From 01066ea3bb1b2e38283d8d247bb3b268b4ea2dab Mon Sep 17 00:00:00 2001
From: Arthur Baars <aibaars@github.com>
Date: Mon, 30 Nov 2020 16:14:34 +0100
Subject: [PATCH] CFG: case expression

---
 .../controlflow/internal/Completion.qll       |  2 +
 .../internal/ControlFlowGraphImpl.qll         | 72 +++++++++++++++++++
 2 files changed, 74 insertions(+)

diff --git a/ql/src/codeql_ruby/controlflow/internal/Completion.qll b/ql/src/codeql_ruby/controlflow/internal/Completion.qll
index 78684220c91..abdc02b3358 100644
--- a/ql/src/codeql_ruby/controlflow/internal/Completion.qll
+++ b/ql/src/codeql_ruby/controlflow/internal/Completion.qll
@@ -137,6 +137,8 @@ private predicate inBooleanContext(AstNode n) {
   n = any(LogicalNotAstNode parent | inBooleanContext(parent)).getOperand()
   or
   n = any(ParenthesizedStatement parent | inBooleanContext(parent)).getChild()
+  or
+  n instanceof Pattern
 }
 
 /**
diff --git a/ql/src/codeql_ruby/controlflow/internal/ControlFlowGraphImpl.qll b/ql/src/codeql_ruby/controlflow/internal/ControlFlowGraphImpl.qll
index a842532eeb1..9a1534cad08 100644
--- a/ql/src/codeql_ruby/controlflow/internal/ControlFlowGraphImpl.qll
+++ b/ql/src/codeql_ruby/controlflow/internal/ControlFlowGraphImpl.qll
@@ -276,6 +276,44 @@ private module Trees {
     }
   }
 
+  private class CaseTree extends PreOrderTree, Case {
+    final override predicate propagatesAbnormal(AstNode child) {
+      child = this.getValue() or child = this.getChild(_).(When).getPattern(_)
+    }
+
+    final override predicate last(AstNode last, Completion c) {
+      last(this.getValue(), last, c) and not exists(this.getChild(_))
+      or
+      last(this.getChild(_), last, c) and c instanceof SimpleCompletion
+      or
+      exists(int i, ControlFlowTree lastBranch |
+        lastBranch = this.getChild(i) and not exists(this.getChild(i + 1))
+      |
+        last(lastBranch, last, c) and
+        c instanceof FalseCompletion
+      )
+    }
+
+    final override predicate succ(AstNode pred, AstNode succ, Completion c) {
+      pred = this and
+      first(this.getValue(), succ) and
+      c instanceof SimpleCompletion
+      or
+      pred = this and
+      first(this.getChild(0), succ) and
+      not exists(this.getValue()) and
+      c instanceof SimpleCompletion
+      or
+      last(this.getValue(), pred, c) and
+      first(this.getChild(0), succ) and
+      c instanceof SimpleCompletion
+      or
+      exists(int i, ControlFlowTree branch | branch = this.getChild(i) |
+        last(branch, pred, c) and first(this.getChild(i + 1), succ) and c instanceof FalseCompletion
+      )
+    }
+  }
+
   private class ClassTree extends StandardPreOrderTree, Class {
     final override AstNode getChildNode(int i) { result = this.getChild(i) }
 
@@ -529,6 +567,10 @@ private module Trees {
     final override AstNode getChildNode(int i) { result = this.getChild(i) }
   }
 
+  private class PatternTree extends StandardPostOrderTree, Pattern {
+    final override AstNode getChildNode(int i) { result = this.getChild() and i = 0 }
+  }
+
   private class ProgramTree extends StandardPreOrderTree, Program {
     final override AstNode getChildNode(int i) { result = this.getChild(i) }
 
@@ -569,6 +611,36 @@ private module Trees {
     final override AstNode getChildNode(int i) { result = this.getOperand() and i = 0 }
   }
 
+  private class WhenTree extends PreOrderTree, When {
+    final override predicate propagatesAbnormal(AstNode child) { child = this.getPattern(_) }
+
+    final override predicate last(AstNode last, Completion c) {
+      exists(int i |
+        not exists(this.getPattern(i + 1)) and
+        last(this.getPattern(i), last, c) and
+        c instanceof FalseCompletion
+      )
+      or
+      last(this.getBody(), last, c)
+    }
+
+    final override predicate succ(AstNode pred, AstNode succ, Completion c) {
+      pred = this and
+      first(this.getPattern(0), succ) and
+      c instanceof SimpleCompletion
+      or
+      exists(int i, Pattern p |
+        p = this.getPattern(i) and
+        last(p, pred, c)
+      |
+        c instanceof TrueCompletion and first(this.getBody(), succ)
+        or
+        c instanceof FalseCompletion and
+        first(this.getPattern(i + 1), succ)
+      )
+    }
+  }
+
   private class ConditionalLoopTree extends PreOrderTree, ConditionalLoopAstNode {
     final override predicate propagatesAbnormal(AstNode child) { child = this.getCondition() }