[documentation] [feature]: sample-lnd.conf makes misleading or wrong statement about env var expansion

[dienummer]

Background

sample-lnd.conf contains the statement "Environment variables are expanded so they may be used" near the top of the file:

lnd/sample-lnd.conf

Line 24 in 2b74a34

; on Plan9. Environment variables are expanded so they may be used. NOTE:

However, this is only true for path-type variables, which call CleanAndExpandPath when they are parsed:

lnd/config.go

Line 1749 in 7af1957

func CleanAndExpandPath(path string) string {

, and for the special case of the rpc user and rpc password, parsed by supplyEnvValue

lnd/config.go

Line 1951 in 7af1957

func supplyEnvValue(value string) string {

lnd/config.go

Lines 1831 to 1834 in 7af1957

	// Resolves environment variable references in RPCUser
	// and RPCPass fields.
	conf.RPCUser = supplyEnvValue(conf.RPCUser)
	conf.RPCPass = supplyEnvValue(conf.RPCPass)

It could be extremely important to allow env var substitution also for backend RPC hostname, ZMQ host and port, etc. when running in a container orchestration system or other professional setup.

In general, storing critical elements of the config in the environment is preferred (https://12factor.net/config)

Your environment

• version of lnd: v0.17.4-beta
• which operating system (uname -a on *Nix) Linux ubuntu 6.8.0-22-generic #22-Ubuntu SMP PREEMPT_DYNAMIC Thu Apr 4 22:30:32 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux
• version of btcd, bitcoind, or other backend 26.1
• any other relevant environment details (N/A)

Steps to reproduce

N/A; see https://github.com/lightningnetwork/lnd/blob/7af195768aa3e06ec447615ce25dd32343d8bd59/config.go and in particular review lines

lnd/config.go

Lines 927 to 945 in 7af1957

	cfg.DataDir = CleanAndExpandPath(cfg.DataDir)
	cfg.TLSCertPath = CleanAndExpandPath(cfg.TLSCertPath)
	cfg.TLSKeyPath = CleanAndExpandPath(cfg.TLSKeyPath)
	cfg.LetsEncryptDir = CleanAndExpandPath(cfg.LetsEncryptDir)
	cfg.AdminMacPath = CleanAndExpandPath(cfg.AdminMacPath)
	cfg.ReadMacPath = CleanAndExpandPath(cfg.ReadMacPath)
	cfg.InvoiceMacPath = CleanAndExpandPath(cfg.InvoiceMacPath)
	cfg.LogDir = CleanAndExpandPath(cfg.LogDir)
	cfg.BtcdMode.Dir = CleanAndExpandPath(cfg.BtcdMode.Dir)
	cfg.BitcoindMode.Dir = CleanAndExpandPath(cfg.BitcoindMode.Dir)
	cfg.BitcoindMode.ConfigPath = CleanAndExpandPath(
	cfg.BitcoindMode.ConfigPath,
	)
	cfg.BitcoindMode.RPCCookie = CleanAndExpandPath(cfg.BitcoindMode.RPCCookie)
	cfg.Tor.PrivateKeyPath = CleanAndExpandPath(cfg.Tor.PrivateKeyPath)
	cfg.Tor.WatchtowerKeyPath = CleanAndExpandPath(cfg.Tor.WatchtowerKeyPath)
	cfg.Watchtower.TowerDir = CleanAndExpandPath(cfg.Watchtower.TowerDir)
	cfg.BackupFilePath = CleanAndExpandPath(cfg.BackupFilePath)
	cfg.WalletUnlockPasswordFile = CleanAndExpandPath(

which are distinct from non-path type config settings (with the exception of RPCuser and RPCpass, which are the only other elements parsed from environment)

Expected behaviour

I expect env var expansion / substitution to work for all config settings so that I can more easily deploy LND in a professionally-managed container orchestration setup.

I would also expect that RPCUser and RPCPass config item env var expansion be documented.

Actual behaviour

Documentation suggests any config item could undergo variable expansion.

Only path-type vars, plus RPCUser and RPCPass undergo env var substitution.

Documentation does not mention that RPCUser and RPCPass undergo env var substitution.