C#/C++/Java: Unbreak partial data flow support

Partial data flow had a semantic merge conflict with this branch. The
problem is that partial data flow doesn't (and shouldn't) cause the
initial pruning steps to run, but the length-2 access paths depend on
the `consCand` information that comes from that initial pruning. The
solution is to restore the old `AccessPath` class, now called
`PartialAccessPath` for use only by partial data flow.

With this change, partial data flow will in some cases allow more field
flow than non-partial data flow.

Author: jbj

cpp/ql/src/semmle/code/cpp/dataflow/internal/DataFlowImpl.qll

@@ -2048,11 +2048,64 @@ private module FlowExploration {
     result = distSrcExt(TCallable(c, config)) - 1
   }
 
+  private newtype TPartialAccessPath =
+    TPartialNil(DataFlowType t) or
+    TPartialCons(Content f, int len) { len in [1 .. 5] }
+
+  /**
+   * Conceptually a list of `Content`s followed by a `Type`, but only the first
+   * element of the list and its length are tracked. If data flows from a source to
+   * a given node with a given `AccessPath`, this indicates the sequence of
+   * dereference operations needed to get from the value in the node to the
+   * tracked object. The final type indicates the type of the tracked object.
+   */
+  private class PartialAccessPath extends TPartialAccessPath {
+    abstract string toString();
+
+    Content getHead() { this = TPartialCons(result, _) }
+
+    int len() {
+      this = TPartialNil(_) and result = 0
+      or
+      this = TPartialCons(_, result)
+    }
+
+    DataFlowType getType() {
+      this = TPartialNil(result)
+      or
+      exists(Content head | this = TPartialCons(head, _) | result = head.getContainerType())
+    }
+
+    abstract AccessPathFront getFront();
+  }
+
+  private class PartialAccessPathNil extends PartialAccessPath, TPartialNil {
+    override string toString() {
+      exists(DataFlowType t | this = TPartialNil(t) | result = ppReprType(t))
+    }
+
+    override AccessPathFront getFront() {
+      exists(DataFlowType t | this = TPartialNil(t) | result = TFrontNil(t))
+    }
+  }
+
+  private class PartialAccessPathCons extends PartialAccessPath, TPartialCons {
+    override string toString() {
+      exists(Content f, int len | this = TPartialCons(f, len) |
+        result = f.toString() + ", ... (" + len.toString() + ")"
+      )
+    }
+
+    override AccessPathFront getFront() {
+      exists(Content f | this = TPartialCons(f, _) | result = TFrontHead(f))
+    }
+  }
+
   private newtype TPartialPathNode =
-    TPartialPathNodeMk(Node node, CallContext cc, AccessPath ap, Configuration config) {
+    TPartialPathNodeMk(Node node, CallContext cc, PartialAccessPath ap, Configuration config) {
       config.isSource(node) and
       cc instanceof CallContextAny and
-      ap = TNil(getErasedRepr(node.getType())) and
+      ap = TPartialNil(getErasedRepr(node.getType())) and
       not fullBarrier(node, config) and
       exists(config.explorationLimit())
       or
@@ -2062,7 +2115,7 @@ private module FlowExploration {
 
   pragma[nomagic]
   private predicate partialPathNodeMk0(
-    Node node, CallContext cc, AccessPath ap, Configuration config
+    Node node, CallContext cc, PartialAccessPath ap, Configuration config
   ) {
     exists(PartialPathNode mid |
       partialPathStep(mid, node, cc, ap, config) and
@@ -2138,7 +2191,7 @@ private module FlowExploration {
 
     CallContext cc;
 
-    AccessPath ap;
+    PartialAccessPath ap;
 
     Configuration config;
 
@@ -2148,7 +2201,7 @@ private module FlowExploration {
 
     CallContext getCallContext() { result = cc }
 
-    AccessPath getAp() { result = ap }
+    PartialAccessPath getAp() { result = ap }
 
