[bug] In Esti check that mtimes match on lakeFS and underlying store

Azure and GCS do _not_ use the time of create-MPU.  The important thing is
that the two share the same time.

Anything else confuses not only people but also programs.  For instance
DataBricks with presigned URLs can get confused by getting one time from
lakeFS and another from underlying storage.

esti/multipart_test.go

@@ -24,11 +24,12 @@ const (
 )
 
 func TestMultipartUpload(t *testing.T) {
-	// timeSlippage is a bound on the time difference between the local server and the S3
-	// server.  It is used to verify that the "Last-Modified" time is actually close to the
-	// CreateMultipartUpload time.  BUT S3 provides this time only at a 1-second resolution.
-	// So a 1-second difference is possible.
-	const timeSlippage = time.Second
+	// timeResolution is a duration greater than the timestamp resolution of the backing
+	// store.  Multipart object on S3 is the time of create-MPU, waiting before completion
+	// ensures that lakeFS did not use the current time.  For other blockstores MPU
+	// completion time is used, meaning it will be hard to detect if the underlying and
+	// lakeFS objects share the same time.
+	const timeResolution = time.Second
 
 	ctx, logger, repo := setupTest(t)
 	defer tearDownTest(repo)
@@ -43,8 +44,6 @@ func TestMultipartUpload(t *testing.T) {
 	require.NoError(t, err, "failed to create multipart upload")
 	logger.Info("Created multipart upload request")
 
-	uploadTime := time.Now()
-
 	parts := make([][]byte, multipartNumberOfParts)
 	var partsConcat []byte
 	for i := 0; i < multipartNumberOfParts; i++ {
@@ -54,9 +53,9 @@ func TestMultipartUpload(t *testing.T) {
 
 	completedParts := uploadMultipartParts(t, ctx, logger, resp, parts, 0)
 
-	// Object should have Last-Modified time at around time of MPU creation.  Server times
-	// after this Sleep will be more than timeSlippage away from uploadTime.
-	time.Sleep(2 * timeSlippage)
+	// On S3 the object should have Last-Modified time at around time of MPU creation.
+	// Ensure lakeFS fails the test if it fakes it by using the current time.
+	time.Sleep(2 * timeResolution)
 
 	completeResponse, err := uploadMultipartComplete(ctx, svc, resp, completedParts)
 	require.NoError(t, err, "failed to complete multipart upload")
@@ -70,13 +69,27 @@ func TestMultipartUpload(t *testing.T) {
 		t.Fatalf("uploaded object did not match")
 	}
 
-	statResp, err := client.StatObjectWithResponse(ctx, repo, mainBranch, &apigen.StatObjectParams{Path: file})
+	statResp, err := client.StatObjectWithResponse(ctx, repo, mainBranch, &apigen.StatObjectParams{Path: file, Presign: aws.Bool(true)})
 	require.NoError(t, err, "failed to get object")
-	require.Equal(t, http.StatusOK, getResp.StatusCode())
-	lastModified := time.Unix(statResp.JSON200.Mtime, 0)
-	require.Less(t, lastModified.Sub(uploadTime).Abs(), timeSlippage,
-		"(remote) last modified time %s too far away from (local) upload time %s",
-		lastModified, uploadTime)
+	require.Equal(t, http.StatusOK, getResp.StatusCode(), getResp.Status())
+
+	// Get last-modified from the underlying store.
+
+	presignedGetURL := statResp.JSON200.PhysicalAddress
+	res, err := http.Get(presignedGetURL)
+	require.NoError(t, err, "GET underlying")
+	// The presigned URL is usable only for GET, but we don't actually care about its body.
+	_ = res.Body.Close()
+	require.Equal(t, http.StatusOK, res.StatusCode, "%s: %s", presignedGetURL, res.Status)
+	lastModifiedString := res.Header.Get("Last-Modified")
+	underlyingLastModified, err := time.Parse(time.RFC1123, lastModifiedString)
+	require.NoError(t, err, "Last-Modified %s", lastModifiedString)
+	// Last-Modified header includes a timezone, which is typically "GMT" on AWS.  Now GMT
+	// _is equal to_ UTC!.  But Go is nothing if not cautious, and considers UTC and GMT to
+	// be different timezones.  So cannot compare with "==" and must use time.Time.Equal.
+	lakeFSMTime := time.Unix(statResp.JSON200.Mtime, 0)
+	require.True(t, lakeFSMTime == underlyingLastModified,
+		"lakeFS mtime %s should be same as on underlying object %s", lakeFSMTime, underlyingLastModified)
 }
 
 func TestMultipartUploadAbort(t *testing.T) {