Templating: use default datasource when missing (#21495)

public/app/features/templating/datasource_variable.ts

@@ -3,6 +3,7 @@ import { stringToJsRegex } from '@grafana/data';
 import { VariableSrv } from './variable_srv';
 import { TemplateSrv } from './template_srv';
 import { DatasourceSrv } from '../plugins/datasource_srv';
+import { config } from '@grafana/runtime';
 
 export class DatasourceVariable implements VariableActions {
   regex: any;
@@ -84,7 +85,8 @@ export class DatasourceVariable implements VariableActions {
     if (this.includeAll) {
       this.addAllOption();
     }
-    return this.variableSrv.validateVariableSelectionState(this);
+    const { defaultDatasource } = config.bootData.settings;
+    return this.variableSrv.validateVariableSelectionState(this, defaultDatasource);
   }
 
   addAllOption() {

public/app/features/templating/variable_srv.ts

@@ -181,7 +181,7 @@ export class VariableSrv {
     return selected;
   }
 
-  validateVariableSelectionState(variable: any) {
+  validateVariableSelectionState(variable: any, defaultValue?: string) {
     if (!variable.current) {
       variable.current = {};
     }
@@ -205,17 +205,33 @@ export class VariableSrv {
 
       return variable.setValue(selected);
     } else {
-      const currentOption: any = _.find(variable.options, {
+      let option: any = undefined;
+
+      // 1. find the current value
+      option = _.find(variable.options, {
         text: variable.current.text,
       });
-      if (currentOption) {
-        return variable.setValue(currentOption);
-      } else {
-        if (!variable.options.length) {
-          return Promise.resolve();
+      if (option) {
+        return variable.setValue(option);
+      }
+
+      // 2. find the default value
+      if (defaultValue) {
+        option = _.find(variable.options, {
+          text: defaultValue,
+        });
+        if (option) {
+          return variable.setValue(option);
         }
+      }
+
+      // 3. use the first value
+      if (variable.options) {
         return variable.setValue(variable.options[0]);
       }
+
+      // 4... give up
+      return Promise.resolve();
     }
   }