     override Configuration getConfiguration() { result = config }
 
@@ -2161,7 +2214,7 @@ private module FlowExploration {
   }
 
   private predicate partialPathStep(
-    PartialPathNodePriv mid, Node node, CallContext cc, AccessPath ap, Configuration config
+    PartialPathNodePriv mid, Node node, CallContext cc, PartialAccessPath ap, Configuration config
   ) {
     localFlowStep(mid.getNode(), node, config) and
     cc = mid.getCallContext() and
@@ -2170,8 +2223,8 @@ private module FlowExploration {
     or
     additionalLocalFlowStep(mid.getNode(), node, config) and
     cc = mid.getCallContext() and
-    mid.getAp() instanceof AccessPathNil and
-    ap = TNil(getErasedRepr(node.getType())) and
+    mid.getAp() instanceof PartialAccessPathNil and
+    ap = TPartialNil(getErasedRepr(node.getType())) and
     config = mid.getConfiguration()
     or
     jumpStep(mid.getNode(), node, config) and
@@ -2181,15 +2234,15 @@ private module FlowExploration {
     or
     additionalJumpStep(mid.getNode(), node, config) and
     cc instanceof CallContextAny and
-    mid.getAp() instanceof AccessPathNil and
-    ap = TNil(getErasedRepr(node.getType())) and
+    mid.getAp() instanceof PartialAccessPathNil and
+    ap = TPartialNil(getErasedRepr(node.getType())) and
     config = mid.getConfiguration()
     or
     partialPathStoreStep(mid, _, _, node, ap) and
     cc = mid.getCallContext() and
     config = mid.getConfiguration()
     or
-    exists(AccessPath ap0, Content f |
+    exists(PartialAccessPath ap0, Content f |
       partialPathReadStep(mid, ap0, f, node, cc, config) and
       apConsFwd(ap, f, ap0, config)
     )
@@ -2210,7 +2263,7 @@ private module FlowExploration {
 
   pragma[inline]
   private predicate partialPathStoreStep(
-    PartialPathNodePriv mid, AccessPath ap1, Content f, Node node, AccessPath ap2
+    PartialPathNodePriv mid, PartialAccessPath ap1, Content f, Node node, PartialAccessPath ap2
   ) {
     ap1 = mid.getAp() and
     store(mid.getNode(), f, node) and
@@ -2220,7 +2273,9 @@ private module FlowExploration {
   }
 
   pragma[nomagic]
-  private predicate apConsFwd(AccessPath ap1, Content f, AccessPath ap2, Configuration config) {
+  private predicate apConsFwd(
+    PartialAccessPath ap1, Content f, PartialAccessPath ap2, Configuration config
+  ) {
     exists(PartialPathNodePriv mid |
       partialPathStoreStep(mid, ap1, f, _, ap2) and
       config = mid.getConfiguration()
@@ -2229,7 +2284,7 @@ private module FlowExploration {
 
   pragma[nomagic]
   private predicate partialPathReadStep(
-    PartialPathNodePriv mid, AccessPath ap, Content f, Node node, CallContext cc,
+    PartialPathNodePriv mid, PartialAccessPath ap, Content f, Node node, CallContext cc,
     Configuration config
   ) {
     ap = mid.getAp() and
@@ -2240,7 +2295,7 @@ private module FlowExploration {
   }
 
   private predicate partialPathOutOfCallable0(
-    PartialPathNodePriv mid, ReturnPosition pos, CallContext innercc, AccessPath ap,
+    PartialPathNodePriv mid, ReturnPosition pos, CallContext innercc, PartialAccessPath ap,
     Configuration config
   ) {
     pos = getReturnPosition(mid.getNode()) and
@@ -2252,8 +2307,8 @@ private module FlowExploration {
 
   pragma[noinline]
   private predicate partialPathOutOfCallable1(
-    PartialPathNodePriv mid, DataFlowCall call, ReturnKind kind, CallContext cc, AccessPath ap,
-    Configuration config
+    PartialPathNodePriv mid, DataFlowCall call, ReturnKind kind, CallContext cc,
+    PartialAccessPath ap, Configuration config
   ) {
     exists(ReturnPosition pos, DataFlowCallable c, CallContext innercc |
       partialPathOutOfCallable0(mid, pos, innercc, ap, config) and
@@ -2266,7 +2321,7 @@ private module FlowExploration {
   }
 
   private predicate partialPathOutOfCallable(
-    PartialPathNodePriv mid, OutNode out, CallContext cc, AccessPath ap, Configuration config
+    PartialPathNodePriv mid, OutNode out, CallContext cc, PartialAccessPath ap, Configuration config
   ) {
     exists(ReturnKind kind, DataFlowCall call |
       partialPathOutOfCallable1(mid, call, kind, cc, ap, config)
@@ -2276,7 +2331,7 @@ private module FlowExploration {
   }
 
