"CAP LS" may only respond 1 line, we now advertise less. "CAP LS 302" unaffected.

When not using version 302, such as with "CAP LS", the specification does not
allow us to use continuation lines. This means all advertised caps must fit
into one line. That is no longer always the case, especially if you load 3rd
party capabilities. So we need to scratch advertising some capabilities to
<302 clients.

"CAP LS 302" is unaffected. Note that version 302 in the specification exists
since at least November 2017, so most clients use that one.

According to https://ircv3.net/software/clients the following clients are
affected by this change:

Desktop Clients
* KVIrc
* Circe
* catgirl
* BitchX
* Pidgin
* LimeChat

Mobile Clients
* IRC for Android
* LimeChat

And various older versions of other clients (obviously).

NOTE: The source is only that IRCv3 page. I did not check manually.

For this particular commit. We filter out various unrealircd.org informative
CAPs and the vendor specific json-log. So that isn't much of a problem.
However, in the future we may be forced to filter out more capabilities to
make room. It would be much better if all clients are on >=302. Also, I
should mention we are not the only IRCd out there, so I can't vouch on what
other IRCds (will) do when hitting this non-302-limit.

Reported by ProgVal in https://bugs.unrealircd.org/view.php?id=6630

doc/RELEASE-NOTES.md

@@ -120,6 +120,12 @@ has been improved.
   telling the server "does not support remote JSON-RPC".
 
 ### Developers and protocol:
+* If you use `CAP LS` instead of `CAP LS 302` then you will now miss various
+  capabilities. We had to trim it down because only 302 and later allow
+  responses that span multiple lines. If your client is still not using 302
+  then please do so soon (the IRCv3 spec is from Nov'2017). For this first
+  change, you won't miss out much, but somewhere in the future this can
+  become a real problem for you.
 * URL API: The OutgoingWebRequest `max_size` (introduced last release) now
   also caps file-backed downloads. Default for file-backed when left at 0
   is 50MB (`DOWNLOAD_MAX_SIZE_FILE_BACKED`). For memory-backed, it stays

include/modules.h

@@ -517,6 +517,7 @@ struct ClientCapability {
 	MessageTagHandler *mtag_handler;         /**< For reverse dependency */
 	Module *owner;                           /**< Module introducing this CAP. */
 	char unloaded;                           /**< Internal flag to indicate module is being unloaded */
+	int minimum_cap_version;                 /**< Minimum CAP version to show this CAP */
 };
 
 typedef struct {
@@ -524,6 +525,7 @@ typedef struct {
 	int flags;
 	int (*visible)(Client *);
 	const char *(*parameter)(Client *);
+	int minimum_cap_version;
 } ClientCapabilityInfo;
 
 /** @defgroup MessagetagAPI Message tag API

src/api-clicap.c

@@ -204,6 +204,7 @@ ClientCapability *ClientCapabilityAdd(Module *module, ClientCapabilityInfo *clic
 	/* Add or update the following fields: */
 	clicap->owner = module;
 	clicap->flags = clicap_request->flags;
+	clicap->minimum_cap_version = clicap_request->minimum_cap_version;
 	clicap->visible = clicap_request->visible;
 	clicap->parameter = clicap_request->parameter;
 

src/modules/cap.c

@@ -165,6 +165,9 @@ static void clicap_generate(Client *client, const char *subcmd, int flags)
 		if (cap->visible && !cap->visible(client))
 			continue; /* hidden */
 
+		if (cap->minimum_cap_version && (client->local->cap_protocol < cap->minimum_cap_version))
+			continue; /* skip: doesn't meet minimum CAP version */
+
 		if (flags)
 		{
 			if (!cap->cap || !(client->local->caps & cap->cap))

src/modules/history_backend_mem.c

@@ -428,6 +428,7 @@ static void init_history_storage(ModuleInfo *modinfo)
 	cap.name = "unrealircd.org/history-storage";
 	cap.flags = CLICAP_FLAGS_ADVERTISE_ONLY;
 	cap.parameter = history_storage_capability_parameter;
+	cap.minimum_cap_version = 302;
 	ClientCapabilityAdd(modinfo->handle, &cap, NULL);
 }
 

src/modules/json-log-tag.c

@@ -54,6 +54,7 @@ MOD_INIT()
 
 	memset(&cap, 0, sizeof(cap));
 	cap.name = "unrealircd.org/json-log";
+	cap.minimum_cap_version = 302;
 	c = ClientCapabilityAdd(modinfo->handle, &cap, &CAP_JSON_LOG);
 
 	memset(&mtag, 0, sizeof(mtag));

src/modules/link-security.c

@@ -80,6 +80,7 @@ MOD_LOAD()
 	cap.name = "unrealircd.org/link-security";
 	cap.flags = CLICAP_FLAGS_ADVERTISE_ONLY;
 	cap.parameter = link_security_capability_parameter;
+	cap.minimum_cap_version = 302;
 	ClientCapabilityAdd(modinfo->handle, &cap, NULL);
 
 	EventAdd(modinfo->handle, "checklinksec", checklinksec, NULL, 2000, 0);

src/modules/plaintext-policy.c

@@ -71,5 +71,6 @@ void init_plaintext_policy(ModuleInfo *modinfo)
 	cap.name = "unrealircd.org/plaintext-policy";
 	cap.flags = CLICAP_FLAGS_ADVERTISE_ONLY;
 	cap.parameter = plaintext_policy_capability_parameter;
+	cap.minimum_cap_version = 302;
 	ClientCapabilityAdd(modinfo->handle, &cap, NULL);
 }