Python: remove EssaNodes

This commit removes SSA nodes from the data flow graph. Specifically, for a definition and use such as
```python
  x = expr
  y = x + 2
```
we used to have flow from `expr` to an SSA variable representing x and from that SSA variable to the use of `x` in the definition of `y`. Now we instead have flow from `expr` to the control flow node for `x` at line 1 and from there to the control flow node for `x` at line 2.

Specific changes:
- `EssaNode` from the data flow layer no longer exists.
- Several glue steps between `EssaNode`s and `CfgNode`s have been deleted.
- Entry nodes are now admitted as `CfgNodes` in the data flow layer (they were filtered out before).
- Entry nodes now have a new `toString` taking into account that the module name may be ambigous.
- Some tests have been rewritten to accomodate the changes, but only `python/ql/test/experimental/dataflow/basic/maximalFlowsConfig.qll` should have semantic changes.
- Comments have been updated
- Test output has been updated, but apart from `python/ql/test/experimental/dataflow/basic/maximalFlows.expected` only `python/ql/test/experimental/dataflow/typetracking-summaries/summaries.py` should have a semantic change. This is a bonus fix, probably meaning that something was never connected up correctly.

python/ql/lib/semmle/python/Flow.qll

@@ -126,7 +126,10 @@ class ControlFlowNode extends @py_flow_node {
   cached
   string toString() {
     Stages::AST::ref() and
-    exists(Scope s | s.getEntryNode() = this | result = "Entry node for " + s.toString())
+    // Since modules can have ambigous names, entry nodes can too, if we do not collate them.
+    exists(Scope s | s.getEntryNode() = this |
+      result = "Entry node for " + concat( | | s.toString(), ",")
+    )
     or
     exists(Scope s | s.getANormalExit() = this | result = "Exit node for " + s.toString())
     or

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

@@ -281,28 +281,33 @@ class DataFlowExpr = Expr;
 /**
  * A module to compute local flow.
  *
- * Flow will generally go from control flow nodes into essa variables at definitions,
+ * Flow will generally go from control flow nodes for expressions into
+ * control flow nodes for variables at definitions,
  * and from there via use-use flow to other control flow nodes.
  *
  * Some syntaxtic constructs are handled separately.
  */
 module LocalFlow {
-  /** Holds if `nodeFrom` is the control flow node defining the essa variable `nodeTo`. */
+  /** Holds if `nodeFrom` is the expression defining the value for the variable `nodeTo`. */
   predicate definitionFlowStep(Node nodeFrom, Node nodeTo) {
     // Definition
     //   `x = f(42)`
-    //   nodeFrom is `f(42)`, cfg node
-    //   nodeTo is `x`, essa var
-    nodeFrom.(CfgNode).getNode() =
-      nodeTo.(EssaNode).getVar().getDefinition().(AssignmentDefinition).getValue()
+    //   nodeFrom is `f(42)`
+    //   nodeTo is `x`
+    exists(AssignmentDefinition def |
+      nodeFrom.(CfgNode).getNode() = def.getValue() and
+      nodeTo.(CfgNode).getNode() = def.getDefiningNode()
+    )
     or
     // With definition
     //   `with f(42) as x:`
-    //   nodeFrom is `f(42)`, cfg node
-    //   nodeTo is `x`, essa var
-    exists(With with, ControlFlowNode contextManager, ControlFlowNode var |
+    //   nodeFrom is `f(42)`
+    //   nodeTo is `x`
+    exists(With with, ControlFlowNode contextManager, WithDefinition withDef, ControlFlowNode var |
+      var = withDef.getDefiningNode()
+    |
       nodeFrom.(CfgNode).getNode() = contextManager and
-      nodeTo.(EssaNode).getVar().getDefinition().(WithDefinition).getDefiningNode() = var and
+      nodeTo.(CfgNode).getNode() = var and
       // see `with_flow` in `python/ql/src/semmle/python/dataflow/Implementation.qll`
       with.getContextExpr() = contextManager.getNode() and
       with.getOptionalVars() = var.getNode() and
@@ -313,34 +318,6 @@ module LocalFlow {
       // * `foo = x.foo(); await foo.async_method(); foo.close()` and
       // * `async with x.foo() as foo: await foo.async_method()`.
     )
-    or
-    // Async with var definition
-    //  `async with f(42) as x:`
-    //  nodeFrom is `x`, cfg node
-    //  nodeTo is `x`, essa var
-    //
-    // This makes the cfg node the local source of the awaited value.
-    //
-    // We have this step in addition to the step above, to handle cases where the QL
-    // modeling of `f(42)` requires a `.getAwaited()` step (in API graphs) when not
-    // using `async with`, so you can do both:
-    // * `foo = await x.foo(); await foo.async_method(); foo.close()` and
-    // * `async with x.foo() as foo: await foo.async_method()`.
-    exists(With with, ControlFlowNode var |
-      nodeFrom.(CfgNode).getNode() = var and
-      nodeTo.(EssaNode).getVar().getDefinition().(WithDefinition).getDefiningNode() = var and
-      with.getOptionalVars() = var.getNode() and
-      with.isAsync()
-    )
-    or
-    // Parameter definition
-    //   `def foo(x):`
-    //   nodeFrom is `x`, cfgNode
-    //   nodeTo is `x`, essa var
-    exists(ParameterDefinition pd |
-      nodeFrom.(CfgNode).getNode() = pd.getDefiningNode() and
-      nodeTo.(EssaNode).getVar() = pd.getVariable()
-    )
   }
 
