Return 0 instead of -1 from hash_table getBytes()

The hash_table implementation cannot accurately track memory usage and was
returning -1 from ConflictSet::getBytes() and ConflictSet_getBytes(). That
violates the API contract that getBytes() returns a non-negative byte count.

Change both entry points to return 0 and document in ConflictSet.h that
implementations which do not track memory usage may return 0.

Add a regression test in test_conflict_set.py that loads the hash_table
implementation and verifies getBytes() is non-negative.

Closes #62

HashTable.cpp

@@ -96,7 +96,9 @@ void ConflictSet::setOldestVersion(int64_t oldestVersion) {
   return impl->setOldestVersion(oldestVersion);
 }
 
-int64_t ConflictSet::getBytes() const { return -1; }
+// The hash_table implementation does not track memory usage, so return 0 to
+// satisfy the API contract that getBytes() returns a non-negative value.
+int64_t ConflictSet::getBytes() const { return 0; }
 
 void ConflictSet::getMetricsV1(MetricsV1 **metrics, int *count) const {
   *metrics = nullptr;
@@ -161,7 +163,10 @@ __attribute__((__visibility__("default"))) void ConflictSet_destroy(void *cs) {
 }
 __attribute__((__visibility__("default"))) int64_t
 ConflictSet_getBytes(void *cs) {
-  using Impl = ConflictSet::Impl;
+  (void)cs;
-  return -1;
+  // The hash_table implementation does not track memory usage, so return 0 to
+  // satisfy the API contract that ConflictSet_getBytes returns a non-negative
+  // value.
+  return 0;
 }
 }

include/ConflictSet.h

@@ -88,7 +88,8 @@ struct __attribute__((__visibility__("default"))) ConflictSet {
 
   ~ConflictSet();
 
-  /** Returns the total bytes in use by this ConflictSet */
+  /** Returns the total bytes in use by this ConflictSet. Implementations that
+   * do not track memory usage return 0. */
   int64_t getBytes() const;
 
   /** Experimental! */
@@ -218,7 +219,8 @@ ConflictSet *ConflictSet_create(int64_t oldestVersion);
 
 void ConflictSet_destroy(ConflictSet *cs);
 
-/** Returns the total bytes in use by this ConflictSet */
+/** Returns the total bytes in use by this ConflictSet. Implementations that
+ * do not track memory usage return 0. */
 int64_t ConflictSet_getBytes(const ConflictSet *cs);
 
 #endif

test_conflict_set.py

@@ -57,6 +57,17 @@ def test_conflict_set():
         assert cs.check(read(0, key), read(1, key)) == [Result.TOO_OLD, Result.COMMIT]
 
 
+def test_hash_table_getBytes():
+    # Regression test for issue #62: the hash_table implementation is
+    # point-query only and does not track memory usage, but getBytes() must
+    # still return a non-negative value rather than -1.
+    with ConflictSet(0, build_dir=build_dir, implementation="hash_table") as cs:
+        assert cs.getBytes() == 0
+        cs.addWrites(1, write(b"key"))
+        assert cs.getBytes() >= 0
+        assert cs.check(read(0, b"key")) == [Result.CONFLICT]
+
+
 def test_write_read_without_outer_reference():
     # Regression test for issue #42: WriteRange/ReadRange must keep their
     # backing key buffers alive, because the C library reads the pointer