RFR: 8257140: Crash in JvmtiTagMap::flush_object_free_events()

[Coleen Phillimore]

The JvmtiTagMap can be deleted while it is posting.  The JvmtiEnv is still on the list but the env is "disposed" of and the tag_map is deleted to save memory until the JvmtiEnv can be reclaimed at a safepoint and taken off the list.

There's a check JvmtiEnvBase::check_for_periodic_clean_up() that determines whether the JvmtiEnv can be taken off the list and that check looks for whether threads are in the middle of JvmtiEnvIterator which sets Thread::jvmti_env_iteration_count.  So this part is safe for the JvmtiTagMap changes.

The fix is to clear the JvmtiTagMapTable of the entries inside the table lock to save memory but leave JvmtiTagMap intact so that we can load a valid pointer when iterating through JvmtiEnvIterator.  We could also not do this at all and let the JvmtiTagMap destructor delete the table when the JvmtiEnv is also deleted, but we don't know what customer situation led to this code in the first place.

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Commit messages:
 - Rename JvmtiTagMap::clear() and fix JvmtiTagMapTable::clear() to null out deleted entries.
 - 8257140: Crash in JvmtiTagMap::flush_object_free_events()

Changes: https://git.openjdk.java.net/jdk/pull/1539/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1539&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257140
  Stats: 25 lines in 5 files changed: 17 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1539.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1539/head:pull/1539

PR: https://git.openjdk.java.net/jdk/pull/1539