Validate that the operation's root type is valid

Turns out neither we (nor anyone else) were validating this, because the
spec didn't say explicitly to validate it.  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.

In graphql/graphql-spec#955 I'm adding it to the spec, and in
graphql/graphql-js#3592 someone else is adding it to graphql-js.  So we
should too, once those land!  At that point we should also likely
reimport the relevant tests, instead of the ones I wrote here, but I
figured I'd put the PR up now so folks can review the non-test parts if
you like.

Fixes vektah#221.

Author: benjaminjkraft

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"