fix manual frame counting

libbeat/publisher/queue/diskqueue/segments.go

@@ -371,14 +371,26 @@ func readSegmentHeaderWithFrameCount(path string) (*segmentHeader, error) {
 			// All other errors mean we are done scanning, exit the loop.
 			break
 		}
-		// Try to advance to the next frame.
-		_, err = file.Seek(int64(frameLength), os.SEEK_CUR)
+		// Length is encoded in both the first and last four bytes of a frame. To
+		// detect truncated / corrupted frames, seek to the last four bytes of
+		// the current frame to make sure the trailing length matches before
+		// advancing to the next frame (otherwise we might accept an impossible
+		// length).
+		_, err = file.Seek(int64(frameLength-8), os.SEEK_CUR)
 		if err != nil {
-			// An error in seeking probably means an invalid length, which
-			// indicates a truncated frame or data corruption, so end the
-			// loop without including it in our count.
 			break
 		}
+		var duplicateLength uint32
+		err = binary.Read(reader, binary.LittleEndian, &duplicateLength)
+		if err != nil {
+			break
+		}
+		if frameLength != duplicateLength {
+			err = fmt.Errorf(
+				"mismatched frame length: %v vs %v", frameLength, duplicateLength)
+			break
+		}
+
 		header.frameCount++
 	}
 	// If we ended up here instead of returning directly, then