Global constraint search ignores timeout when trajectories are not connected

[MichaelGrupp]

Following scenario:

• load frozen state with multiple trajectories 0 & 1 (e.g. assembled in cloud)
• start a trajectory 3 with initial pose relative to trajectory 0 of those frozen trajectories

Now 3 & 0 are connected, but 3 & 1 are not. This means that LastConnectionTime(3, 0) returns a 0 time value by convention, and thus we can trigger a global constraint search in this code path (regardless of how you set global_constraint_search_after_n_seconds):

https://github.com/googlecartographer/cartographer/blob/ea2bf3b1b2fc9ea00f094ccfca24c4d17f10763a/cartographer/mapping/internal/2d/pose_graph_2d.cc#L267-L287

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My question: is this a bug? The name global_constraint_search_after_n_seconds implies that the search is done after n seconds, but here it's scheduled immediately if the sampler pulses.

[MichaelGrupp]

This discussion is related, however it only considers the single trajectory case: #1142 (comment)

Citing @wohe :

without an initial pose there is not guaranteed connection between the two coordinate systems of the robot and the map

In the multi trajectory scenario I outlined here, there is however a connection. I'm fully aware that setting global_sampling_ratio to 0 finally disables global searches, but this is not obvious in this case.