feat: ability to override the default allocator

include/rcutils/allocator.h

@@ -108,6 +108,21 @@ RCUTILS_WARN_UNUSED
 rcutils_allocator_t
 rcutils_get_default_allocator(void);
 
+/// Override the default allocator returned by rcutils_get_default_allocator.
+/**
+ * Attribute          | Adherence
+ * ------------------ | -------------
+ * Allocates Memory   | No
+ * Thread-Safe        | No
+ * Uses Atomics       | No
+ * Lock-Free          | Yes
+ *
+ * \param[in] override_allocator The allocator to set as the default.
+ */
+RCUTILS_PUBLIC
+void
+rcutils_set_default_allocator(rcutils_allocator_t override_allocator);
+
 /// Return true if the given allocator has non-null function pointers.
 /**
  * \param[in] allocator to be checked by the function

src/allocator.c

@@ -69,16 +69,31 @@ rcutils_get_zero_initialized_allocator(void)
   return zero_allocator;
 }
 
+static rcutils_allocator_t rcutils_override_default_allocator = {0};
+
+void
+rcutils_set_default_allocator(rcutils_allocator_t override_allocator)
+{
+  if (rcutils_allocator_is_valid(&override_allocator)) {
+    rcutils_override_default_allocator = override_allocator;
+  }
+}
+
 rcutils_allocator_t
 rcutils_get_default_allocator(void)
 {
+  if (rcutils_allocator_is_valid(&rcutils_override_default_allocator)) {
+    return rcutils_override_default_allocator;
+  }
+
   static rcutils_allocator_t default_allocator = {
     .allocate = __default_allocate,
     .deallocate = __default_deallocate,
     .reallocate = __default_reallocate,
     .zero_allocate = __default_zero_allocate,
     .state = NULL,
   };
+
   return default_allocator;
 }