rustup-dist: Use Download notifications to track install

People have requested some indication of progress for long-running
install steps.  This commit re-uses the download tracker logic to
provide a progress bar (the length is the compressed component
tarball but should be good enough) to provide such feedback.

Fixes: rust-lang#1458, rust-lang#1557

src/rustup-dist/src/manifestation.rs

@@ -74,6 +74,55 @@ pub enum UpdateStatus {
     Unchanged,
 }
 
+struct FileReaderWithProgress<'a> {
+    fh: std::fs::File,
+    notify_handler: &'a Fn(Notification),
+    sent_start: bool,
+    nbytes: u64,
+    flen: u64,
+}
+
+impl<'a> FileReaderWithProgress<'a> {
+    fn new_file(path: &Path, notify_handler: &'a Fn(Notification)) -> Result<Self> {
+        Ok(FileReaderWithProgress {
+            fh: std::fs::File::open(path)?,
+            notify_handler,
+            sent_start: false,
+            nbytes: 0,
+            flen: 0,
+        })
+    }
+}
+
+impl<'a> std::io::Read for FileReaderWithProgress<'a> {
+    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
+        if !self.sent_start {
+            // Send the start notifications
+            let flen = self.fh.metadata()?.len();
+            self.flen = flen;
+            (self.notify_handler)(Notification::Utils(
+                rustup_utils::Notification::DownloadContentLengthReceived(flen),
+            ));
+        }
+        match self.fh.read(buf) {
+            Ok(nbytes) => {
+                self.nbytes += nbytes as u64;
+                if (nbytes == 0) || (self.flen == self.nbytes) {
+                    (self.notify_handler)(Notification::Utils(
+                        rustup_utils::Notification::DownloadFinished,
+                    ));
+                } else {
+                    (self.notify_handler)(Notification::Utils(
+                        rustup_utils::Notification::DownloadDataReceived(&buf[0..nbytes]),
+                    ));
+                }
+                Ok(nbytes)
+            }
+            Err(e) => Err(e),
+        }
+    }
+}
+
 impl Manifestation {
     /// Open the install prefix for updates from a distribution
     /// channel.  The install prefix directory does not need to exist;
@@ -200,13 +249,14 @@ impl Manifestation {
 
             let gz;
             let xz;
+            let reader = FileReaderWithProgress::new_file(&installer_file, notify_handler)?;
             let package: &Package = match format {
                 Format::Gz => {
-                    gz = TarGzPackage::new_file(&installer_file, temp_cfg)?;
+                    gz = TarGzPackage::new(reader, temp_cfg)?;
                     &gz
                 }
                 Format::Xz => {
-                    xz = TarXzPackage::new_file(&installer_file, temp_cfg)?;
+                    xz = TarXzPackage::new(reader, temp_cfg)?;
                     &xz
                 }
             };