fix(prometheus): Change aligment of range queries (#16110)

- future alignment cause issues with rate charts and the display of last
values
- this change modifies the alignment to use the last available aligned
end date and no longer possibly requests data from the future

public/app/plugins/datasource/prometheus/datasource.ts

@@ -224,7 +224,8 @@ export class PrometheusDatasource implements DataSourceApi<PromQuery> {
     query.expr = this.templateSrv.replace(expr, scopedVars, this.interpolateQueryExpr);
     query.requestId = options.panelId + target.refId;
 
-    // Align query interval with step
+    // Align query interval with step to allow query caching and to ensure
+    // that about-same-time query results look the same.
     const adjusted = alignRange(start, end, query.step);
     query.start = adjusted.start;
     query.end = adjusted.end;
@@ -497,8 +498,15 @@ export class PrometheusDatasource implements DataSourceApi<PromQuery> {
   }
 }
 
-export function alignRange(start, end, step) {
-  const alignedEnd = Math.ceil(end / step) * step;
+/**
+ * Align query range to step.
+ * Rounds start and end down to a multiple of step.
+ * @param start Timestamp marking the beginning of the range.
+ * @param end Timestamp marking the end of the range.
+ * @param step Interval to align start and end with.
+ */
+export function alignRange(start: number, end: number, step: number): { end: number; start: number } {
+  const alignedEnd = Math.floor(end / step) * step;
   const alignedStart = Math.floor(start / step) * step;
   return {
     end: alignedEnd,

public/app/plugins/datasource/prometheus/specs/datasource.test.ts

@@ -206,12 +206,12 @@ describe('PrometheusDatasource', () => {
     it('does align intervals that are a multiple of steps', () => {
       const range = alignRange(1, 4, 3);
       expect(range.start).toEqual(0);
-      expect(range.end).toEqual(6);
+      expect(range.end).toEqual(3);
     });
     it('does align intervals that are not a multiple of steps', () => {
       const range = alignRange(1, 5, 3);
       expect(range.start).toEqual(0);
-      expect(range.end).toEqual(6);
+      expect(range.end).toEqual(3);
     });
   });
 
@@ -360,7 +360,7 @@ describe('PrometheusDatasource', () => {
     };
     // Interval alignment with step
     const urlExpected =
-      'proxied/api/v1/query_range?query=' + encodeURIComponent('test{job="testjob"}') + '&start=60&end=240&step=60';
+      'proxied/api/v1/query_range?query=' + encodeURIComponent('test{job="testjob"}') + '&start=60&end=180&step=60';
 
     beforeEach(async () => {
       const response = {
@@ -788,7 +788,7 @@ describe('PrometheusDatasource', () => {
         interval: '5s',
       };
       // times get rounded up to interval
-      const urlExpected = 'proxied/api/v1/query_range?query=test&start=50&end=450&step=50';
+      const urlExpected = 'proxied/api/v1/query_range?query=test&start=50&end=400&step=50';
       backendSrv.datasourceRequest = jest.fn(() => Promise.resolve(response));
       ctx.ds = new PrometheusDatasource(instanceSettings, q, backendSrv as any, templateSrv, timeSrv);
       await ctx.ds.query(query);
@@ -831,7 +831,7 @@ describe('PrometheusDatasource', () => {
         interval: '10s',
       };
       // times get aligned to interval
-      const urlExpected = 'proxied/api/v1/query_range?query=test' + '&start=0&end=500&step=100';
+      const urlExpected = 'proxied/api/v1/query_range?query=test' + '&start=0&end=400&step=100';
       backendSrv.datasourceRequest = jest.fn(() => Promise.resolve(response));
       ctx.ds = new PrometheusDatasource(instanceSettings, q, backendSrv as any, templateSrv, timeSrv);
       await ctx.ds.query(query);
@@ -996,7 +996,7 @@ describe('PrometheusDatasource', () => {
       const urlExpected =
         'proxied/api/v1/query_range?query=' +
         encodeURIComponent('rate(test[$__interval])') +
-        '&start=0&end=500&step=100';
+        '&start=0&end=400&step=100';
       backendSrv.datasourceRequest = jest.fn(() => Promise.resolve(response));
       templateSrv.replace = jest.fn(str => str);
       ctx.ds = new PrometheusDatasource(instanceSettings, q, backendSrv as any, templateSrv, timeSrv);
@@ -1041,7 +1041,7 @@ describe('PrometheusDatasource', () => {
       const urlExpected =
         'proxied/api/v1/query_range?query=' +
         encodeURIComponent('rate(test[$__interval])') +
-        '&start=50&end=450&step=50';
+        '&start=50&end=400&step=50';
 
       templateSrv.replace = jest.fn(str => str);
       backendSrv.datasourceRequest = jest.fn(() => Promise.resolve(response));
@@ -1166,7 +1166,7 @@ describe('PrometheusDatasource for POST', () => {
     const dataExpected = {
       query: 'test{job="testjob"}',
       start: 1 * 60,
-      end: 3 * 60,
+      end: 2 * 60,
       step: 60,
     };
     const query = {