   predicate expressionFlowStep(Node nodeFrom, Node nodeTo) {
@@ -372,9 +349,12 @@ module LocalFlow {
     // First use after definition
     //   `y = 42`
     //   `x = f(y)`
-    //   nodeFrom is `y` on first line, essa var
-    //   nodeTo is `y` on second line, cfg node
-    defToFirstUse(nodeFrom.asVar(), nodeTo.asCfgNode())
+    //   nodeFrom is `y` on first line
+    //   nodeTo is `y` on second line
+    exists(EssaDefinition def |
+      nodeFrom.(CfgNode).getNode() = def.(EssaNodeDefinition).getDefiningNode() and
+      AdjacentUses::firstUse(def, nodeTo.(CfgNode).getNode())
+    )
     or
     // Next use after use
     //   `x = f(y)`
@@ -565,11 +545,7 @@ predicate neverSkipInPathGraph(Node n) {
   // ```
   // we would end up saying that the path MUST not skip the x in `y = x`, which is just
   // annoying and doesn't help the path explanation become clearer.
-  n.asVar() instanceof EssaDefinition and
-  // For a parameter we have flow from ControlFlowNode to SSA node, and then onwards
-  // with use-use flow, and since the CFN is already part of the path graph, we don't
-  // want to force showing the SSA node as well.
-  not n.asVar() instanceof ParameterDefinition
+  n.asCfgNode() = any(EssaNodeDefinition def).getDefiningNode()
 }
 
 /**
@@ -916,7 +892,7 @@ predicate subscriptReadStep(CfgNode nodeFrom, Content c, CfgNode nodeTo) {
 predicate forReadStep(CfgNode nodeFrom, Content c, Node nodeTo) {
   exists(ForTarget target |
     nodeFrom.asExpr() = target.getSource() and
-    nodeTo.asVar().(EssaNodeDefinition).getDefiningNode() = target
+    nodeTo.asCfgNode() = target
   ) and
   (
     c instanceof ListElementContent

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

@@ -23,13 +23,13 @@ private import FlowSummaryImpl as FlowSummaryImpl
  * The current implementation of these cross flows can be seen in `EssaTaintTracking`.
  */
 newtype TNode =
-  /** A node corresponding to an SSA variable. */
-  TEssaNode(EssaVariable var) or
   /** A node corresponding to a control flow node. */
   TCfgNode(ControlFlowNode node) {
     isExpressionNode(node)
     or
     node.getNode() instanceof Pattern
+    or
+    node = any(ScopeEntryDefinition def).getDefiningNode()
   } or
   /**
    * A synthetic node representing the value of an object before a state change.
@@ -155,9 +155,6 @@ class Node extends TNode {
     this.getLocation().hasLocationInfo(filepath, startline, startcolumn, endline, endcolumn)
   }
 
