Handle ancient indexes (with no codec present) with graceful message

Author: rahulgoswami

lucene/core/src/java/org/apache/lucene/index/SegmentInfos.java

@@ -394,7 +394,33 @@ private static void parseSegmentInfos(
       String segName = input.readString();
       byte[] segmentID = new byte[StringHelper.ID_LENGTH];
       input.readBytes(segmentID, 0, segmentID.length);
-      Codec codec = readCodec(input);
+      Codec codec = null;
+      try {
+        codec = readCodec(input);
+      } catch (IllegalArgumentException e) {
+        if (e.getMessage() != null && e.getMessage().contains("Could not load codec")) {
+          // maybe we tried loading an old default codec which isn't present in backward-codecs
+          // anymore.
+          // aka index is too old
+          throw new IndexFormatTooOldException(
+              input,
+              "Index has segments derived from Lucene version "
+                  + infos.indexCreatedVersionMajor
+                  + ".x and is not supported by Lucene "
+                  + Version.LATEST
+                  + ". This Lucene version only supports indexes with major version "
+                  + minSupportedMajorVersion
+                  + " or later (found: "
+                  + infos.indexCreatedVersionMajor
+                  + ", minimum supported: "
+                  + minSupportedMajorVersion
+                  + "). To resolve this issue re-index your data using Lucene "
+                  + minSupportedMajorVersion
+                  + ".x or later.");
+        } else {
+          throw e;
+        }
+      }
       SegmentInfo info =
           codec.segmentInfoFormat().read(directory, segName, segmentID, IOContext.READONCE);
       info.setCodec(codec);