Fix reload behaviors + Add OnLoadError event for file based config providers

[HaoK]

Fixes #489

No sugar currently:

                Exception jsonError = null;
                Action<Exception> jsonLoadError = ex => jsonError = ex;
                new ConfigurationBuilder()
                    .Add(new JsonConfigurationSource { Path = "error.json", OnLoadError = jsonLoadError })
                    .Build();

This at least enables some way to see what failures are occuring during load/reload

@divega

[HaoK]

@divega ping for review

[HaoK]

We could consider making this a Func that returns bool to control whether we want to rethrow the exception. That might be a nice quality of life improvement, but not sure how useful it is always...

[divega]

I like the idea, although not sure the meaning of returning true vs. false is clear. Maybe passing a "load error context" object that contains the exception, the sender (provider) and a bool to decide the outcome?

[HaoK]

@divega updated adding the FileLoadErrorContext and renamed LoadError => LoadException as we discussed.

[HaoK]

SetFileLoadExceptionHandler

[HaoK]

@divega I don't think our CreateInstance/Build hack is going to work, as it is a breaking change.

Lets say they derived from our current sources today and in their Build(), they all base.Build(), they will now get a NotImplemented exception since they didn't implement CreateInstance.

I think we should just introduce a new method in FileConfigurationSource and call it in each of the Builds, its not the end of the world...

   // in FileConfigurationSource
   public void EnsureDefaults(IConfigurationBuilder builder) {
            FileProvider = FileProvider ?? builder.GetFileProvider();
            OnLoadException = OnLoadException ?? builder.GetFileLoadExceptionHandler();
   }

        // boilerplate builds in all FileProviders
        public override IConfigurationProvider Build(IConfigurationBuilder builder)
        {
            EnsureDefaults();
            return new JsonConfigurationProvider(this);
        }

[HaoK]

Updated with new IConfigurationBuilder.Get/SetFileLoadExceptionHandler sugar

[divega]

Lets say they derived from our current sources today and in their Build(), they all base.Build(),

They can't. You get a compiler error if you try to call base.Build() if Build() is abstract on the base.

[divega]

... anyway, I am not against what you did here. My only concern right now is the inconsistent naming between the Set/Get methods and the property on the file sources.

[divega]

I think we should consider renaming OnLoadException to LoadExceptionHandler or FileLoadExceptionHandler.

[HaoK]

@divega @glennc updated PR with fixes to make reload behavior more reliable:

• we now clear the configuration keys at the start of reload (old keys will now be dropped)
• we add a configurable delay (FileConfigurationSource.ReloadDelay) before starting reload (default 250 ms)
• Reenabled all linux tests, lowered sleeps to 1100 ms in the tests(from 2000), since we should be more reliable with the build in delay as well

[HaoK]

This PR now also fixes #486

[HaoK]

36ec5b0