From 9c48d926b9e3cb7208bef3deaea3cebc75f47bc0 Mon Sep 17 00:00:00 2001
From: Kenton Varda <kenton@cloudflare.com>
Date: Fri, 29 Nov 2024 14:38:25 -0600
Subject: [PATCH] Replace `logging` with `tails` in workerd.capnp.

Using a simple array seems strictly easier to understand than a union with zero/one/many variants.

REVIEW: Can we simply remove `logging`? It was introduced relatively recently. Is anyone depending on it yet?
---
 src/workerd/server/server-test.c++ |  4 +--
 src/workerd/server/server.c++      | 44 +++++++++++++++++++-----------
 src/workerd/server/workerd.capnp   |  6 +++-
 3 files changed, 34 insertions(+), 20 deletions(-)

diff --git a/src/workerd/server/server-test.c++ b/src/workerd/server/server-test.c++
index 46548c212c0..63437be3f1b 100644
--- a/src/workerd/server/server-test.c++
+++ b/src/workerd/server/server-test.c++
@@ -2310,9 +2310,7 @@ KJ_TEST("Server: tail workers") {
                 `}
             )
           ],
-          logging = (
-            toServices = ["tail", "tail2"]
-          ),
+          tails = ["tail", "tail2"],
         )
       ),
       ( name = "tail",
diff --git a/src/workerd/server/server.c++ b/src/workerd/server/server.c++
index 4d2d4fb5aa9..9fcba74da3e 100644
--- a/src/workerd/server/server.c++
+++ b/src/workerd/server/server.c++
@@ -3228,24 +3228,36 @@ kj::Own<Server::Service> Server::makeWorker(kj::StringPtr name,
       }
     }
 
-    auto logging = conf.getLogging();
-    switch (logging.which()) {
-      case config::Worker::Logging::Which::NONE: {
-        break;
-      }
-      case config::Worker::Logging::Which::TO_SERVICE: {
-        result.loggingServices = kj::arr(&lookupService(logging.getToService(),
-            kj::str("Worker \"", name, "\"'s logging")));
-        break;
+    result.loggingServices = KJ_MAP(tail, conf.getTails()) {
+      return &lookupService(tail, kj::str("Worker \"", name, "\"'s tails"));
+    };
+
+    // Also check deprecated `logging` field.
+    if (conf.hasTails()) {
+      if (!conf.getLogging().isNone()) {
+        reportConfigError(kj::str("service ", name,
+            ": Config specifies both `logging` and `tails`. Please use only `tails`."));
       }
-      case config::Worker::Logging::Which::TO_SERVICES: {
-        auto list = logging.getToServices();
-        auto builder = kj::heapArrayBuilder<Service*>(list.size());
-        for (auto svc: list) {
-          builder.add(&lookupService(svc, kj::str("Worker \"", name, "\"'s logging")));
+    } else {
+      auto logging = conf.getLogging();
+      switch (logging.which()) {
+        case config::Worker::Logging::Which::NONE: {
+          break;
+        }
+        case config::Worker::Logging::Which::TO_SERVICE: {
+          result.loggingServices = kj::arr(
+              &lookupService(logging.getToService(), kj::str("Worker \"", name, "\"'s logging")));
+          break;
+        }
+        case config::Worker::Logging::Which::TO_SERVICES: {
+          auto list = logging.getToServices();
+          auto builder = kj::heapArrayBuilder<Service*>(list.size());
+          for (auto svc: list) {
+            builder.add(&lookupService(svc, kj::str("Worker \"", name, "\"'s logging")));
+          }
+          result.loggingServices = builder.finish();
+          break;
         }
-        result.loggingServices = builder.finish();
-        break;
       }
     }
 
diff --git a/src/workerd/server/workerd.capnp b/src/workerd/server/workerd.capnp
index a2453d7454f..56cb54ef3c7 100644
--- a/src/workerd/server/workerd.capnp
+++ b/src/workerd/server/workerd.capnp
@@ -630,8 +630,12 @@ struct Worker {
 
   moduleFallback @13 :Text;
 
-  # Tail worker configuration
+  tails @17 :List(ServiceDesignator);
+  # List of tail worker services that should receive tail events for this worker.
+  # See: https://developers.cloudflare.com/workers/observability/logs/tail-workers/
+
   logging :union {
+    # DEPRECATED: Use `tails` instead. `logging` does exactly the same thing.
     none @14 :Void;
     toService @15 :ServiceDesignator;
     toServices @16 :List(ServiceDesignator);