8278965: crash in SymbolTable::do_lookup

[pengxiaolong]

Backport of 582b943 (openjdk/jdk@582b943). Keeps the superclass name alive while a parallel thread loads the superclass, preventing a refcount underflow.

Adaptation for 17u: Mainline's fix only needed to change systemDictionary.cpp because the placeholder-table supername refcounting was already correct there (introduced by JDK-8292286, Convert PlaceholderTable to ResourceHashtable, which is not in 17u). Rather than backport that structural refactor, this PR open-codes the equivalent behavior on the legacy Hashtable-based PlaceholderTable:

• set_supername() now decrements the replaced supername before adopting the new one.
• find_and_remove() keeps the supername alive until the whole entry is removed (released by free_entry), instead of clearing it as soon as the super-load
  queue empties.
• new_entry() initializes _supername to NULL before the first set_supername(), since the legacy table allocates entries as raw, unconstructed memory.

The added gtest declares its symbols as raw Symbol* (not TempNewSymbol) so the TempSymbol cleanup delayer (JDK-8315559) doesn't bump their starting refcount, and releases them manually.

Testing:

• gtest:PlaceholderTable passes; full gtest:all (908 tests) passes.
• tier1~tier4

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Progress

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Warning

 ⚠️ Found leading lowercase letter in issue title for 8278965: crash in SymbolTable::do_lookup

Issue

• JDK-8278965: crash in SymbolTable::do_lookup (Bug - P3)

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[bridgekeeper]

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[openjdk]

This backport pull request has now been updated with issue from the original commit.

[mlbridge]

Webrevs

• 01: Full - Incremental (41ced4fd)
• 00: Full (354b7342)

[pengxiaolong]

Hi @coleenp, could you review this backport? It fixes the crash in SymbolTable::do_lookup.
It's not a clean backport: the fix landed in JDK 21 on top of JDK-8267935 (https://bugs.openjdk.org/browse/JDK-8267935), which replaced Hashtable with ResourceHashtable. Since that change isn't in JDK 17, I had to adapt both the fix and the test to the older Hashtable-based code.