Interface change! Allow decreasing setOldestVersion

ConflictSet.cpp

@@ -3204,11 +3204,7 @@ Iterator firstGeqLogical(Node *n, const std::span<const uint8_t> key) {
       } else {
         n = nextSibling(n);
         if (n == nullptr) {
-          //  This line is genuinely unreachable from any entry point of the
-          //  final library, since we can't remove a key without introducing a
-          //  key after it, and the only production caller of firstGeq is for
-          //  resuming the setOldestVersion scan.
-          return {nullptr, 1}; // GCOVR_EXCL_LINE
+          return {nullptr, 1};
         }
         goto downLeftSpine;
       }
@@ -3489,8 +3485,8 @@ void checkVersionsGeqOldestExtant(Node *n,
       assert(m >= oldestExtantVersion);
     }
   } break;
-  default:                   // GCOVR_EXCL_LINE
-    __builtin_unreachable(); // GCOVR_EXCL_LINE
+  default:
+    abort();
   }
 }
 

SkipList.cpp

@@ -634,7 +634,9 @@ struct __attribute__((visibility("hidden"))) ConflictSet::Impl {
   }
 
   void setOldestVersion(int64_t oldestVersion) {
-    assert(oldestVersion >= this->oldestVersion);
+    if (oldestVersion < this->oldestVersion) {
+      return;
+    }
     this->oldestVersion = oldestVersion;
     SkipList::Finger finger;
     int temp;

include/ConflictSet.h

@@ -77,8 +77,8 @@ struct __attribute__((__visibility__("default"))) ConflictSet {
    * versions. */
   void addWrites(const WriteRange *writes, int count, int64_t writeVersion);
 
-  /** Reads where readVersion < oldestVersion will result in `TooOld`. Must be
-   * greater than or equal to all previous oldest versions. */
+  /** Reads where readVersion < oldestVersion will result in `TooOld`. Ignored
+   * if not greater than or equal to all previous oldest versions. */
   void setOldestVersion(int64_t oldestVersion);
 
   /** Reads where readVersion < oldestVersion will result in `TooOld`. There are