logger: make AddSystemTarget idempotent

Subscribe re-registered the console target on every console-log subscription, producing duplicate minio_logger_webhook_* series on each /minio/metrics/v3 scrape. Fixes #150

Signed-off-by: nikitapogromsky <129324283+nikitapogromsky@users.noreply.github.com>

internal/logger/targets.go

@@ -59,11 +59,35 @@ func (tl *targetsList) get() []Target {
 	return tl.list
 }
 
-func (tl *targetsList) add(t Target) {
+// contains reports whether t is already registered.
+func (tl *targetsList) contains(t Target) bool {
+	tl.mu.RLock()
+	defer tl.mu.RUnlock()
+
+	return tl.indexOf(t) >= 0
+}
+
+// addIfAbsent appends t unless it is already registered.
+// Returns true if t was added.
+func (tl *targetsList) addIfAbsent(t Target) bool {
 	tl.mu.Lock()
 	defer tl.mu.Unlock()
 
+	if tl.indexOf(t) >= 0 {
+		return false
+	}
 	tl.list = append(tl.list, t)
+	return true
+}
+
+// indexOf must be called with tl.mu held.
+func (tl *targetsList) indexOf(t Target) int {
+	for i, existing := range tl.list {
+		if existing == t {
+			return i
+		}
+	}
+	return -1
 }
 
 func (tl *targetsList) set(tgts []Target) {
@@ -125,8 +149,14 @@ func CurrentStats() map[string]types.TargetStats {
 }
 
 // AddSystemTarget adds a new logger target to the
-// list of enabled loggers
+// list of enabled loggers. Adding a target that is already
+// registered is a no-op, so callers may safely re-register
+// long-lived targets such as the console logger.
 func AddSystemTarget(ctx context.Context, t Target) error {
+	if systemTargets.contains(t) {
+		return nil
+	}
+
 	if err := t.Init(ctx); err != nil {
 		return err
 	}
@@ -137,7 +167,7 @@ func AddSystemTarget(ctx context.Context, t Target) error {
 		}
 	}
 
-	systemTargets.add(t)
+	systemTargets.addIfAbsent(t)
 	return nil
 }
 

internal/logger/targets_test.go

@@ -0,0 +1,105 @@
+// Copyright (c) 2015-2026 MinIO, Inc.
+//
+// This file is part of MinIO Object Storage stack
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package logger
+
+import (
+	"context"
+	"sync"
+	"sync/atomic"
+	"testing"
+
+	types "github.com/minio/minio/internal/logger/target/loggertypes"
+)
+
+// fakeTarget is a minimal Target used to exercise the registry.
+type fakeTarget struct {
+	name  string
+	kind  types.TargetType
+	inits atomic.Int32
+}
+
+func (f *fakeTarget) String() string                  { return f.name }
+func (f *fakeTarget) Endpoint() string                { return "" }
+func (f *fakeTarget) Stats() types.TargetStats        { return types.TargetStats{} }
+func (f *fakeTarget) Init(context.Context) error      { f.inits.Add(1); return nil }
+func (f *fakeTarget) IsOnline(context.Context) bool   { return true }
+func (f *fakeTarget) Cancel()                         {}
+func (f *fakeTarget) Send(context.Context, any) error { return nil }
+func (f *fakeTarget) Type() types.TargetType          { return f.kind }
+
+// swapSystemTargets isolates the package-level registry for a test.
+func swapSystemTargets(t *testing.T) {
+	t.Helper()
+	prevTargets, prevConsole := systemTargets, consoleTgt
+	systemTargets, consoleTgt = newTargetsList(), nil
+	t.Cleanup(func() {
+		systemTargets, consoleTgt = prevTargets, prevConsole
+	})
+}
+
+// AddSystemTarget must not register the same target twice: the console
+// logger is re-added on every console-log subscription, and duplicates
+// surface as identical series in the /logger/webhook metrics collector.
+func TestAddSystemTargetIdempotent(t *testing.T) {
+	swapSystemTargets(t)
+	ctx := context.Background()
+
+	console := &fakeTarget{name: "console+http", kind: types.TargetConsole}
+	for range 3 {
+		if err := AddSystemTarget(ctx, console); err != nil {
+			t.Fatalf("AddSystemTarget: %v", err)
+		}
+	}
+	if got := len(SystemTargets()); got != 1 {
+		t.Fatalf("expected 1 system target after repeated add, got %d", got)
+	}
+	if got := console.inits.Load(); got != 1 {
+		t.Fatalf("expected Init to run once, ran %d times", got)
+	}
+	if consoleTgt != console {
+		t.Fatalf("consoleTgt not set to the console target")
+	}
+
+	other := &fakeTarget{name: "other", kind: types.TargetHTTP}
+	if err := AddSystemTarget(ctx, other); err != nil {
+		t.Fatalf("AddSystemTarget: %v", err)
+	}
+	if got := len(SystemTargets()); got != 2 {
+		t.Fatalf("expected 2 distinct system targets, got %d", got)
+	}
+}
+
+func TestAddSystemTargetConcurrent(t *testing.T) {
+	swapSystemTargets(t)
+	ctx := context.Background()
+
+	console := &fakeTarget{name: "console+http", kind: types.TargetConsole}
+	var wg sync.WaitGroup
+	for range 32 {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			_ = AddSystemTarget(ctx, console)
+		}()
+	}
+	wg.Wait()
+
+	if got := len(SystemTargets()); got != 1 {
+		t.Fatalf("expected 1 system target after concurrent adds, got %d", got)
+	}
+}