fix: use %s placeholders in HTTPError to prevent Tornado from doubling % in gateway URLs#1620
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krassowski merged 2 commits intoApr 30, 2026
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…g % in gateway URLs Tornado's HTTPError escapes '%' as '%%' in the log_message string when no positional args are provided. This caused gateway URLs containing percent- encoded characters (e.g. 'http%3A%2F%2F') to appear doubled in error responses and logs. Replace f-string log_message arguments in the three web.HTTPError raises in send_request_to_gateway() with '%s' placeholder strings and pass URL/message values through the args parameter. Tornado only escapes log_message when args is empty, so URL percent characters are preserved correctly. Add test_gateway_httperror_percent_not_doubled() to demonstrate the old f-string path doubles '%' and the new placeholder+args path does not. Closes jupyter-server#1503
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+1, this issue is also affecting me. FYI, this was also fixed recently in a newer version of Tornado (6.5.0+), with a new get_message method, but the original log_message method and behavior is unchanged for backwards compatibility: |
ptch314
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Apr 29, 2026
ptch314
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LGTM. Note that this PR adds regression tests for when %s are correctly left alone. But we do not need test cases for the original behavior when %s are replaced with %%s, because Jupyter Server does not intentionally utilize this behavior of log_message anywhere.
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Problem
Closes #1503.
When the Gateway server is unreachable or returns an error, send_request_to_gateway() raises web.HTTPError passing the error message as an f-string. Tornado's HTTPError.init escapes % → %% in log_message when no positional args are provided. This caused URLs containing percent-encoded characters (e.g. http://host/?redirect=http%3A%2F%2Fexample.com) to appear with doubled percent signs in error responses and logs.
From Tornado's source:
\\python
if log_message and not args:
self.log_message = log_message.replace('%', '%%')
\\
Fix
Replace the three f-string log_message arguments in send_request_to_gateway() with %s format strings and pass URLs / messages as positional �rgs. When �rgs is non-empty Tornado skips the escaping, so percent-encoded characters in gateway URLs are preserved correctly.
Changes
Test
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29 passed in 5.10s
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