s3 uploads: Refactor to access objects via `get_public_upload_url`.

Our current logic only allows S3 block storage providers whose
upload URL matches with the format used by AWS. This also allows
other styles such as the "virtual host" format used by Oracle cloud.

Fixes #17762.

zerver/lib/upload.py

@@ -380,15 +380,37 @@ def get_signed_upload_url(path: str) -> str:
 class S3UploadBackend(ZulipUploadBackend):
     def __init__(self) -> None:
         self.session = boto3.Session(settings.S3_KEY, settings.S3_SECRET_KEY)
-
         self.avatar_bucket = get_bucket(settings.S3_AVATAR_BUCKET, self.session)
-        network_location = urllib.parse.urlparse(
-            self.avatar_bucket.meta.client.meta.endpoint_url
-        ).netloc
-        self.avatar_bucket_url = f"https://{self.avatar_bucket.name}.{network_location}"
-
         self.uploads_bucket = get_bucket(settings.S3_AUTH_UPLOADS_BUCKET, self.session)
 
+    def get_public_upload_url(
+        self,
+        key: str,
+    ) -> str:
+        # Return the public URL for a key in the S3 Avatar bucket.
+        # For Amazon S3 itself, this will return the following:
+        #     f"https://{self.avatar_bucket.name}.{network_location}/{key}"
+        #
+        # However, we need this function to properly handle S3 style
+        # file upload backends that Zulip supports, which can have a
+        # different URL format. Configuring no signature and providing
+        # no access key makes `generate_presigned_url` just return the
+        # normal public URL for a key.
+        config = Config(signature_version=botocore.UNSIGNED)
+        return self.session.client(
+            "s3",
+            region_name=settings.S3_REGION,
+            endpoint_url=settings.S3_ENDPOINT_URL,
+            config=config,
+        ).generate_presigned_url(
+            ClientMethod="get_object",
+            Params={
+                "Bucket": self.avatar_bucket.name,
+                "Key": key,
+            },
+            ExpiresIn=0,
+        )
+
     def delete_file_from_s3(self, path_id: str, bucket: ServiceResource) -> bool:
         key = bucket.Object(path_id)
 
@@ -512,12 +534,14 @@ class S3UploadBackend(ZulipUploadBackend):
 
     def get_avatar_url(self, hash_key: str, medium: bool = False) -> str:
         medium_suffix = "-medium.png" if medium else ""
+        public_url = self.get_public_upload_url(f"{hash_key}{medium_suffix}")
+
         # ?x=x allows templates to append additional parameters with &s
-        return f"{self.avatar_bucket_url}/{hash_key}{medium_suffix}?x=x"
+        return public_url + "?x=x"
 
     def get_export_tarball_url(self, realm: Realm, export_path: str) -> str:
         # export_path has a leading /
-        return f"{self.avatar_bucket_url}{export_path}"
+        return self.get_public_upload_url(export_path[1:])
 
     def realm_avatar_and_logo_path(self, realm: Realm) -> str:
         return os.path.join(str(realm.id), "realm")
@@ -547,8 +571,8 @@ class S3UploadBackend(ZulipUploadBackend):
         # that users use gravatar.)
 
     def get_realm_icon_url(self, realm_id: int, version: int) -> str:
-        # ?x=x allows templates to append additional parameters with &s
-        return f"{self.avatar_bucket_url}/{realm_id}/realm/icon.png?version={version}"
+        public_url = self.get_public_upload_url(f"{realm_id}/realm/icon.png")
+        return public_url + f"?version={version}"
 
     def upload_realm_logo_image(
         self, logo_file: File, user_profile: UserProfile, night: bool
@@ -581,12 +605,12 @@ class S3UploadBackend(ZulipUploadBackend):
         # that users use gravatar.)
 
     def get_realm_logo_url(self, realm_id: int, version: int, night: bool) -> str:
-        # ?x=x allows templates to append additional parameters with &s
         if not night:
             file_name = "logo.png"
         else:
             file_name = "night_logo.png"
-        return f"{self.avatar_bucket_url}/{realm_id}/realm/{file_name}?version={version}"
+        public_url = self.get_public_upload_url(f"{realm_id}/realm/{file_name}")
+        return public_url + f"?version={version}"
 
     def ensure_avatar_image(self, user_profile: UserProfile, is_medium: bool = False) -> None:
         # BUG: The else case should be user_avatar_path(user_profile) + ".png".
@@ -640,7 +664,7 @@ class S3UploadBackend(ZulipUploadBackend):
         emoji_path = RealmEmoji.PATH_ID_TEMPLATE.format(
             realm_id=realm_id, emoji_file_name=emoji_file_name
         )
-        return f"{self.avatar_bucket_url}/{emoji_path}"
+        return self.get_public_upload_url(emoji_path)
 
     def upload_export_tarball(
         self,