Reduce file existence checks when servicing directory exists

[elinor-fung]

The presence of a servicing directory (even empty) would enable file existence checks for all files in the deps.json. We always look in the servicing directory first (if it exists) - and only care about the file existence there, not in the regular app directory. This change limits file existence checks to only when probing the servicing directory itself.

This reduces I/O operations during startup for applications that have a servicing directory present (and don't use additional probe paths or shared stores).

For the staticconsoletemplate startup test in dotnet/performance, running locally on Windows, if an empty servicing directory exists:

Metric	Before	After	Delta
Average	69.812 ms	58.895 ms	-10.92 ms (-15.6%)
Min	69.392 ms	58.467 ms	-10.93 ms
Max	70.772 ms	59.327 ms	-11.45 ms
Std Dev	0.393 ms	0.290 ms	-0.10 ms (26% tighter)
Range	1.38 ms	0.86 ms	38% smaller

@dotnet/appmodel @AaronRobinsonMSFT

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Pull request overview

Reduces hostpolicy startup I/O when a servicing directory exists by limiting file existence checks to servicing probes instead of enabling them globally.

Changes:

• Stop setting the global m_needs_file_existence_checks flag solely due to the presence of the servicing directory.
• When probing a servicing config, explicitly enable is_servicing + file_existence search options for that probe only.

[AaronRobinsonMSFT]

Why is the file_existence flag needed now? Was it added elsewhere previously?

[elinor-fung]

It is also added if additional probe paths or a shared store is specified (m_needs_file_existence_check). In those cases, it is added for all probe configs.

[AaronRobinsonMSFT]

So what you're saying is setting m_needs_file_existence_checks to true sets the file_existence flag and we could technically assert the old code that that flag was set? If so, does is_servicing imply file_existence?

[elinor-fung]

setting m_needs_file_existence_checks to true sets the file_existence flag

Yes. We initialize search_options based on m_needs_file_existence_checks.

we could technically assert the old code that that flag was set

Do you mean assert that m_needs_file_existence_checks is the same as whether search_options has the file_existence flag? Then yes. But it is initialized exactly to a couple lines above in the same function, so I don't know that it would be useful.

does is_servicing imply file_existence?

Yes. I do think is_servicing could go away - it doesn't seem so much a search option as just added in there specifically for a single-file scenario, which I think we could do differently.