fix: close executor when closing pool

[olavloite]

Shutdown the executor that maintains the pool when the pool is closing.

[olavloite]

The only real change here is the addition of the finally block, but GitHub thinks there is a lot more changed because of the changed indentation and formatting.

[olavloite]

cc @jjfox15

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@691a23c). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #501   +/-   ##
=========================================
  Coverage          ?   82.26%           
  Complexity        ?     2395           
=========================================
  Files             ?      138           
  Lines             ?    13414           
  Branches          ?     1240           
=========================================
  Hits              ?    11035           
  Misses            ?     1883           
  Partials          ?      496           

Impacted Files	Coverage Δ	Complexity Δ
...m/google/cloud/spanner/connection/SpannerPool.java	87.20% <100.00%> (ø)	31.00 <0.00> (?)

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[thiagotnunes]

Can this throw an exception?

[olavloite]

Normally, no. It does not declare any checked exceptions, and it also does not wait until it has actually been shutdown, so the chance that anything goes wrong is relatively low.

The method can be invoked in two ways:

• If a client application calls SpannerPool#closeSpannerPool() to explicitly close the pool. Any exception from closerService.shutdown() would in that case be propagated to the client application. Considering the fact that it would not be a checked exception, I would say that that is a reasonable thing to do, as it might indicate a problem somewhere, for example linked to the state of the client application at that moment.
• It is called from the shutdown hook that closes the pool automatically when the application terminates. The shutdown hook catches and ignores any errors, as any errors at that moment are (probably) not something that a client application could do anything with. The alternative would be to bubble it up, which would cause the JVM (by default) to print the error to System.err.

[thiagotnunes]

Cool, that is fine, I was just worried that we could bubble up exceptions that we previously did not.