Prevent variable nullification on later imports

[m-dzianishchyts]

What does this PR do?

Prevents newlines from causing unnecessary JShell wrapper redefinitions and losing previously initialized variables.

The normalization is applied immediately before each snippet is evaluated by JShell, while the original source remains available for completion analysis and snippet processing.

Details

JShell preserves the newline separating snippets when returning CompletionInfo.source(). Passing that leading newline directly to JShell changes the generated wrapper source layout and its class file line-number metadata.

For imports that do not alter executable code, this creates a bytecode difference that incorrectly triggers ExecutionControl.redefine(). Since JJavaExecutionControl does not support in-place redefinition, JShell creates a replacement wrapper with default-initialized fields, causing previously assigned variables to become null.

Stripping surrounding whitespace characters before evaluation removes the accidental source-layout difference while preserving the actual import and code semantics. Legitimate changes to executable bytecode remain unaffected.

The regression test for later imports is enabled, and coverage for variable redeclaration remains intact.

Testing

Verified with existing integration tests and manual testing in Jupyter Notebook.

[andrus]

@m-dzianishchyts , @stariy95 : guys, I tried to understand how this PR is different from the original #120. So I manually reverted all #120 commits (already pushed to "main") and then did a merge --squash of this one on top. The only change that I see is introduction of KernelExecutionIT.java test. But the original problem is still there when I run it manually. It was not fixed.

[andrus]

Ok, I was able to rewrite KernelExecutionIT in a way that reproduces the issue. It is committed to main (see a @Disabled test). The reason the original test didn't fail was that each line was executed in a separate cell. Somehow combining things in cells in a certain way causes this problem. Here are Claude findings on which combos work and which don't:

  ┌──────────────────────────────────────────────────────────┬────────┐
  │                          cells                           │ result │
  ├──────────────────────────────────────────────────────────┼────────┤
  │ 5 separate cells (what the IT does)                      │ ✅ ok  │
  ├──────────────────────────────────────────────────────────┼────────┤
  │ all 5 in one cell                                        │ ❌ NPE │
  ├──────────────────────────────────────────────────────────┼────────┤
  │ %maven alone, then remaining 4 lines in one cell         │ ❌ NPE │
  ├──────────────────────────────────────────────────────────┼────────┤
  │ %maven / import+var om+import / om.getClass()            │ ❌ NPE │
  ├──────────────────────────────────────────────────────────┼────────┤
  │ %maven / import+var om / import+om.getClass()            │ ✅ ok  │
  └──────────────────────────────────────────────────────────┴────────┘

One little thing left - figure out how to fix it 🙂

[m-dzianishchyts]

Hi, @andrus! There is one more clue, requesting review 👋

[andrus]

It works for me now. Amazing how trimming the code would make a difference. Thanks for digging it up! 🎉