tuftool: Add download_root module

This change adds a `download_root` module, which abstracts the logic of
downloading `root.json` out of `download` (the subcommand).  It also
makes the HTTP request a bit more robust by handling the initial request
error as well as a bad response.  Additionally, it avoids creating the
file until after the request has succeeded to avoid creating cruft on
the user's system.

tuftool/src/download_root.rs

@@ -0,0 +1,51 @@
+// Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//! The `download_root` module owns the logic for downloading a given version of `root.json`.
+
+use crate::error::{self, Result};
+use snafu::ResultExt;
+use std::fs::File;
+use std::num::NonZeroU64;
+use std::path::{Path, PathBuf};
+use url::Url;
+
+/// Download the given version of `root.json`
+/// This is an unsafe operation, and doesn't establish trust. It should only be used for testing!
+pub(crate) fn download_root<P>(
+    metadata_base_url: &Url,
+    version: NonZeroU64,
+    outdir: P,
+) -> Result<PathBuf>
+where
+    P: AsRef<Path>,
+{
+    let name = format!("{}.root.json", version);
+
+    let path = outdir.as_ref().join(&name);
+    let url = metadata_base_url.join(&name).context(error::UrlParse {
+        url: format!("{}/{}", metadata_base_url.as_str(), name),
+    })?;
+    root_warning(&path);
+
+    let mut root_request = reqwest::blocking::get(url.as_str())
+        .context(error::ReqwestGet)?
+        .error_for_status()
+        .context(error::BadResponse { url })?;
+
+    let mut f = File::create(&path).context(error::OpenFile { path: &path })?;
+    root_request.copy_to(&mut f).context(error::ReqwestCopy)?;
+
+    Ok(path)
+}
+
+/// Print a very noticeable warning message about the unsafe nature of downloading `root.json`
+/// without verification
+fn root_warning<P: AsRef<Path>>(path: P) {
+    #[rustfmt::skip]
+    eprintln!("\
+=================================================================
+WARNING: Downloading root.json to {}
+This is unsafe and will not establish trust, use only for testing
+=================================================================",
+              path.as_ref().display());
+}

tuftool/src/error.rs

@@ -243,6 +243,13 @@ pub(crate) enum Error {
         backtrace: Backtrace,
     },
 
+    #[snafu(display("Response '{}' from '{}': {}", get_status_code(source), url, source))]
+    BadResponse {
+        url: String,
+        source: reqwest::Error,
+        backtrace: Backtrace,
+    },
+
     #[snafu(display("Failed to sign repository: {}", source))]
     SignRepo {
         source: tough::error::Error,
@@ -357,3 +364,12 @@ pub(crate) enum Error {
         backtrace: Backtrace,
     },
 }
+
+// Extracts the status code from a reqwest::Error and converts it to a string to be displayed
+fn get_status_code(source: &reqwest::Error) -> String {
+    source
+        .status()
+        .as_ref()
+        .map_or("Unknown", |i| i.as_str())
+        .to_string()
+}