provider: ensuring the providers slice isn't a nil pointer

[tombuildsstuff]

This was a fun one to track down.

Turns out due to the way that parallelization works, init() gets called after
the resource.ParallelTest has been called - which means that testAccProviders
could be an empty slice, which caused an unhelpful crash in the Plugin SDK.

Before this change:

2019/12/09 07:55:18 [TRACE] LoadSchemas: retrieving schema for provider type "azurerm"
2019/12/09 07:55:18 [TRACE] GRPCProvider: GetSchema
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x2725f14]
goroutine 25 [running]:
github.com/hashicorp/terraform-plugin-sdk/internal/helper/plugin.(*GRPCProviderServer).getProviderSchemaBlock(...)
	/Users/tharvey/code/pkg/mod/github.com/hashicorp/terraform-plugin-sdk@v1.1.1/internal/helper/plugin/grpc_provider.go:76
github.com/hashicorp/terraform-plugin-sdk/internal/helper/plugin.(*GRPCProviderServer).GetSchema(0xc0000b71f8, 0x5aace60, 0xc00094c300, 0xc000548ee0, 0xc0000b71f8, 0xc00094c300, 0xc000074a80)
	/Users/tharvey/code/pkg/mod/github.com/hashicorp/terraform-plugin-sdk@v1.1.1/internal/helper/plugin/grpc_provider.go:55 +0x94
github.com/hashicorp/terraform-plugin-sdk/internal/tfplugin5._Provider_GetSchema_Handler(0x4dfec00, 0xc0000b71f8, 0x5aace60, 0xc00094c300, 0xc0009520c0, 0x0, 0x5aace60, 0xc00094c300, 0x0, 0x0)
	/Users/tharvey/code/pkg/mod/github.com/hashicorp/terraform-plugin-sdk@v1.1.1/internal/tfplugin5/tfplugin5.pb.go:3045 +0x217
google.golang.org/grpc.(*Server).processUnaryRPC(0xc0000fa2c0, 0x5b073c0, 0xc00088c300, 0xc00097e000, 0xc000727e30, 0x8736660, 0x0, 0x0, 0x0)
	/Users/tharvey/code/pkg/mod/google.golang.org/grpc@v1.23.0/server.go:995 +0x460
google.golang.org/grpc.(*Server).handleStream(0xc0000fa2c0, 0x5b073c0, 0xc00088c300, 0xc00097e000, 0x0)
	/Users/tharvey/code/pkg/mod/google.golang.org/grpc@v1.23.0/server.go:1275 +0xd97
google.golang.org/grpc.(*Server).serveStreams.func1.1(0xc0004a83e0, 0xc0000fa2c0, 0x5b073c0, 0xc00088c300, 0xc00097e000)
	/Users/tharvey/code/pkg/mod/google.golang.org/grpc@v1.23.0/server.go:710 +0xbb
created by google.golang.org/grpc.(*Server).serveStreams.func1
	/Users/tharvey/code/pkg/mod/google.golang.org/grpc@v1.23.0/server.go:708 +0xa1
FAIL	github.com/terraform-providers/terraform-provider-azurerm/azurerm	0.050s
FAIL

Tests pass:

$ acctests azurerm TestAccAzureRMResourceGroup_basic
=== RUN   TestAccAzureRMResourceGroup_basic
=== PAUSE TestAccAzureRMResourceGroup_basic
=== CONT  TestAccAzureRMResourceGroup_basic
--- PASS: TestAccAzureRMResourceGroup_basic (77.08s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	77.132s

[katbyte]

LGTM 👍

[ghost]

This has been released in version 1.39.0 of the provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading. As an example:

provider "azurerm" {
    version = "~> 1.39.0"
}
# ... other configuration ...

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. If you feel I made an error 🤖 🙉 , please reach out to my human friends 👉 hashibot-feedback@hashicorp.com. Thanks!