-  /** Gets the ESSA variable corresponding to this node, if any. */
-  EssaVariable asVar() { none() }
-
   /** Gets the control-flow node corresponding to this node, if any. */
   ControlFlowNode asCfgNode() { none() }
 
@@ -170,25 +167,6 @@ class Node extends TNode {
   LocalSourceNode getALocalSource() { result.flowsTo(this) }
 }
 
-/** A data-flow node corresponding to an SSA variable. */
-class EssaNode extends Node, TEssaNode {
-  EssaVariable var;
-
-  EssaNode() { this = TEssaNode(var) }
-
-  /** Gets the `EssaVariable` represented by this data-flow node. */
-  EssaVariable getVar() { result = var }
-
-  override EssaVariable asVar() { result = var }
-
-  /** Gets a textual representation of this element. */
-  override string toString() { result = var.toString() }
-
-  override Scope getScope() { result = var.getScope() }
-
-  override Location getLocation() { result = var.getLocation() }
-}
-
 /** A data-flow node corresponding to a control-flow node. */
 class CfgNode extends Node, TCfgNode {
   ControlFlowNode node;
@@ -411,8 +389,8 @@ class ModuleVariableNode extends Node, TModuleVariableNode {
   }
 
   /** Gets an `EssaNode` that corresponds to an assignment of this global variable. */
-  EssaNode getAWrite() {
-    result.getVar().getDefinition().(EssaNodeDefinition).definedBy(var, any(DefinitionNode defn))
+  Node getAWrite() {
+    any(EssaNodeDefinition def).definedBy(var, result.asCfgNode().(DefinitionNode))
   }
 
   /** Gets the possible values of the variable at the end of import time */

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

@@ -112,7 +112,7 @@ module ImportResolution {
       not allowedEssaImportStep(_, firstDef)
     |
       not LocalFlow::defToFirstUse(firstDef, _) and
-      val.asVar() = firstDef
+      val.asCfgNode() = firstDef.getDefinition().(EssaNodeDefinition).getDefiningNode()
       or
       exists(ControlFlowNode mid, ControlFlowNode end |
         LocalFlow::defToFirstUse(firstDef, mid) and
@@ -320,11 +320,11 @@ module ImportResolution {
     // name as a submodule, we always consider that this attribute _could_ be a
     // reference to the submodule, even if we don't know that the submodule has been
     // imported yet.
-    exists(string submodule, Module package |
-      submodule = result.asVar().getName() and
-      SsaSource::init_module_submodule_defn(result.asVar().getSourceVariable(),
-        package.getEntryNode()) and
-      m = getModuleFromName(package.getPackageName() + "." + submodule)
+    exists(string submodule, Module package, EssaVariable var |
+      submodule = var.getName() and
+      SsaSource::init_module_submodule_defn(var.getSourceVariable(), package.getEntryNode()) and
+      m = getModuleFromName(package.getPackageName() + "." + submodule) and
+      result.asCfgNode() = var.getDefinition().(EssaNodeDefinition).getDefiningNode()
     )
   }
 

