Add periodic callhome functionality (#14918)

* Add periodic callhome functionality

Periodically (every 24hrs by default), fetch callhome information and
upload it to SUBNET.

New config keys under the `callhome` subsystem:

enable - Set to `on` for enabling callhome. Default `off`
frequency - Interval between callhome cycles. Default `24h`

* Improvements based on review comments

- Update `enableCallhome` safely
- Rename pctx to ctx
- Block during execution of callhome
- Store parsed proxy URL in global subnet config
- Store callhome URL(s) in constants
- Use existing global transport
- Pass auth token to subnetPostReq
- Use `config.EnableOn` instead of `"on"`

* Use atomic package instead of lock

* Use uber atomic package

* Use `Cancel` instead of `cancel`

Co-authored-by: Harshavardhana <harsha@minio.io>

Co-authored-by: Harshavardhana <harsha@minio.io>
Co-authored-by: Aditya Manthramurthy <donatello@users.noreply.github.com>

internal/config/callhome/callhome.go

@@ -0,0 +1,65 @@
+// Copyright (c) 2015-2022 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 callhome
+
+import (
+	"time"
+
+	"github.com/minio/minio/internal/config"
+	"github.com/minio/pkg/env"
+)
+
+// Callhome related keys
+const (
+	Enable    = "enable"
+	Frequency = "frequency"
+)
+
+// DefaultKVS - default KV config for subnet settings
+var DefaultKVS = config.KVS{
+	config.KV{
+		Key:   Enable,
+		Value: "off",
+	},
+	config.KV{
+		Key:   Frequency,
+		Value: "24h",
+	},
+}
+
+// Config represents the subnet related configuration
+type Config struct {
+	// Flag indicating whether callhome is enabled.
+	Enable bool `json:"enable"`
+
+	// The interval between callhome cycles
+	Frequency time.Duration `json:"frequency"`
+}
+
+// LookupConfig - lookup config and override with valid environment settings if any.
+func LookupConfig(kvs config.KVS) (cfg Config, err error) {
+	if err = config.CheckValidKeys(config.CallhomeSubSys, kvs, DefaultKVS); err != nil {
+		return cfg, err
+	}
+
+	cfg.Enable = env.Get(config.EnvMinIOCallhomeEnable,
+		kvs.GetWithDefault(Enable, DefaultKVS)) == config.EnableOn
+	cfg.Frequency, err = time.ParseDuration(env.Get(config.EnvMinIOCallhomeFrequency,
+		kvs.GetWithDefault(Frequency, DefaultKVS)))
+	return cfg, err
+}