Add ability to customize worker prefix

src/Distributed.jl

@@ -68,6 +68,7 @@ export
     worker_id_from_socket,
     cluster_cookie,
     start_worker,
+    worker_output_hook,
 
 # Used only by shared arrays.
     check_same_host

src/cluster.jl

@@ -294,15 +294,53 @@ function start_worker(out::IO, cookie::AbstractString=readline(stdin); close_std
 end
 
 
+"""
+    worker_output_hook
+
+A `Ref` holding an optional callback function for customizing worker output display.
+When set to a function `f(ident::AbstractString, line::AbstractString) -> Bool`,
+it will be called for each line of worker output. If the function returns `true`,
+the line is considered handled and will not be printed with the default prefix.
+If the function returns `false` or if `worker_output_hook[]` is `nothing`,
+the default `"From worker \$ident: "` prefix is used.
+
+This allows test harnesses or other callers to provide richer context about
+what each worker is doing (e.g., which test is running) instead of just the worker id.
+
+# Example
+```julia
+Distributed.worker_output_hook[] = (ident, line) -> begin
+    test_name = get_current_test_for_worker(parse(Int, ident))
+    if test_name !== nothing
+        println("[", test_name, "] (worker ", ident, "): ", line)
+        return true
+    end
+    return false
+end
+```
+"""
+const worker_output_hook = Ref{Union{Nothing, Function}}(nothing)
+
 function redirect_worker_output(ident, stream)
     t = @async while !eof(stream)
         line = readline(stream)
-        if startswith(line, "      From worker ")
+        if startswith(line, "From worker ")
             # stdout's of "additional" workers started from an initial worker on a host are not available
             # on the master directly - they are routed via the initial worker's stdout.
             println(line)
         else
-            println("      From worker $(ident):\t$line")
+            handled = false
+            hook = worker_output_hook[]
+            if hook !== nothing
+                try
+                    handled = hook(ident, line)::Bool
+                catch
+                    handled = false
+                end
+            end
+            if !handled
+                println("  From worker $(ident):\t$line")
+            end
         end
     end
     errormonitor(t)

src/managers.jl

@@ -631,7 +631,7 @@ function connect(manager::ClusterManager, pid::Int, config::WorkerConfig)
 
     if config.io !== nothing
         let pid = pid
-            redirect_worker_output(pid, notnothing(config.io))
+            redirect_worker_output("$pid", notnothing(config.io))
         end
     end