Document thread-safety in terms of constness

ConflictSet.cpp

@@ -2639,6 +2639,12 @@ ConflictSet_getBytes(void *cs) {
 }
 }
 
+// Make sure abi is well-defined
+static_assert(std::is_standard_layout_v<ConflictSet::Result>);
+static_assert(std::is_standard_layout_v<ConflictSet::Key>);
+static_assert(std::is_standard_layout_v<ConflictSet::ReadRange>);
+static_assert(std::is_standard_layout_v<ConflictSet::WriteRange>);
+
 namespace {
 
 std::string getSearchPathPrintable(Node *n) {

include/ConflictSet.h

@@ -26,7 +26,7 @@ limitations under the License.
  *   - It's safe to operate on two different ConflictSets in two different
  * threads concurrently
  *   - It's safe to have multiple threads operating on the same ConflictSet
- * concurrently if and only if all threads only call `check`.
+ * concurrently if and only if all threads only call const methods
  */
 struct __attribute__((__visibility__("default"))) ConflictSet {
   enum Result {
@@ -110,7 +110,8 @@ private:
  *   - It's safe to operate on two different ConflictSets in two different
  * threads concurrently
  *   - It's safe to have multiple threads operating on the same ConflictSet
- * concurrently if and only if all threads only call `check`.
+ * concurrently if and only if all threads only call functions that accept a
+ * const ConflictSet pointer
  */
 typedef struct ConflictSet ConflictSet;
 
@@ -150,7 +151,8 @@ typedef struct {
 } ConflictSet_WriteRange;
 
 /** The result of checking reads[i] is written in results[i] */
-void ConflictSet_check(ConflictSet *cs, const ConflictSet_ReadRange *reads,
+void ConflictSet_check(const ConflictSet *cs,
+                       const ConflictSet_ReadRange *reads,
                        ConflictSet_Result *results, int count);
 
 /** `writes` must be sorted ascending, and must not have adjacent or
@@ -173,6 +175,6 @@ ConflictSet *ConflictSet_create(int64_t oldestVersion);
 void ConflictSet_destroy(ConflictSet *cs);
 
 /** Returns the total bytes in use by this ConflictSet */
-int64_t ConflictSet_getBytes(ConflictSet *cs);
+int64_t ConflictSet_getBytes(const ConflictSet *cs);
 
 #endif