Add CodeQL query to detect redundant assignments

Signed-off-by: Richard Yao <[email protected]>

.github/codeql-cpp.yml

@@ -0,0 +1,5 @@
+name: "Custom CodeQL Analysis"
+
+queries:
+  - uses: ./.github/codeql/custom-queries/cpp/redundantAssignment.ql
+  - uses: ./.github/codeql/openzfs-code-scanning.qls

.github/codeql-python.yml

@@ -0,0 +1,4 @@
+name: "Custom CodeQL Analysis"
+
+paths-ignore:
+  - tests

.github/codeql/custom-queries/cpp/qlpack.yml

@@ -0,0 +1,4 @@
+name: openzfs-cpp-queries
+version: 0.0.0
+libraryPathDependencies: codeql-cpp
+suites: openzfs-cpp-suite

.github/codeql/custom-queries/cpp/redundantAssignment.ql

@@ -0,0 +1,18 @@
+import cpp
+
+from AssignExpr firstAssign, AssignExpr secondAssign
+where
+  // The first assignment is from 'a' to 'b'
+  firstAssign.getDest().(VarAccess).getTarget() = secondAssign.getSource().(VarAccess).getTarget() and
+  firstAssign.getSource().(VarAccess).getTarget() = secondAssign.getDest().(VarAccess).getTarget() and
+  // Ensure 'a' and 'b' are not modified in between these assignments
+  not exists(Expr anyExpr |
+    anyExpr.getEnclosingFunction() = firstAssign.getEnclosingFunction() and
+    anyExpr.isBetween(firstAssign, secondAssign) and
+    (
+      anyExpr.(AssignExpr).getDest().(VarAccess).getTarget() = firstAssign.getDest().(VarAccess).getTarget() or
+      anyExpr.(AssignExpr).getDest().(VarAccess).getTarget() = firstAssign.getSource().(VarAccess).getTarget()
+    )
+  )
+select secondAssign, "This assignment is redundant."
+

.github/codeql/openzfs-code-scanning.qls

@@ -0,0 +1,3 @@
+# Reusing existing QL Pack
+- import: codeql-suites/cpp-code-scanning.qls
+  from: codeql-cpp

.github/workflows/codeql.yml

@@ -29,6 +29,7 @@ jobs:
     - name: Initialize CodeQL
       uses: github/codeql-action/init@v2
       with:
+        config-file: .github/codeql-${{ matrix.language }}.yml
         languages: ${{ matrix.language }}
 
     - name: Autobuild