Check expected miniconda.sh hash, redownloads if necessary

This MR introduces a hash check on the expected version of the miniconda installer.

It re-downloads the installer if the hash does not match or if it is not cached.

This solves an issue in which more recent versions of the conda (4.7.x) take very long to bootstrap bob.devtools.

bob/devtools/bootstrap.py

@@ -200,18 +200,40 @@ def merge_conda_cache(cache, prefix, name):
         os.rename(cached_conda_bld, dst)
 
 
-def get_miniconda_sh():
-    """Retrieves the miniconda3 installer for the current system."""
+def ensure_miniconda_sh():
+    """Retrieves the miniconda3 installer for the current system.
 
-    import http.client
+    Checks the hash of the miniconda3 installer against the expected version,
+    if that does not match, erase existing installer and re-downloads new
+    installer.
+    """
 
     server = "repo.continuum.io"  # https
+
+    # WARNING: if you update this version, remember to update hahes below
     path = "/miniconda/Miniconda3-4.6.14-%s-x86_64.sh"
     if platform.system() == "Darwin":
+        md5sum = 'ffa5f0eead5576fb26b7e6902f5eed09'
         path = path % "MacOSX"
     else:
+        md5sum = '718259965f234088d785cad1fbd7de03'
         path = path % "Linux"
 
+    if os.path.exists("miniconda.sh"):
+        logger.info("(check) miniconda.sh md5sum (== %s?)", md5sum)
+        import hashlib
+        actual_md5 = hashlib.md5(open("miniconda.sh", "rb").read()).hexdigest()
+        if actual_md5 == md5sum:
+            logger.info("Re-using cached miniconda3 installer (hash matches)")
+            return
+        else:
+            logger.info("Erasing cached miniconda3 installer (%s NOT match)",
+                actual_md5)
+            os.unlink("miniconda.sh")
+
+    # re-downloads installer
+    import http.client
+
     logger.info("Connecting to https://%s...", server)
     conn = http.client.HTTPSConnection(server)
     conn.request("GET", path)
@@ -238,11 +260,7 @@ def install_miniconda(prefix, name):
     """
 
     logger.info("Installing miniconda in %s...", prefix)
-
-    if not os.path.exists("miniconda.sh"):  # re-downloads installer
-        get_miniconda_sh()
-    else:
-        logger.info("Re-using cached miniconda3 installer")
+    ensure_miniconda_sh()
 
     cached = None
     if os.path.exists(prefix):  # this is the previous cache, move it