HD-4220: fix concurrency potential bug

Author: fferrandis

backend.c

@@ -100,10 +100,14 @@ bool check_matrix_fragment(char *frag, int frag_len, int piecesize) {
   size_t offset = 0;
 
   while (offset < frag_len) {
+    if (offset + sizeof(fragment_header_t) > frag_len) {
+      // there is some remaining bytes, but not enough to read at least the header
+      return false;
+    }
     if (is_invalid_fragment_header((fragment_header_t *)&frag[offset])) {
       return false;
     }
-    offset += piecesize + getHeaderSize();
+    offset += ((fragment_header_t *)&frag[offset])->meta.size + getHeaderSize();
   }
 
   return true;

backend.go

@@ -282,7 +282,7 @@ func (backend *Backend) EncodeMatrixWithBufferMatrix(bm *BufferMatrix, chunkSize
 		go func(nth int) {
 			fragLen := C.size_t(chunkSize)
 			// last subgroup has a different size
-			if i == int(ctx.number_of_subgroup)-1 {
+			if nth == int(ctx.number_of_subgroup)-1 {
 				fragLen = C.size_t(bm.FragLenLastSubGroup())
 			}
 			atomic.AddInt64(&totLen, int64(fragLen)+int64(backend.headerSize))
@@ -690,7 +690,7 @@ func (backend *Backend) GetRangeMatrix(startIncl, endIncl, cellDataSize, fragSiz
 	   all fragments (see (2) in the function's comment above). */
 	linearizedRangeStartIncl := startIncl - dataOffset
 
-	/* Decoding always works on a group boundary. */
+	/* Decoding always works on a column boundary. */
 	decodedRangeStartIncl := startIncl - lineStart*lineSize
 
 	return &RangeMatrix{

backend_test.go

@@ -913,7 +913,7 @@ func TestLinearizeMatrixAndReconstruct(t *testing.T) {
 				frags2 = append(frags2, encoded.Data[i][rangeM.InFragRangeStartIncl:rangeM.InFragRangeEndExcl])
 			}
 
-			// now do the same test, but this time, insted of linearizing, we are going to reconstruct the stripes
+			// now do the same test, but this time, instead of linearizing, we are going to reconstruct the stripes
 			reconstructed, err := backend.ReconstructMatrix(frags2[1:], 0, currentChunkSize)
 
 			require.NoError(t, err)