Fix T90026: attributes added in editmode are lost on modeswitch

When exiting editmode, customdata [and thus attributes] are copied from
**BMesh** to **Mesh** (in `BM_mesh_bm_to_me`).
When adding attributes, these were always added via
`CustomData_add_layer_named` only.
Instead (if we are in mesh editmode), we need to go through
`BM_data_layer_add_named` to properly update the **BMesh** (see
`update_data_blocks`) so we have stuff available to copy back to
**Mesh** when exiting editmode.
Same is done for removing (also going through BMesh).

This is now done, DomainInfo is updated to point to BMesh (instead of
Mesh) customdata when in editmode.

Maniphest Tasks: T90026

Differential Revision: https://developer.blender.org/D11998

source/blender/blenkernel/intern/attribute.c

@@ -39,6 +39,7 @@
 
 #include "BKE_attribute.h"
 #include "BKE_customdata.h"
+#include "BKE_editmesh.h"
 #include "BKE_hair.h"
 #include "BKE_pointcloud.h"
 #include "BKE_report.h"
@@ -63,6 +64,19 @@ static void get_domains(ID *id, DomainInfo info[ATTR_DOMAIN_NUM])
     }
     case ID_ME: {
       Mesh *mesh = (Mesh *)id;
+      BMEditMesh *em = mesh->edit_mesh;
+      if (em != NULL) {
+        BMesh *bm = em->bm;
+        info[ATTR_DOMAIN_POINT].customdata = &bm->vdata;
+        info[ATTR_DOMAIN_POINT].length = bm->totvert;
+        info[ATTR_DOMAIN_EDGE].customdata = &bm->edata;
+        info[ATTR_DOMAIN_EDGE].length = bm->totedge;
+        info[ATTR_DOMAIN_CORNER].customdata = &bm->ldata;
+        info[ATTR_DOMAIN_CORNER].length = bm->totloop;
+        info[ATTR_DOMAIN_FACE].customdata = &bm->pdata;
+        info[ATTR_DOMAIN_FACE].length = bm->totface;
+      }
+      else {
         info[ATTR_DOMAIN_POINT].customdata = &mesh->vdata;
         info[ATTR_DOMAIN_POINT].length = mesh->totvert;
         info[ATTR_DOMAIN_EDGE].customdata = &mesh->edata;
@@ -71,6 +85,7 @@ static void get_domains(ID *id, DomainInfo info[ATTR_DOMAIN_NUM])
         info[ATTR_DOMAIN_CORNER].length = mesh->totloop;
         info[ATTR_DOMAIN_FACE].customdata = &mesh->pdata;
         info[ATTR_DOMAIN_FACE].length = mesh->totpoly;
+      }
       break;
     }
     case ID_HA: {
@@ -146,7 +161,24 @@ CustomDataLayer *BKE_id_attribute_new(
     return NULL;
   }
 
+  switch (GS(id->name)) {
+    case ID_ME: {
+      Mesh *me = (Mesh *)id;
+      BMEditMesh *em = me->edit_mesh;
+      if (em != NULL) {
+        BM_data_layer_add_named(em->bm, customdata, type, name);
+      }
+      else {
         CustomData_add_layer_named(customdata, type, CD_DEFAULT, NULL, info[domain].length, name);
+      }
+      break;
+    }
+    default: {
+      CustomData_add_layer_named(customdata, type, CD_DEFAULT, NULL, info[domain].length, name);
+      break;
+    }
+  }
+
   const int index = CustomData_get_named_layer_index(customdata, type, name);
   return (index == -1) ? NULL : &(customdata->layers[index]);
 }
@@ -168,8 +200,26 @@ bool BKE_id_attribute_remove(ID *id, CustomDataLayer *layer, ReportList *reports
     return false;
   }
 
+  switch (GS(id->name)) {
+    case ID_ME: {
+      Mesh *me = (Mesh *)id;
+      BMEditMesh *em = me->edit_mesh;
+      if (em != NULL) {
+        BM_data_layer_free(em->bm, customdata, layer->type);
+      }
+      else {
         const int length = BKE_id_attribute_data_length(id, layer);
         CustomData_free_layer(customdata, layer->type, length, index);
+      }
+      break;
+    }
+    default: {
+      const int length = BKE_id_attribute_data_length(id, layer);
+      CustomData_free_layer(customdata, layer->type, length, index);
+      break;
+    }
+  }
+
   return true;
 }
 
@@ -316,7 +366,7 @@ CustomData *BKE_id_attributes_iterator_next_domain(ID *id, CustomDataLayer *laye
 
   for (AttributeDomain domain = 0; domain < ATTR_DOMAIN_NUM; domain++) {
     CustomData *customdata = info[domain].customdata;
-    if (customdata && customdata->layers) {
+    if (customdata && customdata->layers && customdata->totlayer) {
       if (customdata->layers == layers) {
         use_next = true;
       }