Add test for check_exists following redirects (#455)

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <[email protected]>

schema_salad/fetcher.py

@@ -101,7 +101,9 @@ def check_exists(self, url: str) -> bool:
         split = urllib.parse.urlsplit(url)
         scheme, path = split.scheme, split.path
 
-        if scheme in ["http", "https"] and self.session is not None:
+        if scheme in ["http", "https"]:
+            if self.session is None:
+                raise ValidationException(f"Can't check {scheme} URL, session is None")
             try:
                 resp = self.session.head(url, allow_redirects=True)
                 resp.raise_for_status()
@@ -113,7 +115,7 @@ def check_exists(self, url: str) -> bool:
             return os.path.exists(urllib.request.url2pathname(str(path)))
         if scheme == "mailto":
             return True
-        raise ValidationException(f"Unsupported scheme in url: {url}")
+        raise ValidationException(f"Unsupported scheme '{scheme}' in url: {url}")
 
     def urljoin(self, base_url: str, url: str) -> str:
         if url.startswith("_:"):

schema_salad/tests/test_ref_resolver.py

@@ -249,3 +249,13 @@ def aa2(item: Union[CommentedMap, CommentedSeq]) -> bool:
     with open(path) as f:
         content = f.read()
         assert {"foo": "bar", "baz": content, "quux": content} == r3
+
+
+def test_check_exists_follows_redirects() -> None:
+    fetcher = DefaultFetcher({}, Session())
+    # This tests that the redirect from http to https doesn't cause a
+    # false positive, this URL doesn't exist and it should return
+    # False because if it follows the redirect it'll get a 404.  Tests
+    # for previous bug where allow_redirects wasn't set and as a
+    # result it would return True even though the result was 3xx.
+    assert not fetcher.check_exists("http://commonwl.org/does/not/exist")