Fix glDrawElements emulation with FULL_ES2 (#22294)

This patch fixes an issue related to defining the buffer size before
drawing. It introduces a slight overhead of O(buffersize), but is
probably ok in practice, and is necessary for correctness. (Also,
this only affects FULL_ES2, which is not on by default.)

src/library_webgl.js

@@ -3803,19 +3803,47 @@ for (/**@suppress{duplicate}*/var i = 0; i <= {{{ GL_POOL_TEMP_BUFFERS_SIZE }}};
   glDrawElements: (mode, count, type, indices) => {
 #if FULL_ES2
     var buf;
+    var vertexes = 0;
     if (!GLctx.currentElementArrayBufferBinding) {
       var size = GL.calcBufLength(1, type, 0, count);
       buf = GL.getTempIndexBuffer(size);
       GLctx.bindBuffer(0x8893 /*GL_ELEMENT_ARRAY_BUFFER*/, buf);
       GLctx.bufferSubData(0x8893 /*GL_ELEMENT_ARRAY_BUFFER*/,
                           0,
                           HEAPU8.subarray(indices, indices + size));
+
+      // Calculating vertex count if shader's attribute data is on client side
+      if (count > 0) {
+        for (var i = 0; i < GL.currentContext.maxVertexAttribs; ++i) {
+          var cb = GL.currentContext.clientBuffers[i];
+          if (cb.clientside && cb.enabled) {
+            let arrayClass;
+            switch(type) {
+              case 0x1401 /* GL_UNSIGNED_BYTE */: arrayClass = Uint8Array; break;
+              case 0x1403 /* GL_UNSIGNED_SHORT */: arrayClass = Uint16Array; break;
+#if FULL_ES3
+              case 0x1405 /* GL_UNSIGNED_INT */: arrayClass = Uint32Array; break;
+#endif
+              default:
+                GL.recordError(0x502 /* GL_INVALID_OPERATION */);
+#if GL_ASSERTIONS
+                err('type is not supported in glDrawElements');
+#endif
+                return;
+            }
+
+            vertexes = new arrayClass(HEAPU8.buffer, indices, count).reduce((max, current) => Math.max(max, current)) + 1;
+            break;
+          }
+        }
+      }
+
       // the index is now 0
       indices = 0;
     }
 
     // bind any client-side buffers
-    GL.preDrawHandleClientVertexAttribBindings(count);
+    GL.preDrawHandleClientVertexAttribBindings(vertexes);
 #endif
 
     GLctx.drawElements(mode, count, type, indices);

test/third_party/cubegeom/cubegeom_pre_vao_es.c

@@ -311,12 +311,19 @@ int main(int argc, char *argv[])
     glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, (void*)12);
     glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, (void*) 0);
     glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, (void*)24);
-    glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, (void*)36);
 
     glBindVertexArray(0);
 
     glDeleteVertexArrays(1, &vao);
 
+    // Validate glDrawElements when a client side vertex array is used.
+    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 32, arrayData);
+    glEnableVertexAttribArray(0);
+    glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, 32, arrayData + 16);
+    glEnableVertexAttribArray(1);
+
+    glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, elementData + 18);
+
     // END
 
     SDL_GL_SwapBuffers();