 = 0x30010200,
    JVMTI_VERSION_9   = 0x30090000,
    JVMTI_VERSION_11  = 0x300B0000,
    JVMTI_VERSION_19  = 0x30130000,
    JVMTI_VERSION = 0x30000000 + (21 * 0x10000) + ( 0 * 0x100) + 0  /* version: 21.0.0 */
};

So it seems right now there is no difference in the end result between using JVMTI_VERSION and JVMTI_VERSION_<n>, assuming the compile and runtime versions match (you aren't dropping the debug agent into a different JDK build).

Now for jvmti agents that possibly could end up running in various JDK releases, the difference is more meaningful. They probably do not want to use JDK_VERSION because this could cause the agent to fail to initialize with older JDK releases that the agent author intends for agent to work with. Instead they should specify the minimum JVMTI version that they require.

Having said that it may at first seem odd that the debug agent would want to request JVMTI_VERSION_1, since it actually requires something newer, but the check to make sure that the runtime and compile time versions are the same will produce an error if indeed the runtime version is older than what we built against.  So the advantage of requesting an old version and then doing a runtime version check is that is that you can produce a very specific error message if the versioning is not right. If you rely on GetEnv to catch versioning issues, all you know is that you got a JNI_EVERSION error code.

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PR: https://git.openjdk.org/jdk/pull/11602