Document and test thread safety properties

Closes #2

Internal.h

@@ -10,10 +10,12 @@
 #include <cstdlib>
 #include <cstring>
 #include <inttypes.h>
+#include <latch>
 #include <map>
 #include <set>
 #include <span>
 #include <string>
+#include <thread>
 #include <unordered_set>
 #include <utility>
 #include <vector>
@@ -652,32 +654,60 @@ template <class ConflictSetImpl> struct TestDriver {
       auto *results2 =
           new (arena) ConflictSet::Result[numPointReads + numRangeReads];
 
+#ifdef THREAD_TEST
+      auto *results3 =
+          new (arena) ConflictSet::Result[numPointReads + numRangeReads];
+      std::latch ready{1};
+      std::thread thread2{[&]() {
+        ready.count_down();
+        cs.check(reads, results3, numPointReads + numRangeReads);
+      }};
+      ready.wait();
+#endif
+
       CALLGRIND_START_INSTRUMENTATION;
       cs.check(reads, results1, numPointReads + numRangeReads);
       CALLGRIND_STOP_INSTRUMENTATION;
 
       refImpl.check(reads, results2, numPointReads + numRangeReads);
-      for (int i = 0; i < numPointReads + numRangeReads; ++i) {
-        if (results1[i] != results2[i]) {
-          if (reads[i].end.len == 0) {
-            fprintf(stderr,
-                    "Expected %s, got %s for read of {%s} at version %" PRId64
-                    "\n",
-                    resultToStr(results2[i]), resultToStr(results1[i]),
-                    printable(reads[i].begin).c_str(), reads[i].readVersion);
-          } else {
-            fprintf(
-                stderr,
-                "Expected %s, got %s for read of [%s, %s) at version %" PRId64
-                "\n",
-                resultToStr(results2[i]), resultToStr(results1[i]),
-                printable(reads[i].begin).c_str(),
-                printable(reads[i].end).c_str(), reads[i].readVersion);
+
+      auto compareResults = [reads](ConflictSet::Result *results1,
+                                    ConflictSet::Result *results2, int count) {
+        for (int i = 0; i < count; ++i) {
+          if (results1[i] != results2[i]) {
+            if (reads[i].end.len == 0) {
+              fprintf(stderr,
+                      "Expected %s, got %s for read of {%s} at version %" PRId64
+                      "\n",
+                      resultToStr(results2[i]), resultToStr(results1[i]),
+                      printable(reads[i].begin).c_str(), reads[i].readVersion);
+            } else {
+              fprintf(
+                  stderr,
+                  "Expected %s, got %s for read of [%s, %s) at version %" PRId64
+                  "\n",
+                  resultToStr(results2[i]), resultToStr(results1[i]),
+                  printable(reads[i].begin).c_str(),
+                  printable(reads[i].end).c_str(), reads[i].readVersion);
+            }
+            return false;
           }
-          ok = false;
-          return true;
         }
+        return true;
+      };
+
+      if (!compareResults(results1, results2, numPointReads + numRangeReads)) {
+        ok = false;
+        return true;
       }
+
+#ifdef THREAD_TEST
+      thread2.join();
+      if (!compareResults(results3, results2, numPointReads + numRangeReads)) {
+        ok = false;
+        return true;
+      }
+#endif
     }
     return false;
   }