Python: fix shared CFG exception-handler reachability

[yoff]

Summary

Fixes exception-handler reachability in the shared Python CFG introduced by #21921 and now present on main. Testing the dataflow switch-over in #21925 surfaced the bug.

Leaf expressions that may throw, including bare yield and plain imports, were not always connected to typed exception-handler entries. The fix uses the canonical shared-CFG injects mapping when starting exceptional completion, so these expressions reach the appropriate catch entry.

Tests

Adds focused inline coverage for:

• bare yield reaching a GeneratorExit handler;
• a plain import reaching an ImportError handler.

DCA impact

The switch-over DCA in #38556 exposed effects of the missing exception edges in mypy, CPython, and youtube-dl. Restoring these edges recovers the affected catch reachability and downstream flow.

Validation

• codeql test run python/ql/test/library-tests/ControlFlow/shared-cfg-exceptions
• codeql query format --check-only python/ql/test/library-tests/ControlFlow/shared-cfg-exceptions/ExceptionReachabilityTest.ql

[Copilot]

Pull request overview

Fixes shared Python CFG/SSA regressions affecting exception-handler reachability and module-export phi uses.

Changes:

• Routes exceptional completion through canonical CFG nodes.
• Preserves synthetic normal-exit uses for generic SSA definitions.
• Adds regression tests for both fixes.

Show a summary per file

File	Description
python/ql/lib/semmle/python/controlflow/internal/AstNodeImpl.qll	Uses canonical CFG-node mapping for exception flow.
python/ql/lib/semmle/python/dataflow/new/internal/SsaImpl.qll	Supports phi definitions when resolving uses.
python/ql/test/library-tests/ControlFlow/shared-cfg-exceptions/test.py	Adds yield and import exception cases.
python/ql/test/library-tests/ControlFlow/shared-cfg-exceptions/ExceptionReachabilityTest.ql	Checks typed-handler reachability.
python/ql/test/library-tests/ControlFlow/shared-cfg-exceptions/ExceptionReachabilityTest.expected	Records expected exception results.
python/ql/test/library-tests/dataflow-new-ssa/test.py	Adds conditional module-export coverage.
python/ql/test/library-tests/dataflow-new-ssa/SsaTest.ql	Checks synthetic exit uses for phi definitions.

Review details

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• Files reviewed: 6/7 changed files
• Comments generated: 0
• Review effort level: Balanced

[aschackmull]

Hmm, I think perhaps you need to add the relevant nodes to postOrInOrder instead. Otherwise, if these are leaf nodes, then you end up without a separate after-node, which means that unconditional exceptions cannot be represented.
(The fact that we have always = false may save us for now, but flipping that boolean wouldn't work, so it seems a bit too brittle.)

[aschackmull]

I guess perhaps we could add a consistency check to verify that beginAbruptCompletion always sit on isIn or isAdditional nodes. I believe that's an implicit assumption by the library.