Auto merge of #34644 - infinity0:master, r=alexcrichton

Avoid redundant downloads when bootstrapping

If the local file is available, then verify it against the hash we just
downloaded, and if it matches then we don't need to download it again.

src/bootstrap/bootstrap.py

@@ -31,8 +31,16 @@ def get(url, path, verbose=False):
 
     try:
         download(sha_path, sha_url, verbose)
+        if os.path.exists(path):
+            if verify(path, sha_path, False):
+                print("using already-download file " + path)
+                return
+            else:
+                print("ignoring already-download file " + path + " due to failed verification")
+                os.unlink(path)
         download(temp_path, url, verbose)
-        verify(temp_path, sha_path, verbose)
+        if not verify(temp_path, sha_path, True):
+            raise RuntimeError("failed verification")
         print("moving {} to {}".format(temp_path, path))
         shutil.move(temp_path, path)
     finally:
@@ -64,13 +72,12 @@ def verify(path, sha_path, verbose):
         found = hashlib.sha256(f.read()).hexdigest()
     with open(sha_path, "r") as f:
         expected, _ = f.readline().split()
-    if found != expected:
-        err = ("invalid checksum:\n"
+    verified = found == expected
+    if not verified and verbose:
+        print("invalid checksum:\n"
                "    found:    {}\n"
                "    expected: {}".format(found, expected))
-        if verbose:
-            raise RuntimeError(err)
-        sys.exit(err)
+    return verified
 
 
 def unpack(tarball, dst, verbose=False, match=None):

src/etc/get-stage0.py

@@ -31,8 +31,6 @@ def main(triple):
     filename = 'rustc-{}-{}.tar.gz'.format(channel, triple)
     url = 'https://static.rust-lang.org/dist/{}/{}'.format(date, filename)
     dst = dl_dir + '/' + filename
-    if os.path.exists(dst):
-        os.unlink(dst)
     bootstrap.get(url, dst)
 
     stage0_dst = triple + '/stage0'