osxkeychain: store: use Apple's proto consts

Commit 4cdcdc2 swapped consts `kSecProtocolTypeHTTPS` and
`kSecProtocolTypeHTTP` with plain-text "https" and "http" strings.

This is causing a regression where credentials stored with prior
versions (< v0.9.0) can't be fetched anymore.

Unfortunately we can't just revert back to using Objective-C consts, as
these are unsigned integers that need to be converted into `CFStringRef`
and then passed to an helper like `keychain.CFStringToString`.

Although `keychain.CFStringToString` is exported, it takes a C type
`C.CFStringRef` so it's not consumable from other packages due to Cgo
restrictions:

> Cgo translates C types into equivalent unexported Go types. Because
> the translations are unexported, a Go package should not expose C
> types in its exported API: a C type used in one Go package is
> different from the same C type used in another.

We could alternatively copy `keychain.CFStringToString` into the
`osxkeychain` package, but this commit takes a simpler approach: just
hardcode the value of `kSecProtocolTypeHTTPS` and `kSecProtocolTypeHTTP`
as strings. (These consts are very unlikely to ever change since it'd
break all existing consumers.)

This is **NOT** handling backward compatibility with v0.9.0, since it
was released only 12hrs ago. So this fix won't work with credentials
created with v0.9.0.

Signed-off-by: Albin Kerouanton <albinker@gmail.com>

osxkeychain/osxkeychain.go

@@ -129,14 +129,22 @@ func (h Osxkeychain) List() (map[string]string, error) {
 	return resp, nil
 }
 
+const (
+	// Hardcoded protocol types matching their Objective-C equivalents.
+	// https://developer.apple.com/documentation/security/ksecattrprotocolhttps?language=objc
+	kSecProtocolTypeHTTPS = "htps" // This is NOT a typo.
+	// https://developer.apple.com/documentation/security/ksecattrprotocolhttp?language=objc
+	kSecProtocolTypeHTTP = "http"
+)
+
 func splitServer(serverURL string, item keychain.Item) error {
 	u, err := registryurl.Parse(serverURL)
 	if err != nil {
 		return err
 	}
-	item.SetProtocol("https")
+	item.SetProtocol(kSecProtocolTypeHTTPS)
 	if u.Scheme == "http" {
-		item.SetProtocol("http")
+		item.SetProtocol(kSecProtocolTypeHTTP)
 	}
 	item.SetServer(u.Hostname())
 	if p := u.Port(); p != "" {