add check for schema map[string]interface{} asssertion

[dbadura]

When you use GoLoader and put something that is not map[string]interface{} the library panic on nil value, because in draft.go, line 95 m := documentNode.(map[string]interface{}), assertion is not checked.
our issue

[dbadura]

After testing this library with errcheck with -assert flag, I found more not checked assertions.

draft.go:95:7:  m := documentNode.(map[string]interface{})
jsonLoader.go:204:15:   defer f.Close()
schema.go:88:8: b := documentNode.(bool)
schema.go:102:7:        m := documentNode.(map[string]interface{})
schema.go:260:21:       arrayOfTypes := m[KEY_TYPE].([]interface{})
schema.go:299:41:       currentSchema.additionalProperties = m[KEY_ADDITIONAL_PROPERTIES].(bool)
schema.go:321:28:       patternPropertiesMap := m[KEY_PATTERN_PROPERTIES].(map[string]interface{})
schema.go:425:36:       currentSchema.additionalItems = m[KEY_ADDITIONAL_ITEMS].(bool)
schema.go:729:22:       requiredValues := m[KEY_REQUIRED].([]interface{})
schema.go:792:32:       currentSchema.uniqueItems = m[KEY_UNIQUE_ITEMS].(bool)
schema.go:1019:7:       m := documentNode.(map[string]interface{})
schema.go:1042:7:       m := documentNode.(map[string]interface{})
schema.go:1049:14:      values := m[k].([]interface{})
schemaPool.go:86:30:    p.parseReferencesRecursive(v, ref, draft)
schemaPool.go:128:33:   p.parseReferencesRecursive(v, *localRef, draft)
schemaPool.go:132:31:   p.parseReferencesRecursive(v, *localRef, draft)
schemaPool.go:203:19:   p.parseReferences(document, refToURL, true)
utils.go:119:16:        jsonNumber := what.(json.Number)
utils.go:137:13:        number := what.(json.Number)
utils.go:160:13:        number := what.(json.Number)
validation.go:119:13:   value := currentNode.(json.Number)
validation.go:173:24:   castCurrentNode := currentNode.([]interface{})
validation.go:198:24:   castCurrentNode = convertDocumentNode(currentNode).(map[string]interface{})
validation.go:231:14:   value := currentNode.(bool)
validation.go:253:14:   value := currentNode.(string)
validation.go:484:30:   additionalItemSchema := currentSubSchema.additionalItems.(*subSchema)
validation.go:712:17:   stringValue := value.(string)
validation.go:776:12:   number := value.(json.Number)

So I am going to fix.

[johandorland]

This is definitely an overlooked oopsie, so thanks for the PR. I have one minor nitpick, but that is about it. 👍

[johandorland]

Most of the code uses the isKind helper method, so to keep the style similar to the rest of the code it's probably better to change this into:

if !isKind(documentNode, reflect.Map) {
    return "", nil, errors.New("schema is invalid")
}

[dbadura]

Fixed : )

[johandorland]

@dbadura Thank you for looking at those issues. I looked at a couple of them and I suspect most of them are false positives. From the couple I quickly looked at all of them had a type check somewhere, although not in the format errcheck expects. If you find another error that definitely needs to be checked, please let me know, but I don't want to clutter the code with redundant error handling because some linting tool said so.

[dbadura]

Thank you. I noticed how you do the assertion type later : )