[FLINK-13387] Can not download taskmanger & jobmanagers logs in the old UI

[yanghua]

What is the purpose of the change

This pull request fixs the problem about taskmanger & jobmanagers logs in the old UI can not be downloaded

Brief change log

• Fixs the problem about taskmanger & jobmanagers logs in the old UI can not be downloaded

Verifying this change

This change is hard to be verified with the test cases, the changes were verified in the local env*.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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Last check on commit 49a8e02 (Tue Aug 06 15:51:26 UTC 2019)

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[yanghua]

@dawidwys To clarify, I originally intended to write test cases, but I found it difficult to verify with test cases. If you have idea about how to verify with test cases, please let me know.
This issue has been fixed and verified in my local old Flink web UI.
cc @aljoscha and @rmetzger

[flinkbot]

CI report:

• 49a8e02 : SUCCESS Build

[dawidwys]

I didn't go through the whole PR, but duplicating a whole endpoint seems a bit unnecessary. Can't we just change the frontend somehow to access the same endpoint?

[zentol]

Couldn't we just set the href in taskmanager.(log|stdout).html to ../taskmanagers/[...]?

[zentol]

yup this actually works, will subsume this PR.

[zentol]

Subsumed in #9238.