Implement SqlException.IsTransient

[tbolon]

Fix #649

Code is based (it's a simple copy/paste for now) on EFCore SqlServerTransientExceptionDetector.cs

The xmldoc is quite empty (and does not respect the convention), I expect some help and/or guidance about how it should be handled: should we document all the cases handled by the code? Or keep it as a black box? Or something between where the a direct link to the code is added in the documentation?

Should we move the code outside the class to keep the file size small?

Should we expose this algorithm outside (make the external class public) so other libraries can use it?

[roji]

What happens if there's both a transient and a non-transient error? This would return IsTransient=true, which doesn't seem right.

[tbolon]

You think it is not? In which cases there are multiple errors returned for an exception? Anyway, if we retry because there is at least one transient error, and that error disappear, we can perhaps expect that other non-transient error will disappear. If we fail early we will not be able to detect if they are related.

[roji]

Anyway, if we retry because there is at least one transient error, and that error disappear, we can perhaps expect that other non-transient error will disappear. If we fail early we will not be able to detect if they are related.

I'm not very knowledgeable about SQL Server/SqlClient error handling (or which situation produces multiple errors), but generally speaking, the whole point of an error being non-transient is that it doesn't just go away when you retry the same operation. So I don't think it makes sense for IsTransient to return true when there's at least one non-transient error.

[tbolon]

I have inverted the check. The code will return true only if all errors are transient. It will return false on first non-transient (or no errors)

[tbolon]

Ping @AndriySvyryd which has handled most of the work on dotnet/efcore.

[ErikEJ]

There already is a list of transient errors in the codebase, let's stick to this one:

SqlClient/src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/Reliability/SqlConfigurableRetryFactory.cs

Line 40 in c7cefee

private static readonly HashSet<int> s_defaultTransientErrors

[tbolon]

There already is a list of transient errors in the codebase, let's stick to this one:

SqlClient/src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/Reliability/SqlConfigurableRetryFactory.cs

Line 40 in c7cefee

private static readonly HashSet<int> s_defaultTransientErrors

What could be the prefered way to reuse that list? Add an internal method that I could call from the property or move out the list of codes in a specific class?

[tbolon]

Ping @ErikEJ to resurect this PR.

[damienhoneyford]

It would be great to progress this PR if possible -- it's really frustrating as a developer to find a field named IsTransient on the exception only to later discover that it isn't wired up!

My 2c would be that the list of errors that @ErikEJ referred to is considerably smaller than the list of errors handled by the EF code, which makes me wonder how the list in SqlConfigurableRetryFactory came about. On the face of it, the EF list is the better choice as it's more exhaustive, but either way some refactoring would be needed -- probably moving that list in to a separate class or turning it in to an enum and then referencing it from both places.

It would be good to get some feedback from some of the frequent contributors to confirm the best course of action to get this change merged and make the IsTransient property actually work.

[stevendarby]

My 2c would be that the list of errors that @ErikEJ referred to is considerably smaller than the list of errors handled by the EF code [...] On the face of it, the EF list is the better choice as it's more exhaustive

Just to add to this, the EF code has been updated very recently, while the SqlClient list is 3 years old. Seems much better to go with the former.

Importantly, the most recent addition to the EF switch statement does a check on more than just the number, so a HashSet of ints isn't going to cut it really.

@tbolon probably worth updating this PR to include the latest changes referred to above :)
https://github.com/dotnet/efcore/blob/main/src/EFCore.SqlServer/Storage/Internal/SqlServerTransientExceptionDetector.cs