Python: Test demonstrating need for phi-read-nodes

Or for a data flow node filling that role, at least.

python/ql/test/experimental/dataflow/use-use-flow/read_explosion.py

@@ -0,0 +1,36 @@
+# This test file is inspired by
+# `csharp/ql/test/library-tests/dataflow/local/UseUseExplosion.cs`
+# but with `n=3` kept small, since we do have the explosion.
+
+cond = ...
+
+# global variables are slightly special,
+# so we go into a function scope
+def scope():
+    x = 0
+
+    if(cond > 3):
+      if(cond > 2):
+        if(cond > 1):
+           pass
+        else:
+          use(x)
+      else:
+        use(x)
+    else:
+      use(x)
+
+    if(cond > 3):
+      if(cond > 2):
+        if(cond > 1):
+           pass
+        else:
+          use(x)
+      else:
+        use(x)
+    else:
+      use(x)
+
+    def use(v):
+        # this could just be `pass` but we do not want it optimized away.
+        y = v+2

python/ql/test/experimental/dataflow/use-use-flow/use-use-counts.expected

@@ -0,0 +1,24 @@
+implicit_use_count
+| 0 |
+implicit_use
+source_use_count
+| 6 |
+source_use
+| read_explosion.py:13:15:13:15 | ControlFlowNode for x |
+| read_explosion.py:15:13:15:13 | ControlFlowNode for x |
+| read_explosion.py:17:11:17:11 | ControlFlowNode for x |
+| read_explosion.py:24:15:24:15 | ControlFlowNode for x |
+| read_explosion.py:26:13:26:13 | ControlFlowNode for x |
+| read_explosion.py:28:11:28:11 | ControlFlowNode for x |
+use_use_edge_count
+| 9 |
+use_use_edge
+| read_explosion.py:13:15:13:15 | ControlFlowNode for x | read_explosion.py:24:15:24:15 | ControlFlowNode for x |
+| read_explosion.py:13:15:13:15 | ControlFlowNode for x | read_explosion.py:26:13:26:13 | ControlFlowNode for x |
+| read_explosion.py:13:15:13:15 | ControlFlowNode for x | read_explosion.py:28:11:28:11 | ControlFlowNode for x |
+| read_explosion.py:15:13:15:13 | ControlFlowNode for x | read_explosion.py:24:15:24:15 | ControlFlowNode for x |
+| read_explosion.py:15:13:15:13 | ControlFlowNode for x | read_explosion.py:26:13:26:13 | ControlFlowNode for x |
+| read_explosion.py:15:13:15:13 | ControlFlowNode for x | read_explosion.py:28:11:28:11 | ControlFlowNode for x |
+| read_explosion.py:17:11:17:11 | ControlFlowNode for x | read_explosion.py:24:15:24:15 | ControlFlowNode for x |
+| read_explosion.py:17:11:17:11 | ControlFlowNode for x | read_explosion.py:26:13:26:13 | ControlFlowNode for x |
+| read_explosion.py:17:11:17:11 | ControlFlowNode for x | read_explosion.py:28:11:28:11 | ControlFlowNode for x |

python/ql/test/experimental/dataflow/use-use-flow/use-use-counts.ql

@@ -0,0 +1,37 @@
+import python
+private import semmle.python.dataflow.new.internal.DataFlowPrivate
+
+query int implicit_use_count() {
+  exists(SsaSourceVariable x | x.getName() = "x" | result = count(x.getAnImplicitUse()))
+}
+
+query ControlFlowNode implicit_use() {
+  exists(SsaSourceVariable x | x.getName() = "x" | result = x.getAnImplicitUse())
+}
+
+query int source_use_count() {
+  exists(SsaSourceVariable x | x.getName() = "x" | result = count(x.getASourceUse()))
+}
+
+query ControlFlowNode source_use() {
+  exists(SsaSourceVariable x | x.getName() = "x" | result = x.getASourceUse())
+}
+
+query int use_use_edge_count() {
+  exists(SsaSourceVariable x | x.getName() = "x" |
+    result =
+      count(NameNode use1, NameNode use2 |
+        use1 = x.getAUse() and
+        use2 = x.getAUse() and
+        LocalFlow::useToNextUse(use1, use2)
+      )
+  )
+}
+
+query predicate use_use_edge(NameNode use1, NameNode use2) {
+  exists(SsaSourceVariable x | x.getName() = "x" |
+    use1 = x.getAUse() and
+    use2 = x.getAUse() and
+    LocalFlow::useToNextUse(use1, use2)
+  )
+}