Launcher: set a download timeout

In opendev, we have seen the launcher shutdown process hang due
to download threads stuck while downloading image artifacts.  This
has also likely wedged the image rotation process in general since
all the upload slots are occupied.

Correct this by setting a requests timeout.

Change-Id: I6cf56eca50d5b119f63ac3ac5ccb99190a82d0e1

zuul/launcher/server.py

@@ -88,6 +88,10 @@ DOWNLOAD_BLOCK_SIZE = 1024 * 64
 # The byte range size for an individual GET operation.  Should be a
 # multiple of the above.  Current value is approx 100MiB.
 DOWNLOAD_CHUNK_SIZE = DOWNLOAD_BLOCK_SIZE * 1525
+# Set a 30 second connection timeout, and 900 second transfer timeout
+# for each chunk.  The transfer timeout works out to a bit less than
+# 1Mbps.
+DOWNLOAD_CHUNK_TIMEOUT = (30, 900)
 
 
 def _normalize_statsd_name(name):
@@ -3199,20 +3203,22 @@ class Launcher:
         headers = {'Range': f'bytes={start}-{end}'}
         with open(path, 'r+b') as f:
             f.seek(start)
-            with requests.get(url, stream=True, headers=headers) as resp:
+            with requests.get(url, stream=True, timeout=DOWNLOAD_CHUNK_TIMEOUT,
+                              headers=headers) as resp:
                 resp.raise_for_status()
                 for chunk in resp.iter_content(chunk_size=DOWNLOAD_BLOCK_SIZE):
                     f.write(chunk)
 
     def downloadArtifact(self, url, path):
         futures = []
-        with requests.head(url) as resp:
+        with requests.head(url, timeout=DOWNLOAD_CHUNK_TIMEOUT) as resp:
             if resp.status_code == 403:
                 self.log.debug(
                     "Received 403 for %s, retrying with range header", url)
                 # This may be a pre-signed url that can't handle a
                 # HEAD request, retry with GET:
                 with requests.get(url, stream=True,
+                                  timeout=DOWNLOAD_CHUNK_TIMEOUT,
                                   headers={"Range": "bytes=0-0"}) as resp:
                     resp.raise_for_status()
                     size = int(resp.headers['content-range'].split('/')[-1])