logger.setBindings() doesn't clear logger[Symbol(pino.parsedChindings)]

[okofish]

#754 added a very useful logger.setBindings() method. But this doesn't play well with prettifiers, which appear to make use of a cached version of the bindings:

> let child=logger.child({foo: 123});

// pino.parsedChindings is undefined at start
> child.bindings()
{ foo: 123 }
> child[Pino.symbols.parsedChindingsSym]
undefined

// when the first log call is made, pino.parsedChindings gets populated
> child.info('hello')
[2020-08-11 23:12:27.349 +0000] INFO : hello
    foo: 123
> child[Pino.symbols.parsedChindingsSym]
{ pid: 16561, hostname: 'nutrimatic', foo: 123 }

// logger.setBindings doesn't update pino.parsedChindings
> child.setBindings({bar: 456})
> child.bindings()
{ foo: 123, bar: 456 }
> child[Pino.symbols.parsedChindingsSym]
{ pid: 16561, hostname: 'nutrimatic', foo: 123 }

// so subsequent logs don't include the new bindings
> child.info('hello')
[2020-08-11 23:13:24.553 +0000] INFO : hello
    foo: 123
> child[Pino.symbols.parsedChindingsSym]
{ pid: 16561, hostname: 'nutrimatic', foo: 123 }

// manually clearing pino.parsedChindings causes it to be regenerated on the next log call
> delete child[Pino.symbols.parsedChindingsSym]
> child.info('hello')
[2020-08-11 23:14:28.240 +0000] INFO : hello
    foo: 123
    bar: 456
> child[Pino.symbols.parsedChindingsSym]
{ pid: 16561, hostname: 'nutrimatic', foo: 123, bar: 456 }

Would it be "safe" (performance-wise) to have logger.setBindings() automatically clear pino.parserChindings so that the next log call causes it to update? I think this may be OK since said updating only happens when a prettifier is enabled, so it won't impact production situations where there's no prettifier. Alternatively, could we add an additional method that manually clears pino.parserChindings?

I can submit a PR for either of these approaches; I'm just not sure which is best.

[mcollina]

Good spot! The first approach (clearing the cache on setBindings()) is perfect.

[okofish]

Great, I've submitted #893.

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