Add availability_zone to the HELLO response (#1487)

It's inconvenient for client implementations to extract the
`availability_zone` information from the `INFO` response. The `INFO`
response contains a lot of information that a client implementation
typically doesn't need.

This PR adds the availability zone to the `HELLO` response. Clients
usually already use the `HELLO` command for protocol negotiation and
also get the server `version` and `role` from its response. To keep the
`HELLO` response small, the field is only added if availability zone is
configured.

---------

Signed-off-by: Rueian <rueiancsie@gmail.com>

src/networking.c

@@ -4206,7 +4206,7 @@ void helloCommand(client *c) {
 
     /* Let's switch to the specified RESP mode. */
     if (ver) c->resp = ver;
-    addReplyMapLen(c, 6 + !server.sentinel_mode);
+    addReplyMapLen(c, 6 + !server.sentinel_mode + (sdslen(server.availability_zone) != 0));
 
     addReplyBulkCString(c, "server");
     addReplyBulkCString(c, server.extended_redis_compat ? "redis" : SERVER_NAME);
@@ -4235,6 +4235,11 @@ void helloCommand(client *c) {
 
     addReplyBulkCString(c, "modules");
     addReplyLoadedModules(c);
+
+    if (sdslen(server.availability_zone) != 0) {
+        addReplyBulkCString(c, "availability_zone");
+        addReplyBulkCBuffer(c, server.availability_zone, sdslen(server.availability_zone));
+    }
 }
 
 /* This callback is bound to POST and "Host:" command names. Those are not

tests/unit/protocol.tcl

@@ -232,6 +232,40 @@ start_server {tags {"protocol network"}} {
 
 }
 
+start_server {tags {"protocol hello"}} {
+    test {HELLO without protover} {
+        set reply [r HELLO 3]
+        assert_equal [dict get $reply proto] 3
+
+        set reply [r HELLO]
+        assert_equal [dict get $reply proto] 3
+
+        set reply [r HELLO 2]
+        assert_equal [dict get $reply proto] 2
+
+        set reply [r HELLO]
+        assert_equal [dict get $reply proto] 2
+    }
+
+    test {HELLO and availability-zone} {
+        r CONFIG SET availability-zone myzone
+
+        set reply [r HELLO 3]
+        assert_equal [dict get $reply availability_zone] myzone
+
+        set reply [r HELLO 2]
+        assert_equal [dict get $reply availability_zone] myzone
+
+        r CONFIG SET availability-zone ""
+
+        set reply [r HELLO 3]
+        assert_equal [dict exists $reply availability_zone] 0
+
+        set reply [r HELLO 2]
+        assert_equal [dict exists $reply availability_zone] 0
+    }
+}
+
 start_server {tags {"regression"}} {
     test "Regression for a crash with blocking ops and pipelining" {
         set rd [valkey_deferring_client]

tests/unit/tracking.tcl

@@ -154,23 +154,6 @@ start_server {tags {"tracking network logreqres:skip"}} {
         assert_equal [dict get $reply proto] 3
     }
 
-    test {HELLO without protover} {
-        set reply [r HELLO 3]
-        assert_equal [dict get $reply proto] 3
-
-        set reply [r HELLO]
-        assert_equal [dict get $reply proto] 3
-
-        set reply [r HELLO 2]
-        assert_equal [dict get $reply proto] 2
-
-        set reply [r HELLO]
-        assert_equal [dict get $reply proto] 2
-
-        # restore RESP3 for next test
-        r HELLO 3
-    }
-
     test {RESP3 based basic invalidation} {
         r CLIENT TRACKING off
         r CLIENT TRACKING on