tls: Restructure and improve certificate management
- Expose the list of Caddy instances through caddy.Instances() - Added arbitrary storage to caddy.Instance - The cache of loaded certificates is no longer global; now scoped per-instance, meaning upon reload (like SIGUSR1) the old cert cache will be discarded entirely, whereas before, aggressively reloading config that added and removed lots of sites would cause unnecessary build-up in the cache over time. - Key certificates in the cache by their SHA-256 hash instead of by their names. This means certificates will not be duplicated in memory (within each instance), making Caddy much more memory-efficient for large-scale deployments with thousands of sites sharing certs. - Perform name-to-certificate lookups scoped per caddytls.Config instead of a single global lookup. This prevents certificates from stepping on each other when they overlap in their names. - Do not allow TLS configurations keyed by the same hostname to be different; this now throws an error. - Updated relevant tests, with a stark awareness that more tests are needed. - Change the NewContext function signature to include an *Instance. - Strongly recommend (basically require) use of caddytls.NewConfig() to create a new *caddytls.Config, to ensure pointers to the instance certificate cache are initialized properly. - Update the TLS-SNI challenge solver (even though TLS-SNI is disabled currently on the CA side). Store temporary challenge cert in instance cache, but do so directly by the ACME challenge name, not the hash. Modified the getCertificate function to check the cache directly for a name match if one isn't found otherwise. This will allow any caddytls.Config to be able to help solve a TLS-SNI challenge, with one extra side-effect that might actually be kind of interesting (and useless): clients could send a certificate's hash as the SNI and Caddy would be able to serve that certificate for the handshake. - Do not attempt to match a "default" (random) certificate when SNI is present but unrecognized; return no certificate so a TLS alert happens instead. - Store an Instance in the list of instances even while the instance is still starting up (this allows access to the cert cache for performing renewals at startup, etc). Will be removed from list again if instance startup fails. - Laid groundwork for ACMEv2 and Let's Encrypt wildcard support. Server type plugins will need to be updated slightly to accommodate minor adjustments to their API (like passing in an Instance). This commit includes the changes for the HTTP server. Certain Caddyfile configurations might error out with this change, if they configured different TLS settings for the same hostname. This change trades some complexity for other complexity, but ultimately this new complexity is more correct and robust than earlier logic. Fixes #1991 Fixes #1994 Fixes #1303
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