Check the Last-Modified header of the remote file

client.go

@@ -187,6 +187,13 @@ func (c *Client) doHTTPRequest(hreq *http.Request, resp *Response) (bool, error)
 		hreq.Header.Set("User-Agent", c.UserAgent)
 	}
 
+	// TODO: set If-Modified-Since header
+	// https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.25
+	// if resp.fi != nil {
+	// 	since := resp.fi.ModTime().Format(http.TimeFormat)
+	// 	hreq.Header.Set("If-Modified-Since", since)
+	// }
+
 	hresp, err := c.HTTPClient.Do(hreq)
 	if err != nil {
 		return false, err

client_test.go

@@ -427,3 +427,44 @@ func TestRemoteTime(t *testing.T) {
 		t.Errorf("expected %v, got %v", time.Unix(lastmod, 0), fi.ModTime())
 	}
 }
+
+// TestTimeModified
+func TestLastModified(t *testing.T) {
+	filename := "./.testLastModified"
+	defer os.Remove(filename)
+
+	oldURL := fmt.Sprintf("%s?lastmod=%d", ts.URL, time.Now().Add(-24*time.Hour).Unix())
+	newURL := fmt.Sprintf("%s?lastmod=%d", ts.URL, time.Now().Unix())
+
+	// download initial file
+	if _, err := Get(filename, oldURL); err != nil {
+		panic(err)
+	}
+
+	// skip unmodifed remote file
+	resp, err := Get(filename, oldURL)
+	if err != nil {
+		panic(err)
+	}
+	if !resp.DidResume || resp.bytesResumed < 1 {
+		t.Errorf("expected client to skip unmodified file")
+	}
+
+	// download modified remote file
+	resp, err = Get(filename, newURL)
+	if err != nil {
+		panic(err)
+	}
+	if resp.DidResume || resp.bytesResumed != 0 {
+		t.Errorf("expected client to download modified file")
+	}
+
+	// skip unmodifed remote file
+	resp, err = Get(filename, oldURL)
+	if err != nil {
+		panic(err)
+	}
+	if !resp.DidResume || resp.bytesResumed < 1 {
+		t.Errorf("expected client to skip unmodified file")
+	}
+}

grab_test.go

@@ -69,7 +69,7 @@ var ts = httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http
 			panic(err)
 		}
 		lastmodt := time.Unix(lastmodi, 0).UTC()
-		lastmod := lastmodt.Format("Mon, 02 Jan 2006 15:04:05") + " GMT"
+		lastmod := lastmodt.Format(http.TimeFormat)
 		w.Header().Set("Last-Modified", lastmod)
 	}
 

request.go

@@ -45,9 +45,10 @@ type Request struct {
 	// exist.
 	NoCreateDirectories bool
 
-	// IgnoreRemoteTime specifies that grab should not attempt to set the
-	// timestamp of the local file to match the remote file.
-	IgnoreRemoteTime bool
+	// NoSetLocalModTime specifies that grab should not attempt to set the
+	// timestamp of the local file to match the remote file. The operating system
+	// will instead set the timestamp to the current system time.
+	NoSetLocalModTime bool
 
 	// Size specifies the expected size of the file transfer if known. If the
 	// server response size does not match, the transfer is cancelled and

response.go

@@ -254,8 +254,6 @@ func (c *Response) readResponse(resp *http.Response) error {
 //
 // If a checksum has been requested, it will be executed on the existing file
 // and an error returned if it fails validation.
-//
-// TODO: check timestamps and/or E-Tags
 func (c *Response) checkExisting() (bool, error) {
 	if c.fi == nil {
 		return false, nil
@@ -265,6 +263,16 @@ func (c *Response) checkExisting() (bool, error) {
 		return true, ErrFileExists
 	}
 
+	// compare last-modified timestamp
+	// BUG: resumed downloads always compute the local partial to be most recent
+	if c.HTTPResponse != nil {
+		// discard error as zero-time is never greater than fi.ModTime
+		lastmod, _ := http.ParseTime(c.HTTPResponse.Header.Get("Last-Modified"))
+		if lastmod.Unix() > c.fi.ModTime().Unix() {
+			return false, nil
+		}
+	}
+
 	// determine expected file size
 	size := c.Request.Size
 	if size == 0 && c.HTTPResponse != nil {
@@ -441,7 +449,7 @@ func (c *Response) copy() error {
 	}
 
 	// set timestamp
-	if !c.Request.IgnoreRemoteTime {
+	if !c.Request.NoSetLocalModTime {
 		if err := setLocalTimestamp(c.HTTPResponse, c.Filename); err != nil {
 			return c.close(err)
 		}

util.go

@@ -15,7 +15,7 @@ func isCanceled(ctx context.Context) error {
 	}
 }
 
-// copyBuffer behaves similarly to io.CopyBuffer except that is checks for
+// copyBuffer behaves similarly to io.CopyBuffer except that it checks for
 // cancelation of the given context.Context.
 func copyBuffer(ctx context.Context, dst io.Writer, src io.Reader, buf []byte) (written int64, err error) {
 	if buf == nil {