Assume malformed head chunk is partially written data

Head chunks and the WAL may not be completely flushed to disk if
e.g., Prometheus crashes.

Partial records may exist at the end of the file; allow this and assume
there is not further valid data.

A future change should validate CRCs also, and stop reading on the first
incorrect CRC.

src/pdu/block/chunk_reference.cc

@@ -43,9 +43,15 @@ std::optional<std::pair<size_t, ChunkReference>> readHeadChunkMeta(
 
     size_t dataLen = dec.read_varuint();
 
-    // skip data and 4 byte crc, leaving decoder at the start of the next
-    // chunk meta header.
-    dec.seek(dataLen + 4, std::ios_base::cur);
+    try {
+        // skip data and 4 byte crc, leaving decoder at the start of the next
+        // chunk meta header.
+        dec.seek(dataLen + 4, std::ios_base::cur);
+    } catch (const std::exception&) {
+        // if we fail to seek that far, assume that this file was partially
+        // written. There should be no later valid entries
+        return {};
+    }
 
     return {{seriesRef, std::move(ref)}};
 }