Comment out most options in the generated config. (#4863)

Make it so that most options in the config are optional, and commented out in
the generated config.

The reasons this is a good thing are as follows:

* If we decide that we should change the default for an option, we can do so,
  and only those admins that have deliberately chosen to override that option
  will be stuck on the old setting.

* It moves us towards a point where we can get rid of the super-surprising
  feature of synapse where the default settings for the config come from the
  generated yaml.

* It makes setting up a test config for unit testing an order of magnitude
  easier (see forthcoming PR).

* It makes the generated config more consistent, and hopefully easier for users
  to understand.

synapse/config/repository.py

@@ -19,6 +19,36 @@ from synapse.util.module_loader import load_module
 
 from ._base import Config, ConfigError
 
+DEFAULT_THUMBNAIL_SIZES = [
+    {
+        "width": 32,
+        "height": 32,
+        "method": "crop",
+    }, {
+        "width": 96,
+        "height": 96,
+        "method": "crop",
+    }, {
+        "width": 320,
+        "height": 240,
+        "method": "scale",
+    }, {
+        "width": 640,
+        "height": 480,
+        "method": "scale",
+    }, {
+        "width": 800,
+        "height": 600,
+        "method": "scale"
+    },
+]
+
+THUMBNAIL_SIZE_YAML = """\
+        #  - width: %(width)i
+        #    height: %(height)i
+        #    method: %(method)s
+"""
+
 MISSING_NETADDR = (
     "Missing netaddr library. This is required for URL preview API."
 )
@@ -77,9 +107,9 @@ def parse_thumbnail_requirements(thumbnail_sizes):
 
 class ContentRepositoryConfig(Config):
     def read_config(self, config):
-        self.max_upload_size = self.parse_size(config["max_upload_size"])
-        self.max_image_pixels = self.parse_size(config["max_image_pixels"])
-        self.max_spider_size = self.parse_size(config["max_spider_size"])
+        self.max_upload_size = self.parse_size(config.get("max_upload_size", "10M"))
+        self.max_image_pixels = self.parse_size(config.get("max_image_pixels", "32M"))
+        self.max_spider_size = self.parse_size(config.get("max_spider_size", "10M"))
 
         self.media_store_path = self.ensure_directory(config["media_store_path"])
 
@@ -139,9 +169,9 @@ class ContentRepositoryConfig(Config):
             )
 
         self.uploads_path = self.ensure_directory(config["uploads_path"])
-        self.dynamic_thumbnails = config["dynamic_thumbnails"]
+        self.dynamic_thumbnails = config.get("dynamic_thumbnails", False)
         self.thumbnail_requirements = parse_thumbnail_requirements(
-            config["thumbnail_sizes"]
+            config.get("thumbnail_sizes", DEFAULT_THUMBNAIL_SIZES),
         )
         self.url_preview_enabled = config.get("url_preview_enabled", False)
         if self.url_preview_enabled:
@@ -178,6 +208,13 @@ class ContentRepositoryConfig(Config):
     def default_config(self, data_dir_path, **kwargs):
         media_store = os.path.join(data_dir_path, "media_store")
         uploads_path = os.path.join(data_dir_path, "uploads")
+
+        formatted_thumbnail_sizes = "".join(
+            THUMBNAIL_SIZE_YAML % s for s in DEFAULT_THUMBNAIL_SIZES
+        )
+        # strip final NL
+        formatted_thumbnail_sizes = formatted_thumbnail_sizes[:-1]
+
         return r"""
         # Directory where uploaded images and attachments are stored.
         #
@@ -204,11 +241,11 @@ class ContentRepositoryConfig(Config):
 
         # The largest allowed upload size in bytes
         #
-        max_upload_size: "10M"
+        #max_upload_size: 10M
 
         # Maximum number of pixels that will be thumbnailed
         #
-        max_image_pixels: "32M"
+        #max_image_pixels: 32M
 
         # Whether to generate new thumbnails on the fly to precisely match
         # the resolution requested by the client. If true then whenever
@@ -216,32 +253,18 @@ class ContentRepositoryConfig(Config):
         # generate a new thumbnail. If false the server will pick a thumbnail
         # from a precalculated list.
         #
-        dynamic_thumbnails: false
+        #dynamic_thumbnails: false
 
         # List of thumbnails to precalculate when an image is uploaded.
         #
-        thumbnail_sizes:
-        - width: 32
-          height: 32
-          method: crop
-        - width: 96
-          height: 96
-          method: crop
-        - width: 320
-          height: 240
-          method: scale
-        - width: 640
-          height: 480
-          method: scale
-        - width: 800
-          height: 600
-          method: scale
+        #thumbnail_sizes:
+%(formatted_thumbnail_sizes)s
 
         # Is the preview URL API enabled?  If enabled, you *must* specify
         # an explicit url_preview_ip_range_blacklist of IPs that the spider is
         # denied from accessing.
         #
-        url_preview_enabled: False
+        #url_preview_enabled: false
 
         # List of IP address CIDR ranges that the URL preview spider is denied
         # from accessing.  There are no defaults: you must explicitly
@@ -306,6 +329,6 @@ class ContentRepositoryConfig(Config):
         #  - netloc: '^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$'
 
         # The largest allowed URL preview spidering size in bytes
-        max_spider_size: "10M"
-
+        #
+        #max_spider_size: 10M
         """ % locals()