Why use reflection here?

thomasmey · November 25, 2024, 5:35pm

Hi, I’m fairly new to golang and I did stumble upon this code in k8s/k8s:

kubernetes/kubernetes/blob/master/pkg/credentialprovider/plugins.go#L55


      
          	providers[name] = provider
          }
          
          // NewDockerKeyring creates a DockerKeyring to use for resolving credentials,
          // which draws from the set of registered credential providers.
          func NewDockerKeyring() DockerKeyring {
          	keyring := &providersDockerKeyring{
          		Providers: make([]DockerConfigProvider, 0),
          	}
          
          	keys := reflect.ValueOf(providers).MapKeys()
          	stringKeys := make([]string, len(keys))
          	for ix := range keys {
          		stringKeys[ix] = keys[ix].String()
          	}
          	sort.Strings(stringKeys)
          
          	for _, key := range stringKeys {
          		provider := providers[key]
          		if provider.Enabled() {
          			klog.V(4).Infof("Registering credential provider: %v", key)

var providers = make(map[string]DockerConfigProvider)
[…]

keys := reflect.ValueOf(providers).MapKeys()
stringKeys := make([]string, len(keys))
for ix := range keys {
	stringKeys[ix] = keys[ix].String()
}
sort.Strings(stringKeys)

Why is reflection used here? The map key type is know and string, so why did the author use reflection here?

Any ideas?
Idiomatic golang would be to iterate over the map.
Looking at the commit it was done for Make CredentialProvider config loading deterministic. · kubernetes/kubernetes@804ee25 · GitHub
For sorting after map key reasons, but I don’t understand why to use reflection here still.

peakedshout · November 26, 2024, 2:15am

From this code snippet, it looks like it’s not using a lock to get a snapshot list of the map because of data races, it just saves some troublesome work. You can try viewing the source code of MapKeys.

lemarkar · November 26, 2024, 9:57am

Git blame sheds some light on the question here. In short, they wanted this algorithm to be more deterministic than simply iterating over map keys, since the order of keys access is random.

thomasmey · November 26, 2024, 11:44am

But why not build an array of all keys and sort it without using reflection?
The missing mutex for access the data structure looks wrong also, not sure how MapKeys would alleviate this problem.

Modestying · November 26, 2024, 11:55am

maybe you can send an email for the answer

thomasmey · November 26, 2024, 12:00pm

FYI

Reading the map source code of golang show that there are actually two different map implementation: the experimental SwissMap and this great blog post I found