feat: allow public buckets, i.e. without auth

It turns out that public buckets, so ones that don't require any auth, should
not have any authentication headers at all. So allowing the `get_credentials`
parameter to be `None`, in which case signing of requests and passing
authentication headers is skipped.

As discussed at #94

README.md

@@ -175,6 +175,26 @@ with \
         print(row)
 ```
 
+
+### Public Buckets
+
+For public buckets where credentials should not be passed, pass `None` as the `get_credentials` parameter.
+
+```python
+query_my_db = partial(sqlite_s3_query,
+    url='https://my-public-bucket.s3.eu-west-2.amazonaws.com/my-db.sqlite',
+    get_credentials=None,
+)
+
+with \
+        query_my_db() as query, \
+        query('SELECT * FROM my_table_2 WHERE my_col = ?', params=('my-value',)) as (columns, rows):
+
+    for row in rows:
+        print(row)
+```
+
+
 ### HTTP Client
 
 The HTTP client can be changed by overriding the the default `get_http_client` parameter, which is shown below.

sqlite_s3_query.py

@@ -74,6 +74,23 @@ def sqlite_s3_query_multi(url, get_credentials=lambda now: (
     local = threading.local()
     local.pending_exception = None
 
+    def get_request_headers_for_private_buckets(method, params, headers, now):
+        region, access_key_id, secret_access_key, session_token = get_credentials(now)
+        to_auth_headers = headers + (
+            (('x-amz-security-token', session_token),) if session_token is not None else \
+            ()
+        )
+        return aws_sigv4_headers(
+            now, access_key_id, secret_access_key, region, method, to_auth_headers, params,
+        )
+
+    def get_request_headers_for_public_buckets(_, __, headers, ___):
+        return headers
+
+    get_request_headers = \
+        get_request_headers_for_private_buckets if get_credentials is not None else \
+        get_request_headers_for_public_buckets
+
     def set_pending_exception(exception):
         local.pending_exception = exception
 
@@ -98,14 +115,7 @@ def run_with_db(db, func, *args):
     @contextmanager
     def make_auth_request(http_client, method, params, headers):
         now = datetime.utcnow()
-        region, access_key_id, secret_access_key, session_token = get_credentials(now)
-        to_auth_headers = headers + (
-            (('x-amz-security-token', session_token),) if session_token is not None else \
-            ()
-        )
-        request_headers = aws_sigv4_headers(
-            now, access_key_id, secret_access_key, region, method, to_auth_headers, params,
-        )
+        request_headers = get_request_headers(method, params, headers, now)
         url = f'{scheme}://{netloc}{path}'
         with http_client.stream(method, url, params=params, headers=request_headers) as response:
             response.raise_for_status()

test.py

@@ -148,6 +148,27 @@ def test_select_with_named_params(self):
 
         self.assertEqual(rows, [(500,)])
 
+    def test_select_with_named_params_public_bucket(self):
+        create_bucket('my-public-bucket')
+        disable_auth('my-public-bucket')
+        with get_db([
+            ("CREATE TABLE my_table (my_col_a text, my_col_b text);", ())
+        ] + [
+            ("INSERT INTO my_table VALUES " + ','.join(["('some-text-a', 'some-text-b')"] * 500), ()),
+            ("INSERT INTO my_table VALUES " + ','.join(["('some-text-c', 'some-text-d')"] * 100), ()),
+        ]) as db:
+            put_object_with_versioning('my-public-bucket', 'my.db', db)
+
+        with sqlite_s3_query(
+                'http://localhost:9000/my-public-bucket/my.db',
+                get_credentials=None,
+                get_libsqlite3=get_libsqlite3
+        ) as query:
+            with query('SELECT COUNT(*) FROM my_table WHERE my_col_a = :first', named_params=((':first', 'some-text-a'),)) as (columns, rows):
+                rows = list(rows)
+
+        self.assertEqual(rows, [(500,)])
+
     def test_select_large(self):
         empty = (bytes(4050),)
 
@@ -840,6 +861,36 @@ def enable_versioning(bucket):
     response = httpx.put(url, content=content, headers=headers)
     response.raise_for_status()
 
+def disable_auth(bucket):
+    content = f'''
+        {{
+            "Version": "2012-10-17",
+            "Statement": [
+                {{
+                    "Sid": "Stmt1405592139000",
+                    "Effect": "Allow",
+                    "Principal": "*",
+                    "Action": [
+                        "s3:GetObject",
+                        "s3:GetObjectVersion"
+                    ],
+                    "Resource": [
+                        "arn:aws:s3:::{bucket}/*"
+                    ]
+                }}
+            ]
+        }}
+    '''.encode()
+    url = f'http://127.0.0.1:9000/{bucket}/?policy'
+    body_hash = hashlib.sha256(content).hexdigest()
+    parsed_url = urllib.parse.urlsplit(url)
+
+    headers = aws_sigv4_headers(
+        'AKIAIOSFODNN7EXAMPLE', 'wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY',
+        (), 's3', 'us-east-1', parsed_url.netloc, 'PUT', parsed_url.path, (('policy', ''),), body_hash,
+    )
+    response = httpx.put(url, content=content, headers=headers)
+    response.raise_for_status()
 
 def aws_sigv4_headers(access_key_id, secret_access_key, pre_auth_headers,
                       service, region, host, method, path, params, body_hash):