Fix Log::Builder mutate user-provided broadcast backend

spec/std/log/builder_spec.cr

@@ -47,6 +47,25 @@ describe Log::Builder do
     log.level.should eq(s(:info))
   end
 
+  it "does not alter user-provided broadcast backend" do
+    builder = Log::Builder.new
+    a = Log::MemoryBackend.new
+    b = Log::MemoryBackend.new
+
+    broadcast = Log::BroadcastBackend.new
+    broadcast.append(a, :fatal)
+    previous_backends = broadcast.@backends.dup
+
+    builder.bind("db", :trace, broadcast)
+    builder.bind("db", :info, b)
+
+    log = builder.for("db")
+
+    backend = log.backend.should be_a(Log::BroadcastBackend)
+    backend.should_not be broadcast
+    broadcast.@backends.should eq(previous_backends)
+  end
+
   it "creates a log for broadcast backend" do
     builder = Log::Builder.new
     a = Log::MemoryBackend.new
@@ -61,9 +80,9 @@ describe Log::Builder do
     log = builder.for("db")
 
     backend = log.backend.should be_a(Log::BroadcastBackend)
-    backend.@backends.should eq({a => s(:fatal), b => s(:info)})
+    backend.@backends.should eq({broadcast => s(:trace), b => s(:info)})
     log.source.should eq("db")
-    log.level.should eq(s(:info))
+    log.level.should eq(s(:trace))
   end
 
   it "creates a log for same broadcast backend added multiple times" do

src/log/broadcast_backend.cr

@@ -9,6 +9,12 @@
 class Log::BroadcastBackend < Log::Backend
   property level : Severity? = nil
 
+  # :nodoc:
+  #
+  # Indicates whether this backend is used by a `Log` instance to distribute
+  # its messages to multiple backends.
+  property? internal = false
+
   @backends = Hash(Log::Backend, Severity).new
 
   def initialize

src/log/builder.cr

@@ -104,19 +104,16 @@ class Log::Builder
       # if the bind applies for the same backend, the last applied
       # level should be used
       log.initial_level = level
-    when BroadcastBackend
-      current_backend.append(backend, level)
-      # initial_level needs to be recomputed since the append_backend
-      # might be called with the same backend as before but with a
-      # different (higher) level
-      log.initial_level = current_backend.min_level
-      current_backend.level = log.changed_level
     else
-      broadcast = BroadcastBackend.new
-      broadcast.append(current_backend, log.initial_level)
+      broadcast = current_backend.as?(BroadcastBackend)
+      unless broadcast && broadcast.internal?
+        broadcast = BroadcastBackend.new
+        broadcast.internal = true
+        broadcast.append(current_backend, log.initial_level)
+        log.backend = broadcast
+      end
       broadcast.append(backend, level)
       broadcast.level = log.changed_level
-      log.backend = broadcast
       log.initial_level = broadcast.min_level
     end
   end