avocado.utils.download: Extend get_file API

Add the possibility of using caching in the get_file
API -> If the dst already exists, we can compare its
file hash (SHA1, MD5, among others) with an expected
hash. If the hashes match, don't try to retrieve the
src again, just return.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

avocado/utils/download.py

@@ -25,6 +25,7 @@ import urllib2
 
 from . import aurl
 from . import output
+from . import crypto
 
 
 log = logging.getLogger('avocado.test')
@@ -113,15 +114,7 @@ def url_download_interactive(url, output_file, title='', chunk_size=102400):
     output_file.close()
 
 
-def get_file(src, dst, permissions=None):
-    """
-    Get a file from src and put it in dest, returning dest path.
-
-    :param src: source path or URL. May be local or a remote file.
-    :param dst: destination path.
-    :param permissions: (optional) set access permissions.
-    :return: destination path.
-    """
+def _get_file(src, dst, permissions=None):
     if src == dst:
         return
 
@@ -133,3 +126,55 @@ def get_file(src, dst, permissions=None):
     if permissions:
         os.chmod(dst, permissions)
     return dst
+
+
+def get_file(src, dst, permissions=None, hash_expected=None,
+             hash_algorithm="md5", download_retries=1):
+    """
+    Gets a file from a source location, optionally using caching.
+
+    If no hash_expected is provided, simply download the file. Else,
+    keep trying to download the file until download_failures exceeds
+    download_retries or the hashes match.
+
+    If the hashes match, return dst. If download_failures exceeds
+    download_retries, raise an EnvironmentError.
+
+    :param src: source path or URL. May be local or a remote file.
+    :param dst: destination path.
+    :param permissions: (optional) set access permissions.
+    :param hash_expected: Hash string that we expect the file downloaded to
+            have.
+    :param hash_algorithm: Algorithm used to calculate the hash string
+            (md5, sha1).
+    :param download_retries: Number of times we are going to retry a failed
+            download.
+    :raise: EnvironmentError.
+    :return: destination path.
+    """
+    def _verify_hash(filename):
+        if os.path.isfile(filename):
+            return crypto.hash_file(filename, algorithm=hash_algorithm)
+        return None
+
+    if hash_expected is None:
+        return _get_file(src, dst, permissions)
+
+    download_failures = 0
+    hash_file = _verify_hash(dst)
+
+    while not hash_file == hash_expected:
+        hash_file = _verify_hash(_get_file(src, dst, permissions))
+        if hash_file != hash_expected:
+            log.error("It seems that dst %s is corrupted" % dst)
+            download_failures += 1
+        if download_failures > download_retries:
+            raise EnvironmentError("Failed to retrieve %s. "
+                                   "Possible reasons - Network connectivity "
+                                   "problems or incorrect hash_expected "
+                                   "provided -> '%s'" %
+                                   (src, hash_expected))
+        else:
+            log.error("Retrying download of src %s", src)
+
+    return dst