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

@@ -87,13 +87,13 @@
  *    This is adequate as the route through `TIterableElement(sequence)` does not transfer precise content.
  *
  * 5. [Read] Content is read from `sequence` to its elements.
- *    a) If the element is a plain variable, the target is the corresponding essa node.
+ *    a) If the element is a plain variable, the target is the corresponding control flow node.
  *
  *    b) If the element is itself a sequence, with control-flow node `seq`, the target is `TIterableSequence(seq)`.
  *
  *    c) If the element is a starred variable, with control-flow node `v`, the target is `TIterableElement(v)`.
  *
- * 6. [Store] Content is stored from `TIterableElement(v)` to the essa variable for `v`, with
+ * 6. [Store] Content is stored from `TIterableElement(v)` to the control flow node for variable `v`, with
  *    content type `ListElementContent`.
  *
  * 7. [Flow, Read, Store] Steps 2 through 7 are repeated for all recursive elements which are sequences.
@@ -313,7 +313,7 @@ predicate iterableUnpackingConvertingStoreStep(Node nodeFrom, Content c, Node no
  * Step 5
  * For a sequence node inside an iterable unpacking, data flows from the sequence to its elements. There are
  * three cases for what `toNode` should be:
- *    a) If the element is a plain variable, `toNode` is the corresponding essa node.
+ *    a) If the element is a plain variable, `toNode` is the corresponding control flow node.
  *
  *    b) If the element is itself a sequence, with control-flow node `seq`, `toNode` is `TIterableSequence(seq)`.
  *
@@ -351,20 +351,25 @@ predicate iterableUnpackingElementReadStep(Node nodeFrom, Content c, Node nodeTo
           nodeTo = TIterableElementNode(element)
         else
           // Step 5a
-          nodeTo.asVar().getDefinition().(MultiAssignmentDefinition).getDefiningNode() = element
+          exists(MultiAssignmentDefinition mad | element = mad.getDefiningNode() |
+            nodeTo.(CfgNode).getNode() = element
+          )
     )
   )
 }
 
 /**
  * Step 6
- * Data flows from `TIterableElement(v)` to the essa variable for `v`, with
+ * Data flows from `TIterableElement(v)` to the control flow node for variable `v`, with
  * content type `ListElementContent`.
  */
 predicate iterableUnpackingStarredElementStoreStep(Node nodeFrom, Content c, Node nodeTo) {
-  exists(ControlFlowNode starred | starred.getNode() instanceof Starred |
+  exists(ControlFlowNode starred, MultiAssignmentDefinition mad |
+    starred.getNode() instanceof Starred and
+    starred = mad.getDefiningNode()
+  |
     nodeFrom = TIterableElementNode(starred) and
-    nodeTo.asVar().getDefinition().(MultiAssignmentDefinition).getDefiningNode() = starred and
+    nodeTo.asCfgNode() = starred and
     c instanceof ListElementContent
   )
 }

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

@@ -71,7 +71,7 @@ class LocalSourceNode extends Node {
     or
     // We include all scope entry definitions, as these act as the local source within the scope they
     // enter.
-    this.asVar() instanceof ScopeEntryDefinition
+    this.asCfgNode() = any(ScopeEntryDefinition def).getDefiningNode()
   }
 
   /** Holds if this `LocalSourceNode` can flow to `nodeTo` in one or more local flow steps. */
@@ -165,7 +165,7 @@ class LocalSourceNodeNotModuleVariableNode extends LocalSourceNode {
   LocalSourceNodeNotModuleVariableNode() {
     this instanceof ExprNode
     or
-    this.asVar() instanceof ScopeEntryDefinition
+    this.asCfgNode() = any(ScopeEntryDefinition def).getDefiningNode()
   }
 }
 

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

@@ -89,8 +89,9 @@ predicate matchAsFlowStep(Node nodeFrom, Node nodeTo) {
     or
     // the interior pattern flows to the alias
     nodeFrom.(CfgNode).getNode().getNode() = subject.getPattern() and
-    nodeTo.(EssaNode).getVar().getDefinition().(PatternAliasDefinition).getDefiningNode().getNode() =
-      alias
+    exists(PatternAliasDefinition pad | pad.getDefiningNode().getNode() = alias |
+      nodeTo.(CfgNode).getNode() = pad.getDefiningNode()
+    )
   )
 }
 
@@ -123,13 +124,9 @@ predicate matchLiteralFlowStep(Node nodeFrom, Node nodeTo) {
 predicate matchCaptureFlowStep(Node nodeFrom, Node nodeTo) {
   exists(MatchCapturePattern capture, Name var | capture.getVariable() = var |
     nodeFrom.(CfgNode).getNode().getNode() = capture and
-    nodeTo
-        .(EssaNode)
-        .getVar()
-        .getDefinition()
-        .(PatternCaptureDefinition)
-        .getDefiningNode()
-        .getNode() = var
+    exists(PatternCaptureDefinition pcd | pcd.getDefiningNode().getNode() = var |
+      nodeTo.(CfgNode).getNode() = pcd.getDefiningNode()
+    )
   )
 }
 

