Python: model from X import * as uncertain SSA writes

Add a 4th disjunct to `SsaImplInput::variableWrite` in the shared-SSA
adapter that mirrors legacy ESSA's `ImportStarRefinement`: every
variable whose scope is the import-star's scope, OR which is used in
the import-star's scope, gets an uncertain write at the `import *`
position.

Uncertain writes do not kill prior definitions; shared SSA's
`SsaUncertainWrite` joins the new value with the immediately-preceding
definition via `uncertainWriteDefinitionInput`. This is the equivalent
of legacy ESSA's two-input refinement.

Cannot depend on `ImportStar` / `ImportResolution` (those modules
import `SsaImpl`), so the predicate uses the structural heuristic on
`Cfg::ImportStarNode` directly.

This closes the two remaining failing dataflow library-tests:

- `import-star/global` — `module_export` chains via `from X import *`
  re-exports now resolve: the importing module has an SSA def of every
  re-exported name, so `lastUseVar` finds the read at the use site.
- `typetracking_imports/highlight_problem` — a direct `from .foo import
  foo` immediately followed by `from .other import *` is now correctly
  marked as dead at the direct import.

Two scope-entry-def noise rows in `highlight_problem.expected` are also
dropped — legacy ESSA needed them as refinement inputs, but shared SSA
handles uncertain writes without an explicit prior def. They were
always tagged `no use to normal exit` (dead).

Dataflow library-tests: 62/64 → 64/64 passing.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/ql/lib/semmle/python/dataflow/new/internal/SsaImpl.qll

@@ -48,14 +48,6 @@ private module CfgForSsa implements BB::CfgSig<Py::Location> {
   predicate dominatingEdge = CfgImpl::Cfg::dominatingEdge/2;
 }
 
-/**
- * A source variable for SSA. Wraps a Python `Variable` (the AST-level
- * notion of a named binding within a scope) so that the shared SSA
- * implementation can use it as a `SourceVariable`.
- *
- * We only track variables that are read at least once in their scope —
- * tracking write-only variables is unnecessary work.
- */
 /**
  * A source variable for SSA, wrapping a Python AST `Variable`.
  *
@@ -113,7 +105,9 @@ class SsaSourceVariable extends TSsaSourceVariable {
    * `SsaSourceVariable.getASourceUse()`.
    */
   Cfg::ControlFlowNode getASourceUse() {
-    exists(Cfg::NameNode n | result = n | n.uses(this.getVariable()) or n.deletes(this.getVariable()))
+    exists(Cfg::NameNode n | result = n |
+      n.uses(this.getVariable()) or n.deletes(this.getVariable())
+    )
   }
 
   /**
@@ -223,6 +217,31 @@ private module SsaImplInput implements SsaImplCommon::InputSig<Py::Location, Cfg
     hasEntryDefIn(v, bb) and
     i = -1 and
     certain = true
+    or
+    // `from X import *` — possibly rebinds every name in the importing
+    // scope. Modelled as an uncertain write at the import-star's CFG
+    // position for every variable that lives in (or is referenced
+    // from) the same scope as the import-star. Mirrors legacy ESSA's
+    // `ImportStarRefinement` (see `essa/SsaDefinitions.qll`'s
+    // `import_star_refinement` predicate). The write is uncertain so
+    // that prior definitions of the variable remain available — the
+    // shared-SSA `SsaUncertainWrite` merges the new value with the
+    // immediately preceding definition.
+    exists(Cfg::ImportStarNode imp |
+      bb.getNode(i) = imp and
+      certain = false and
+      (
+        v.getVariable().getScope() = imp.getScope()
+        or
+        // Variable is defined in some other scope but referenced in
+        // the same scope as the import-star (matches legacy clause 2:
+        // `other.uses(v) and def.getScope() = other.getScope()`).
+        exists(Cfg::NameNode other |
+          other.uses(v.getVariable()) and
+          imp.getScope() = other.getScope()
+        )
+      )
+    )
   }
 
   predicate variableRead(CfgImpl::BasicBlock bb, int i, SourceVariable v, boolean certain) {
@@ -400,13 +419,17 @@ class MultiAssignmentDefinition extends EssaNodeDefinition {
 
