Validate that the operation's root type is valid

Somehow we were not validating this!  So you could do
`mutation { bogus }` even if the schema has no mutation types, or worse,
any syntactically valid query if the schema is totally empty.  Ideally
we'd prohibit schemas without a query type entirely (spec says they need
one) but that caused some problems in tests and it wouldn't surprise me
if it causes problems in real life too (since an extension schema,
validated on its own, may look the same).  So we just prevent it at
query time.

Fixes vektah#221.

validator/imported/spec/schemas.yml

@@ -487,3 +487,4 @@
   }
 
   scalar Any
+- ""

validator/rules/known_root_type.go

@@ -0,0 +1,35 @@
+package validator
+
+import (
+	"fmt"
+
+	"github.com/vektah/gqlparser/v2/ast"
+	. "github.com/vektah/gqlparser/v2/validator"
+)
+
+func init() {
+	AddRule("KnownRootType", func(observers *Events, addError AddErrFunc) {
+		// A query's root must be a valid type.  Surprisingly, this isn't
+		// checked anywhere else!
+		observers.OnOperation(func(walker *Walker, operation *ast.OperationDefinition) {
+			var def *ast.Definition
+			switch operation.Operation {
+			case ast.Query, "":
+				def = walker.Schema.Query
+			case ast.Mutation:
+				def = walker.Schema.Mutation
+			case ast.Subscription:
+				def = walker.Schema.Subscription
+			default:
+				// This shouldn't even parse; if it did we probably need to
+				// update this switch block to add the new operation type.
+				panic(fmt.Sprintf(`got unknown operation type "%s"`, operation.Operation))
+			}
+			if def == nil {
+				addError(
+					Message(`Schema does not support operation type "%s"`, operation.Operation),
+					At(operation.Position))
+			}
+		})
+	})
+}

validator/spec/KnownRootTypeRule.spec.yml

@@ -0,0 +1,19 @@
+- name: Known root type
+  rule: KnownRootType
+  schema: 0
+  query: |
+    query { dog { name } }
+- name: Valid root type but not in schema
+  rule: KnownRootType
+  schema: 0
+  query: |
+    mutation { dog { name } }
+  errors:
+    - message: Schema does not support operation type "mutation"
+- name: Valid root type but schema is entirely empty
+  rule: KnownRootType
+  schema: 20
+  query: |
+    { dog { name } }
+  errors:
+    - message: Schema does not support operation type "query"