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

@@ -216,8 +216,10 @@ predicate awaitStep(DataFlow::Node nodeFrom, DataFlow::Node nodeTo) {
  */
 predicate asyncWithStep(DataFlow::Node nodeFrom, DataFlow::Node nodeTo) {
   exists(With with, ControlFlowNode contextManager, ControlFlowNode var |
+    var = any(WithDefinition wd).getDefiningNode()
+  |
     nodeFrom.(DataFlow::CfgNode).getNode() = contextManager and
-    nodeTo.(DataFlow::EssaNode).getVar().getDefinition().(WithDefinition).getDefiningNode() = var and
+    nodeTo.(DataFlow::CfgNode).getNode() = var and
     // see `with_flow` in `python/ql/src/semmle/python/dataflow/Implementation.qll`
     with.getContextExpr() = contextManager.getNode() and
     with.getOptionalVars() = var.getNode() and

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

@@ -50,8 +50,20 @@ predicate jumpStep(Node nodeFrom, Node nodeTo) {
 }
 
 predicate capturedJumpStep(Node nodeFrom, Node nodeTo) {
-  exists(SsaSourceVariable var, DefinitionNode def | var.hasDefiningNode(def) |
-    nodeTo.asVar().(ScopeEntryDefinition).getSourceVariable() = var and
+  // Jump into a capturing scope.
+  //
+  // var = expr
+  // ...
+  // def f():
+  // ..var is used..
+  //
+  // nodeFrom is `expr`
+  // nodeTo is entry node for `f`
+  exists(ScopeEntryDefinition e, SsaSourceVariable var, DefinitionNode def |
+    e.getSourceVariable() = var and
+    var.hasDefiningNode(def)
+  |
+    nodeTo.asCfgNode() = e.getDefiningNode() and
     nodeFrom.asCfgNode() = def.getValue() and
     var.getScope().getScope*() = nodeFrom.getScope()
   )
@@ -228,8 +240,7 @@ private module SummaryTypeTrackerInput implements SummaryTypeTracker::Input {
     |
       param = FlowSummary::SummaryComponent::parameter(apos) and
       DataFlowDispatch::parameterMatch(ppos, apos) and
-      // pick the SsaNode rather than the CfgNode
-      result.asVar().getDefinition().(ParameterDefinition).getParameter() = p and
+      result.asCfgNode().getNode() = p and
       (
         exists(int i | ppos.isPositional(i) |
           p = callable.getALocalSource().asExpr().(CallableExpr).getInnerScope().getArg(i)

python/ql/lib/semmle/python/frameworks/Django.qll

@@ -2740,7 +2740,7 @@ module PrivateDjango {
       this.asExpr() = list and
       // we look for an assignment to the `MIDDLEWARE` setting
       exists(DataFlow::Node mw |
-        mw.asVar().getName() = "MIDDLEWARE" and
+        mw.asExpr().(Name).getId() = "MIDDLEWARE" and
         DataFlow::localFlow(this, mw)
       |
         // To only include results where CSRF protection matters, we only care about CSRF

python/ql/lib/semmle/python/security/dataflow/ExceptionInfo.qll

@@ -28,8 +28,8 @@ private class TracebackFunctionCall extends ExceptionInfo, DataFlow::CallCfgNode
 /** A caught exception. */
 private class CaughtException extends ExceptionInfo {
   CaughtException() {
-    this.asVar().getDefinition().(EssaNodeDefinition).getDefiningNode().getNode() =
-      any(ExceptStmt s).getName()
+    this.asExpr() = any(ExceptStmt s).getName() and
+    this.asCfgNode() = any(EssaNodeDefinition def).getDefiningNode()
   }
 }