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54fc146432
Nomad agents expect to receive `SIGHUP` to reload their configuration. The signal handler for this is installed fairly late in agent startup, after the client or server components are up and running. This means that configuration management tools can potentially reload the configuration before the agent can handle it, causing the agent to crash. We don't want to allow configuration reload during client or server component startup, because it would significantly complicate initialization. Instead, we'll implement the systemd notify protocol. This causes systemd to block sending configuration reload signals until the agent is actually ready. Users can still bypass this by sending signals directly. Note that there are several Go libraries that implement the sdnotify protocol, but most are part of much larger projects which would create a lot of dependabot burden. The bits of the protocol we need are extremely simple to implement in a just a couple of functions. For non-Linux or non-systemd Linux systems, this feature is a no-op. In future work we could potentially implement service notification for Windows as well. Fixes: https://github.com/hashicorp/nomad/issues/3885
41 lines
1004 B
Go
41 lines
1004 B
Go
// Copyright (c) HashiCorp, Inc.
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// SPDX-License-Identifier: BUSL-1.1
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//go:build linux
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package agent
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import (
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"io"
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"net"
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"os"
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"time"
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)
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const sdNotifySocketEnvVar = "NOTIFY_SOCKET"
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// openNotify opens the systemd notify socket only if the expected env var has
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// been set, because the systemd unit file is Type=notify or Type=notify-reload
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// (systemd 253+). It then unsets the env var in the agent process so that child
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// processes can't accidentally inherit it. This function returns (nil, nil) if
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// the env var isn't set.
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func openNotify() (io.WriteCloser, error) {
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socketPath := os.Getenv(sdNotifySocketEnvVar)
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if socketPath == "" {
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return nil, nil
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}
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defer os.Unsetenv(sdNotifySocketEnvVar)
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conn, err := net.DialTimeout("unixgram", socketPath, time.Second)
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return conn, err
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}
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// sdNotify sends the message on the systemd notify socket, and gracefully
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// handles a nil socket
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func sdNotify(w io.Writer, msg string) {
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if w == nil || msg == "" {
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return
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}
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w.Write([]byte(msg))
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}
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