Which SlotChainBuilder is used is unpredictable, depends on order of resource loading

[xiazuojie]

Issue Description

Which com.alibaba.csp.sentinel.slotchain.SlotChainBuilder to use can be specified by creating a resource file resources/META-INF/services/com.alibaba.csp.sentinel.slotchain.SlotChainBuilder.
However, sentinel-core already ships its own file that uses com.alibaba.csp.sentinel.slots.DefaultSlotChainBuilder.
If an user provides his own resource file, we have multiple files (resources/META-INF/services/com.alibaba.csp.sentinel.slotchain.SlotChainBuilder) in classpath. Which one actually gets used depends on which resource file is loaded first, which is unpredictable and may cause unexpected issues.

Type: bug report or feature request
bug report

Describe what happened (or what feature you want)

If a user provides a resources/META-INF/services/com.alibaba.csp.sentinel.slotchain.SlotChainBuilder, it may not get used because sentinel-core already ships one.

Describe what you expected to happen

Which SlotChainBuilder is used can be specified in a predictable way, maybe a system property.
If there are multiple specifications, raise an error with detailed message on how to specify which one to use.

How to reproduce it (as minimally and precisely as possible)

Tell us your environment

Anything else we need to know?

From Alibaba with love

[sczyh30]

Actually it seems not a good design to make SlotChainBuilder as the SPI and load the first-encountered implementation. We may need a more flexible mechanism like #411 (though compatibility should be considered).

[sczyh30]

Resolved in Sentinel 1.7.2 via #411