Suppress verbose log during table iterator get (#193)

* If no record exists in the iterator range, the very first get may
return failure, and that is expected. We should not leave an error
log for that case, as it may be very verbose.

src/table_file.h

@@ -291,6 +291,7 @@ class TableFile {
         uint64_t maxSeq;
 
         bool safeMode;
+        bool prevNextHappened;
     };
 
     Status updateSnapshot();

src/table_file_iterator.cc

@@ -30,6 +30,7 @@ TableFile::Iterator::Iterator()
     , minSeq(NOT_INITIALIZED)
     , maxSeq(NOT_INITIALIZED)
     , safeMode(false)
+    , prevNextHappened(false)
     {}
 
 TableFile::Iterator::~Iterator() {
@@ -178,7 +179,13 @@ Status TableFile::Iterator::get(Record& rec_out) {
             return Status::KEY_NOT_FOUND;
         }
         // Otherwise, error.
-        _log_err(tFile->myLog, "fdb_iterator_get failed: %d", fs);
+
+        // NOTE: If it is the very first get without moving the cursor,
+        //       an error is expected if no record exists in the range.
+        //       Tolerate this case.
+        if (prevNextHappened) {
+            _log_err(tFile->myLog, "fdb_iterator_get failed: %d", fs);
+        }
         return toJungleStatus(fs);
     }
 
@@ -243,7 +250,11 @@ Status TableFile::Iterator::getMeta(Record& rec_out,
             return Status::KEY_NOT_FOUND;
         }
         // Otherwise, error.
-        _log_err(tFile->myLog, "fdb_iterator_get failed: %d", fs);
+
+        // NOTE: Same as `get()`.
+        if (prevNextHappened) {
+            _log_err(tFile->myLog, "fdb_iterator_get_metaonly failed: %d", fs);
+        }
         return toJungleStatus(fs);
     }
 
@@ -285,6 +296,7 @@ Status TableFile::Iterator::prev() {
         assert(fs == FDB_RESULT_SUCCESS);
         return Status::OUT_OF_RANGE;
     }
+    prevNextHappened = true;
     return Status();
 }
 
@@ -298,6 +310,7 @@ Status TableFile::Iterator::next() {
         assert(fs == FDB_RESULT_SUCCESS);
         return Status::OUT_OF_RANGE;
     }
+    prevNextHappened = true;
     return Status();
 }