stream header flag + export from mongo

packages/api/src/utility/EnsureStreamHeaderStream.js

@@ -0,0 +1,35 @@
+const stream = require('stream');
+
+class EnsureStreamHeaderStream extends stream.Transform {
+  constructor() {
+    super({ objectMode: true });
+    this.wasHeader = false;
+  }
+  _transform(chunk, encoding, done) {
+    if (!this.wasHeader) {
+      if (chunk.__isDynamicStructure) {
+        // ignore dynamic structure header
+        done();
+        return;
+      }
+
+      if (
+        !chunk.__isStreamHeader &&
+        // TODO remove isArray test
+        !Array.isArray(chunk.columns)
+      ) {
+        this.push({
+          __isStreamHeader: true,
+          __isComputedStructure: true,
+          columns: Object.keys(chunk).map(columnName => ({ columnName })),
+        });
+      }
+
+      this.wasHeader = true;
+    }
+    this.push(chunk);
+    done();
+  }
+}
+
+module.exports = EnsureStreamHeaderStream;