Ensure a minimum retry interval of 5 seconds in fetching central configuration

[estolfo]

Description

The central config spec has recently been updated to say that regardless of what the Cache-Control headers are, agents should only retry at a maximum every 5 seconds to fetch the central configuration. I.e. if the Cache-Control headers specify a value less than 5, the default retry interval should be used.

Agent Issues

• [META 690] Ensure a minimum retry interval of 5 seconds in fetching central configuration apm-agent-java#2803
• [META 690] Ensure a minimum retry interval of 5 seconds in fetching central configuration apm-agent-dotnet#1831
• [META 690] Ensure a minimum retry interval of 5 seconds in fetching central configuration apm-agent-nodejs#2941
• [META 690] Ensure a minimum retry interval of 5 seconds in fetching central configuration apm-agent-python#1645
• [META 690] Ensure a minimum retry interval of 5 seconds in fetching central configuration apm-agent-go#1322
• [META 690] Ensure a minimum retry interval of 5 seconds in fetching central configuration apm-agent-php#778

[beniwohli]

A question that came up while implementing this. The spec says

If the Cache-Control header is zero (or less than zero)

However, the relevant RFC says that max-age is of type delta-seconds:

The delta-seconds rule specifies a non-negative integer, representing time in seconds.

So, negative values aren't allowed. Is the intention of the spec to behave reasonably in the face of misbehaving servers/proxies, or could we simplify this a bit?

FWIW, the Python agent behaves as spec'd by accident, as the regex fails to parse negative values and falls back to the default interval of 300s.

[felixbarny]

Is the intention of the spec to behave reasonably in the face of misbehaving servers/proxies, or could we simplify this a bit?

As guarding against negative values doesn't seem to increase the complexity, it seems like a good idea to do that.

FWIW, the Python agent behaves as spec'd by accident, as the regex fails to parse negative values and falls back to the default interval of 300s.

This sounds like a valid way to handle it 👍