pw_clock_tree: Introduce Element may_block()

This change introduces the `may_block()` function for the
`Element` class. The `ElementBlocking` class sets the constructor
argument `may_block` to true, all other derived classes set
`may_block` to false.

In a subsequent change the `ClockTree` class will use the
`may_block()` function call to determine whether to acquire
the `interrupt_spin_lock_` or the `mutex_` to synchronize acceess
to the respective clock tree elements.

Bug: 331672574
Change-Id: I5196ff418f59bd8261f843981d62ab2d250ef71c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/211148
Lint: Lint 🤖 <[email protected]>
Commit-Queue: Austin Foxley <[email protected]>
Reviewed-by: Austin Foxley <[email protected]>

pw_clock_tree/clock_tree_test.cc

@@ -1563,5 +1563,17 @@ TEST(ClockTree, ClockFailureRelease2Blocking) {
 TEST(ClockTree, ClockFailureRelease2NonBlocking) {
   TestFailureRelease2<ElementNonBlockingMightFail>();
 }
+
+TEST(ClockTree, ElementMayBlock) {
+  ClockSourceTest<ElementNonBlockingCannotFail> clock_non_blocking_cannot_fail;
+  EXPECT_FALSE(clock_non_blocking_cannot_fail.may_block());
+
+  ClockSourceTest<ElementNonBlockingMightFail> clock_non_blocking_might_fail;
+  EXPECT_FALSE(clock_non_blocking_might_fail.may_block());
+
+  ClockSourceTest<ElementBlocking> clock_blocking;
+  EXPECT_TRUE(clock_blocking.may_block());
+}
+
 }  // namespace
 }  // namespace pw::clock_tree

pw_clock_tree/public/pw_clock_tree/clock_tree.h

@@ -38,12 +38,15 @@ namespace pw::clock_tree {
 /// `ElementNonBlockingCannotFail` or `ElementNonBlockingMightFail` class.
 class Element {
  public:
-  constexpr Element() {}
+  constexpr Element(bool may_block = false) : may_block_(may_block) {}
   virtual ~Element() = default;
 
   /// Get reference count for this clock tree element.
   uint32_t ref_count() const { return ref_count_; }
 
+  /// Check whether acquiring or releasing the element may block.
+  bool may_block() const { return may_block_; }
+
   // Not copyable or movable
   Element(const Element&) = delete;
   Element(const Element&&) = delete;
@@ -97,9 +100,12 @@ class Element {
   virtual Status DoDisable() { return OkStatus(); }
 
  private:
-  /// Reference count for this child tree element.
+  /// Reference count for this tree element.
   uint32_t ref_count_ = 0;
 
+  /// Whether acquiring or releasing the element may block.
+  const bool may_block_;
+
   friend class ClockTree;
   template <typename ElementType>
   friend class DependentElement;
@@ -109,7 +115,10 @@ class Element {
 
 /// Abstract class of a clock tree element that might need to block to perform
 /// element updates.
-class ElementBlocking : public Element {};
+class ElementBlocking : public Element {
+ public:
+  constexpr ElementBlocking() : Element(/*may_block=*/true) {}
+};
 
 /// Abstract class of a clock tree element that will not block to perform
 /// element updates and will not fail when performing clock updates.