WebKit wants these private key properties to be readable in order to
implement a deserialization function. Currently they are read-only
because at the time GTlsCertificate was originally designed, the plan
was to support PKCS#11-backed private keys: private keys that are stored
on a smartcard, where the private key is completely unreadable. The
design goal was to support both memory-backed and smartcard-backed
private keys with the same GTlsCertificate API, abstracting away the
implementation differences such that code using GTlsCertificate doesn't
need to know the difference.
The original PKCS#11 implementation was never fully baked and at some
point in the past I deleted it all. It has since been replaced with a
new implementation, including a GTlsCertificate:private-key-pkcs11-uri
property, which is readable. So our current API already exposes the
differences between normal private keys and PKCS#11-backed private keys.
The point of making the private-key and private-key-pem properties
write-only was to avoid exposing this difference.
Do we have to make this API function readable? No, because WebKit could
be just as well served if we were to expose serialize and deserialize
functions instead. But WebKit needs to support serializing and
deserializing the non-private portion of GTlsCertificate with older
versions of GLib anyway, so we can do whatever is nicest for GLib. And I
think making this property readable is nicest, since the original design
reason for it to not be readable is now obsolete. The disadvantage to
this approach is that it's now possible for an application to read the
private-key or private-key-pem property, receive NULL, and think "this
certificate must not have a private key," which would be incorrect if
the private-key-pkcs11-uri property is set. That seems like a minor
risk, but it should be documented.
