vim-patch:8.1.0798: changing a blob while iterating over it works strangely

Problem:    Changing a blob while iterating over it works strangely.
Solution:   Make a copy of the Blob before iterating.
dd29ea1805

src/nvim/eval.c

@@ -2599,15 +2599,16 @@ void *eval_for_line(const char_u *arg, bool *errp, char_u **nextcmdp, int skip)
           fi->fi_lw.lw_item = tv_list_first(l);
         }
       } else if (tv.v_type == VAR_BLOB) {
-        blob_T *const b = tv.vval.v_blob;
-        if (b == NULL) {
-          tv_clear(&tv);
-        } else {
-          // No need to increment the refcount, it's already set for
-          // the blob being used in "tv".
-          fi->fi_blob = b;
         fi->fi_bi = 0;
+        if (tv.vval.v_blob != NULL) {
+          typval_T btv;
+
+          // Make a copy, so that the iteration still works when the
+          // blob is changed.
+          tv_blob_copy(&tv, &btv);
+          fi->fi_blob = btv.vval.v_blob;
         }
+        tv_clear(&tv);
       } else {
         EMSG(_(e_listreq));
         tv_clear(&tv);
@@ -9737,17 +9738,7 @@ int var_item_copy(const vimconv_T *const conv,
     }
     break;
   case VAR_BLOB:
-    to->v_type = VAR_BLOB;
+    tv_blob_copy(from, to);
-    if (from->vval.v_blob == NULL) {
-      to->vval.v_blob = NULL;
-    } else {
-      tv_blob_alloc_ret(to);
-      const int len = from->vval.v_blob->bv_ga.ga_len;
-
-      to->vval.v_blob->bv_ga.ga_data
-          = xmemdup(from->vval.v_blob->bv_ga.ga_data, (size_t)len);
-      to->vval.v_blob->bv_ga.ga_len = len;
-    }
     break;
   case VAR_DICT:
     to->v_type = VAR_DICT;

src/nvim/eval/typval.c

@@ -2252,6 +2252,30 @@ void tv_blob_alloc_ret(typval_T *const ret_tv)
   tv_blob_set_ret(ret_tv, b);
 }
 
+/// Copy a blob typval to a different typval.
+///
+/// @param[in]  from  Blob object to copy from.
+/// @param[out]  to  Blob object to copy to.
+void tv_blob_copy(typval_T *const from, typval_T *const to)
+  FUNC_ATTR_NONNULL_ALL
+{
+  assert(from->v_type == VAR_BLOB);
+
+  to->v_type = VAR_BLOB;
+  if (from->vval.v_blob == NULL) {
+    to->vval.v_blob = NULL;
+  } else {
+    tv_blob_alloc_ret(to);
+    const int len = from->vval.v_blob->bv_ga.ga_len;
+
+    if (len > 0) {
+      to->vval.v_blob->bv_ga.ga_data
+          = xmemdup(from->vval.v_blob->bv_ga.ga_data, (size_t)len);
+    }
+    to->vval.v_blob->bv_ga.ga_len = len;
+  }
+}
+
 //{{{3 Clear
 #define TYPVAL_ENCODE_ALLOW_SPECIALS false
 

src/nvim/testdir/test_blob.vim

@@ -154,6 +154,7 @@ func Test_blob_for_loop()
     call assert_equal(i, byte)
     let i += 1
   endfor
+  call assert_equal(4, i)
 
   let blob = 0z00
   call remove(blob, 0)
@@ -161,6 +162,19 @@ func Test_blob_for_loop()
   for byte in blob
     call assert_error('loop over empty blob')
   endfor
+
+  let blob = 0z0001020304
+  let i = 0
+  for byte in blob
+    call assert_equal(i, byte)
+    if i == 1
+      call remove(blob, 0)
+    elseif i == 3
+      call remove(blob, 3)
+    endif
+    let i += 1
+  endfor
+  call assert_equal(5, i)
 endfunc
 
 func Test_blob_concatenate()