sauron: best effort detection of invalid raw blocks

It happens that Esplora will send us trimmed blocks (both my personal
instance and blockstream.info, fwiw). Try to detect it without parsing
the entire block on our side.

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

sauron/sauron.py

@@ -1,6 +1,7 @@
 #!/usr/bin/env python3
-import sys
 import requests
+import sys
+import time
 
 from requests.packages.urllib3.util.retry import Retry
 from requests.adapters import HTTPAdapter
@@ -109,12 +110,25 @@ def getrawblock(plugin, height, **kwargs):
         }
 
     block_url = "{}/block/{}/raw".format(plugin.api_endpoint, blockhash_req.text)
+    while True:
         block_req = fetch(block_url)
         if block_req.status_code != 200:
             return {
                 "blockhash": None,
                 "block": None,
             }
+        # We may download partial/incomplete files for Esplora. Best effort to
+        # not crash lightningd by sending an invalid (trimmed) block.
+        # NOTE: this will eventually be fixed upstream, at which point we should
+        # just reuse the retry handler.
+        content_len = block_req.headers.get("Content-length")
+        if content_len is None:
+            break
+        if int(content_len) == len(block_req.content):
+            break
+        plugin.log("Esplora gave us an incomplete block, retrying in 2s",
+                   level="error")
+        time.sleep(2)
 
     return {
         "blockhash": blockhash_req.text,