   private predicate partialPathOutOfArgument(
-    PartialPathNodePriv mid, PostUpdateNode node, CallContext cc, AccessPath ap,
+    PartialPathNodePriv mid, PostUpdateNode node, CallContext cc, PartialAccessPath ap,
     Configuration config
   ) {
     exists(
@@ -2302,7 +2357,7 @@ private module FlowExploration {
   pragma[noinline]
   private predicate partialPathIntoArg(
     PartialPathNodePriv mid, int i, CallContext cc, DataFlowCall call, boolean emptyAp,
-    AccessPath ap, Configuration config
+    PartialAccessPath ap, Configuration config
   ) {
     exists(ArgumentNode arg |
       arg = mid.getNode() and
@@ -2311,24 +2366,24 @@ private module FlowExploration {
       ap = mid.getAp() and
       config = mid.getConfiguration()
     |
-      ap instanceof AccessPathNil and emptyAp = true
+      ap instanceof PartialAccessPathNil and emptyAp = true
       or
-      ap instanceof AccessPathCons and emptyAp = false
+      ap instanceof PartialAccessPathCons and emptyAp = false
     )
   }
 
   pragma[nomagic]
   private predicate partialPathIntoCallable0(
     PartialPathNodePriv mid, DataFlowCallable callable, int i, CallContext outercc,
-    DataFlowCall call, boolean emptyAp, AccessPath ap, Configuration config
+    DataFlowCall call, boolean emptyAp, PartialAccessPath ap, Configuration config
   ) {
     partialPathIntoArg(mid, i, outercc, call, emptyAp, ap, config) and
     callable = resolveCall(call, outercc)
   }
 
   private predicate partialPathIntoCallable(
     PartialPathNodePriv mid, ParameterNode p, CallContext outercc, CallContextCall innercc,
-    DataFlowCall call, AccessPath ap, Configuration config
+    DataFlowCall call, PartialAccessPath ap, Configuration config
   ) {
     exists(int i, DataFlowCallable callable, boolean emptyAp |
       partialPathIntoCallable0(mid, callable, i, outercc, call, emptyAp, ap, config) and
@@ -2342,7 +2397,8 @@ private module FlowExploration {
 
   pragma[nomagic]
   private predicate paramFlowsThroughInPartialPath(
-    ParameterNode p, ReturnKind kind, CallContextCall cc, AccessPathNil apnil, Configuration config
+    ParameterNode p, ReturnKind kind, CallContextCall cc, PartialAccessPathNil apnil,
+    Configuration config
   ) {
     exists(PartialPathNodePriv mid, ReturnNode ret |
       mid.getNode() = ret and
@@ -2362,17 +2418,18 @@ private module FlowExploration {
   pragma[noinline]
   private predicate partialPathThroughCallable0(
     DataFlowCall call, PartialPathNodePriv mid, ReturnKind kind, CallContext cc,
-    AccessPathNil apnil, Configuration config
+    PartialAccessPathNil apnil, Configuration config
   ) {
-    exists(ParameterNode p, CallContext innercc, AccessPathNil midapnil |
+    exists(ParameterNode p, CallContext innercc, PartialAccessPathNil midapnil |
       partialPathIntoCallable(mid, p, cc, innercc, call, midapnil, config) and
       paramFlowsThroughInPartialPath(p, kind, innercc, apnil, config) and
       not parameterValueFlowsThrough(p, kind, innercc)
     )
   }
 
   private predicate partialPathThroughCallable(
-    PartialPathNodePriv mid, OutNode out, CallContext cc, AccessPathNil apnil, Configuration config
+    PartialPathNodePriv mid, OutNode out, CallContext cc, PartialAccessPathNil apnil,
+    Configuration config
   ) {
     exists(DataFlowCall call, ReturnKind kind |
       partialPathThroughCallable0(call, mid, kind, cc, apnil, config) and
@@ -2382,8 +2439,8 @@ private module FlowExploration {
 
   pragma[noinline]
   private predicate valuePartialPathThroughCallable0(
-    DataFlowCall call, PartialPathNodePriv mid, ReturnKind kind, CallContext cc, AccessPath ap,
-    Configuration config
+    DataFlowCall call, PartialPathNodePriv mid, ReturnKind kind, CallContext cc,
+    PartialAccessPath ap, Configuration config
   ) {
     exists(ParameterNode p, CallContext innercc |
       partialPathIntoCallable(mid, p, cc, innercc, call, ap, config) and
@@ -2392,7 +2449,7 @@ private module FlowExploration {
   }
 
   private predicate valuePartialPathThroughCallable(
-    PartialPathNodePriv mid, OutNode out, CallContext cc, AccessPath ap, Configuration config
+    PartialPathNodePriv mid, OutNode out, CallContext cc, PartialAccessPath ap, Configuration config
   ) {
     exists(DataFlowCall call, ReturnKind kind |
       valuePartialPathThroughCallable0(call, mid, kind, cc, ap, config) and