fix(lsm): account for empty entries when appending data

lsm/src/store/lsm_store.c

@@ -195,7 +195,14 @@ lsm_error lsm_entry_data_append(lsm_store *store, lsm_entry_handle *handle,
 
   // Data is in memory and still fits -> keep it in memory
   if ((new_len <= LSM_STORE_DISK_THRESHOLD) && (!entry->data.on_disk)) {
-    char *buf = realloc(entry->data.value.ptr, new_len * sizeof(char));
+    char *buf;
+
+    // Entries with no data do not have an allocated buffer yet
+    if (entry->data.len == 0) {
+      buf = malloc(new_len * sizeof(char));
+    } else {
+      buf = realloc(entry->data.value.ptr, new_len * sizeof(char));
+    }
 
     if (buf == NULL) {
       return lsm_error_failed_alloc;
@@ -218,17 +225,23 @@ lsm_error lsm_entry_data_append(lsm_store *store, lsm_entry_handle *handle,
         return lsm_error_failed_io;
       }
 
-      size_t written = 0;
+      // If there was data present in memory already, we sync this to disk.
+      // This check is required because it's possible that more than the
+      // treshold is written to an empty entry immediately, meaning there's no
+      // allocated memory buffer present.
+      if (entry->data.len > 0) {
+        size_t written = 0;
 
-      // Write original in-memory data to file
-      while (written < entry->data.len) {
-        written += fwrite(&entry->data.value.ptr[written], sizeof(char),
-                          entry->data.len - written, f);
+        // Write original in-memory data to file
+        while (written < entry->data.len) {
+          written += fwrite(&entry->data.value.ptr[written], sizeof(char),
+                            entry->data.len - written, f);
+        }
+
+        free(entry->data.value.ptr);
+        entry->data.value.ptr = NULL;
       }
 
-      free(entry->data.value.ptr);
-      entry->data.value.ptr = NULL;
-
       handle->f = f;
       entry->data.on_disk = true;
     }