Install Syncthing as a service using NSSM

Install-SyncthingService.ps1

<#
.SYNOPSIS
    Installs Syncthing as a Windows service

.DESCRIPTION
    Uses NSSM (Non-Sucking Service Manager) to set up Syncthing as a Windows
    service. Requires both Syncthing and NSSM to already be installed. nssm.exe
    must be available via the PATH environment variable.

.PARAMETER InstallDir
    The Syncthing installation directory where you can find syncthing.exe

.PARAMETER LogPath
    The location where you want to save log files. Automatic log rotation is
    set up when log file size hits 512 KB. If not specified, defaults to the
    installation directory under the "log" folder.

.PARAMETER ServiceName
    Defaults to "Syncthing"

.EXAMPLE
    .\Install-SyncthingService.ps1 C:\Syncthing

    Installs the Syncthing executable at C:\Syncthing\syncthing.exe as a Windows service.

.NOTES
    MIT License

    Copyright (c) 2019 Pathway Services Inc.

    Permission is hereby granted, free of charge, to any person obtaining a copy
    of this software and associated documentation files (the "Software"), to deal
    in the Software without restriction, including without limitation the rights
    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
    copies of the Software, and to permit persons to whom the Software is
    furnished to do so, subject to the following conditions:

    The above copyright notice and this permission notice shall be included in all
    copies or substantial portions of the Software.

    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
    SOFTWARE.
#>

#Requires -RunAsAdministrator
[CmdletBinding()]
param(
    [Parameter(Mandatory=$True)]
    [string]$InstallDir,

    [Parameter()]
    [string]$LogPath,

    [Parameter()]
    [string]$ServiceName = "Syncthing"
)

$ErrorActionPreference = "Stop"
Set-StrictMode -Version 4.0

if (!(Get-Command "nssm" -ErrorAction SilentlyContinue)) {
    throw "nssm.exe not found. Make sure it is included in the PATH environment variable."
}

function Invoke-Nssm {
    nssm $args
    if ($LASTEXITCODE -ne 0) {
        throw "nssm exited with code $LASTEXITCODE"
    }
}

$syncthingExe = Join-Path $InstallDir "syncthing.exe"
if (!(Test-Path $syncthingExe)) {
    throw "$syncthingExe not found."
}

$homeDir = Join-Path $env:LOCALAPPDATA "Syncthing"

if (!$LogPath) {
    $LogPath = Join-Path $InstallDir "log\log.txt"
    New-Item -Type File $LogPath -Force
}

$serviceCreds = Get-Credential -Message "Enter credentials under which the Syncthing service should run, or press cancel to run as LOCALSYSTEM (not recommended)."

Invoke-Nssm install $ServiceName $syncthingExe
Invoke-Nssm set $ServiceName AppParameters "-no-restart -no-browser -home=""$homeDir"""
Invoke-Nssm set $ServiceName AppDirectory $InstallDir
Invoke-Nssm set $ServiceName AppExit "Default" "Exit"
Invoke-Nssm set $ServiceName AppExit 1 "Restart"
Invoke-Nssm set $ServiceName AppExit 2 "Restart"
Invoke-Nssm set $ServiceName AppExit 3 "Restart"
Invoke-Nssm set $ServiceName AppExit 4 "Restart"
Invoke-Nssm set $ServiceName AppStdout $LogPath
Invoke-Nssm set $ServiceName AppStderr $LogPath
Invoke-Nssm set $ServiceName AppStopMethodConsole 10000
Invoke-Nssm set $ServiceName AppStopMethodWindow 10000
Invoke-Nssm set $ServiceName AppStopMethodThreads 10000
Invoke-Nssm set $ServiceName AppRotateFiles 1
Invoke-Nssm set $ServiceName AppRotateOnline 1
Invoke-Nssm set $ServiceName AppRotateBytes 524288
Invoke-Nssm set $ServiceName DisplayName $ServiceName
Invoke-Nssm set $ServiceName Start SERVICE_DELAYED_AUTO_START
Invoke-Nssm set $ServiceName Type SERVICE_WIN32_OWN_PROCESS

if ($serviceCreds) {
    # This is the simplest way to get the plain-text password that I'm aware of:
    $serviceCreds = New-Object System.Net.NetworkCredential `
        -ArgumentList $serviceCreds.UserName, $serviceCreds.Password

    $plaintextPassword = $serviceCreds.Password
    Invoke-Nssm set $ServiceName ObjectName $serviceCreds.UserName $plaintextPassword
}