cmd: honor zero as an unlimited memory setting (#2598)

Signed-off-by: Jan-Otto Kröpke <mail@jkroepke.de>

cmd/windows_exporter/main.go

@@ -25,6 +25,7 @@ import (
 	"fmt"
 	"log/slog"
 	"maps"
+	"math"
 	"net/http"
 	"net/http/pprof"
 	"os"
@@ -127,7 +128,7 @@ func run(ctx context.Context, args []string) int {
 		).Default("normal").String()
 		memoryLimit = app.Flag(
 			"process.memory-limit",
-			"Limit memory usage in bytes. This is a soft-limit and not guaranteed. 0 means no limit. Read more at https://pkg.go.dev/runtime/debug#SetMemoryLimit .",
+			"Limit memory usage in bytes. This is a soft-limit and not guaranteed. 0 means no limit. Negative values are invalid. Read more at https://pkg.go.dev/runtime/debug#SetMemoryLimit .",
 		).Default("200000000").Int64()
 	)
 
@@ -153,7 +154,11 @@ func run(ctx context.Context, args []string) int {
 		return 1
 	}
 
-	debug.SetMemoryLimit(*memoryLimit)
+	if err := setProcessMemoryLimit(*memoryLimit); err != nil {
+		logStartupError(ctx, "Invalid process memory limit", err)
+
+		return 1
+	}
 
 	logger, err := log.New(logConfig)
 	if err != nil {
@@ -400,3 +405,18 @@ func expandEnabledCollectors(enabled string) []string {
 
 	return slices.Compact(strings.Split(expanded, ","))
 }
+
+// setProcessMemoryLimit applies the CLI contract: zero disables the soft limit.
+func setProcessMemoryLimit(limit int64) error {
+	if limit < 0 {
+		return errors.New("process.memory-limit must be non-negative")
+	}
+
+	if limit == 0 {
+		limit = math.MaxInt64
+	}
+
+	debug.SetMemoryLimit(limit)
+
+	return nil
+}

cmd/windows_exporter/memory_limit_test.go

@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: Apache-2.0
+//
+// Copyright The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build windows
+
+package main
+
+import (
+	"math"
+	"runtime/debug"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestProcessMemoryLimit(t *testing.T) {
+	// This setting is process-wide, so the cases must run sequentially.
+	previous := debug.SetMemoryLimit(-1)
+
+	t.Cleanup(func() { debug.SetMemoryLimit(previous) })
+
+	require.NoError(t, setProcessMemoryLimit(0))
+	require.Equal(t, int64(math.MaxInt64), debug.SetMemoryLimit(-1))
+	require.NoError(t, setProcessMemoryLimit(1<<30))
+	require.Equal(t, int64(1<<30), debug.SetMemoryLimit(-1))
+	require.ErrorContains(t, setProcessMemoryLimit(-1), "non-negative")
+	require.Equal(t, int64(1<<30), debug.SetMemoryLimit(-1))
+}