   override Cfg::ControlFlowNode getDefiningNode() {
     // Default: the underlying `Name` CFG node (where the SSA def lives).
-    not exists(Cfg::StarredNode s | s.getNode().(Py::Starred).getValue() = super.getDefiningNode().getNode()) and
+    not exists(Cfg::StarredNode s |
+      s.getNode().(Py::Starred).getValue() = super.getDefiningNode().getNode()
+    ) and
     result = super.getDefiningNode()
     or
     // Exception: for `*rest`, expose the enclosing `Starred` CFG node
     // so that `IterableUnpacking::iterableUnpackingStarredElementStoreStep`
     // can attach the rest-list to it.
-    exists(Cfg::StarredNode s | s.getNode().(Py::Starred).getValue() = super.getDefiningNode().getNode() |
+    exists(Cfg::StarredNode s |
+      s.getNode().(Py::Starred).getValue() = super.getDefiningNode().getNode()
+    |
       result = s
     )
   }

python/ql/test/library-tests/dataflow/typetracking_imports/highlight_problem.expected

@@ -1,9 +1,7 @@
-| pkg/alias_only_direct.py:0:0:0:0 | Module pkg.alias_only_direct | pkg/alias_only_direct.py:1:22:1:24 | GSSA Variable foo | use to normal exit |
-| pkg/alias_problem.py:0:0:0:0 | Module pkg.alias_problem | pkg/alias_problem.py:1:22:1:24 | GSSA Variable foo | no use to normal exit |
-| pkg/alias_problem.py:0:0:0:0 | Module pkg.alias_problem | pkg/alias_problem.py:2:1:2:20 | GSSA Variable foo | use to normal exit |
-| pkg/alias_problem_fixed.py:0:0:0:0 | Module pkg.alias_problem_fixed | pkg/alias_problem_fixed.py:0:0:0:0 | GSSA Variable foo | no use to normal exit |
-| pkg/alias_problem_fixed.py:0:0:0:0 | Module pkg.alias_problem_fixed | pkg/alias_problem_fixed.py:3:22:3:24 | GSSA Variable foo | use to normal exit |
-| pkg/problem_absolute_import.py:0:0:0:0 | Module pkg.problem_absolute_import | pkg/problem_absolute_import.py:1:25:1:27 | GSSA Variable foo | no use to normal exit |
-| pkg/problem_absolute_import.py:0:0:0:0 | Module pkg.problem_absolute_import | pkg/problem_absolute_import.py:2:1:2:23 | GSSA Variable foo | use to normal exit |
-| pkg/works_absolute_import.py:0:0:0:0 | Module pkg.works_absolute_import | pkg/works_absolute_import.py:0:0:0:0 | GSSA Variable foo | no use to normal exit |
-| pkg/works_absolute_import.py:0:0:0:0 | Module pkg.works_absolute_import | pkg/works_absolute_import.py:2:25:2:27 | GSSA Variable foo | use to normal exit |
+| pkg/alias_only_direct.py:0:0:0:0 | Module pkg.alias_only_direct | pkg/alias_only_direct.py:1:22:1:24 | SSA def(Global Variable foo) | use to normal exit |
+| pkg/alias_problem.py:0:0:0:0 | Module pkg.alias_problem | pkg/alias_problem.py:1:22:1:24 | SSA def(Global Variable foo) | no use to normal exit |
+| pkg/alias_problem.py:0:0:0:0 | Module pkg.alias_problem | pkg/alias_problem.py:2:1:2:20 | SSA def(Global Variable foo) | use to normal exit |
+| pkg/alias_problem_fixed.py:0:0:0:0 | Module pkg.alias_problem_fixed | pkg/alias_problem_fixed.py:3:22:3:24 | SSA def(Global Variable foo) | use to normal exit |
+| pkg/problem_absolute_import.py:0:0:0:0 | Module pkg.problem_absolute_import | pkg/problem_absolute_import.py:1:25:1:27 | SSA def(Global Variable foo) | no use to normal exit |
+| pkg/problem_absolute_import.py:0:0:0:0 | Module pkg.problem_absolute_import | pkg/problem_absolute_import.py:2:1:2:23 | SSA def(Global Variable foo) | use to normal exit |
+| pkg/works_absolute_import.py:0:0:0:0 | Module pkg.works_absolute_import | pkg/works_absolute_import.py:2:25:2:27 | SSA def(Global Variable foo) | use to normal exit |