cmd/syncthing: Add CLI completion functionality (fixes #8616) (#9226)

### Purpose

This implements CLI completion using the Kongplete module. As a side
effect a CLI structure for syncthing/cli was created for kongplete to be
able to parse and implement CLI completion.

### Testing

I've tested the autocompletion manually, and it had worked, but I hadn't
added any tests so as to test it automatically. Additionally, I ran `go
run build.go test` with all tests passing.

3 files changed, 23 insertions, 4 deletions

diff --git a/cmd/syncthing/main.go b/cmd/syncthing/main.go
index ab3c76e73..6d2216290 100644
--- a/cmd/syncthing/main.go
+++ b/cmd/syncthing/main.go
@@ -32,6 +32,7 @@ import (
 
 	"github.com/alecthomas/kong"
 	"github.com/thejerf/suture/v4"
+	"github.com/willabides/kongplete"
 
 	"github.com/syncthing/syncthing/cmd/syncthing/cli"
 	"github.com/syncthing/syncthing/cmd/syncthing/cmdutil"
@@ -133,10 +134,11 @@ var (
 // commands and options here are top level commands to syncthing.
 // Cli is just a placeholder for the help text (see main).
 var entrypoint struct {
-	Serve    serveOptions `cmd:"" help:"Run Syncthing"`
-	Generate generate.CLI `cmd:"" help:"Generate key and config, then exit"`
-	Decrypt  decrypt.CLI  `cmd:"" help:"Decrypt or verify an encrypted folder"`
-	Cli      cli.CLI      `cmd:"" help:"Command line interface for Syncthing"`
+	Serve              serveOptions                 `cmd:"" help:"Run Syncthing"`
+	Generate           generate.CLI                 `cmd:"" help:"Generate key and config, then exit"`
+	Decrypt            decrypt.CLI                  `cmd:"" help:"Decrypt or verify an encrypted folder"`
+	Cli                cli.CLI                      `cmd:"" help:"Command line interface for Syncthing"`
+	InstallCompletions kongplete.InstallCompletions `cmd:"" help:"Print commands to install shell completions"`
 }
 
 // serveOptions are the options for the `syncthing serve` command.
@@ -247,6 +249,7 @@ func main() {
 		log.Fatal(err)
 	}
 
+	kongplete.Complete(parser)
 	ctx, err := parser.Parse(args)
 	parser.FatalIfErrorf(err)
 	ctx.BindTo(l, (*logger.Logger)(nil)) // main logger available to subcommands
diff --git a/go.mod b/go.mod
index 1e1928626..aac37007c 100644
--- a/go.mod
+++ b/go.mod
@@ -37,6 +37,7 @@ require (
 	github.com/thejerf/suture/v4 v4.0.2
 	github.com/urfave/cli v1.22.14
 	github.com/vitrun/qart v0.0.0-20160531060029-bf64b92db6b0
+	github.com/willabides/kongplete v0.3.0
 	golang.org/x/crypto v0.17.0
 	golang.org/x/exp v0.0.0-20240103183307-be819d1f06fc
 	golang.org/x/net v0.19.0
@@ -64,6 +65,8 @@ require (
 	github.com/golang/snappy v0.0.4 // indirect
 	github.com/google/pprof v0.0.0-20231229205709-960ae82b1e42 // indirect
 	github.com/google/uuid v1.5.0 // indirect
+	github.com/hashicorp/errwrap v1.0.0 // indirect
+	github.com/hashicorp/go-multierror v1.0.0 // indirect
 	github.com/klauspost/cpuid/v2 v2.2.6 // indirect
 	github.com/matttproud/golang_protobuf_extensions/v2 v2.0.0 // indirect
 	github.com/nxadm/tail v1.4.11 // indirect
@@ -71,11 +74,13 @@ require (
 	github.com/onsi/gomega v1.30.0 // indirect
 	github.com/oschwald/maxminddb-golang v1.12.0 // indirect
 	github.com/pkg/errors v0.9.1 // indirect
+	github.com/posener/complete v1.2.3 // indirect
 	github.com/power-devops/perfstat v0.0.0-20221212215047-62379fc7944b // indirect
 	github.com/prometheus/client_model v0.5.0 // indirect
 	github.com/prometheus/common v0.45.0 // indirect
 	github.com/prometheus/procfs v0.12.0 // indirect
 	github.com/quic-go/qtls-go1-20 v0.4.1 // indirect
+	github.com/riywo/loginshell v0.0.0-20200815045211-7d26008be1ab // indirect
 	github.com/russross/blackfriday/v2 v2.1.0 // indirect
 	github.com/yusufpapurcu/wmi v1.2.3 // indirect
 	go.uber.org/mock v0.4.0 // indirect
diff --git a/go.sum b/go.sum
index fe1833b54..516ddaa3e 100644
--- a/go.sum
+++ b/go.sum
@@ -92,6 +92,10 @@ github.com/google/uuid v1.5.0 h1:1p67kYwdtXjb0gL0BPiP1Av9wiZPo5A8z2cWkTZ+eyU=
 github.com/google/uuid v1.5.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
 github.com/greatroar/blobloom v0.7.2 h1:F30MGLHOcb4zr0pwCPTcKdlTM70rEgkf+LzdUPc5ss8=
 github.com/greatroar/blobloom v0.7.2/go.mod h1:mjMJ1hh1wjGVfr93QIHJ6FfDNVrA0IELv8OvMHJxHKs=
+github.com/hashicorp/errwrap v1.0.0 h1:hLrqtEDnRye3+sgx6z4qVLNuviH3MR5aQ0ykNJa/UYA=
+github.com/hashicorp/errwrap v1.0.0/go.mod h1:YH+1FKiLXxHSkmPseP+kNlulaMuP3n2brvKWEqk/Jc4=
+github.com/hashicorp/go-multierror v1.0.0 h1:iVjPR7a6H0tWELX5NxNe7bYopibicUzc7uPribsnS6o=
+github.com/hashicorp/go-multierror v1.0.0/go.mod h1:dHtQlpGsu+cZNNAkkCN/P3hoUDHhCYQXV3UM06sGGrk=
 github.com/hashicorp/golang-lru/v2 v2.0.7 h1:a+bsQ5rvGLjzHuww6tVxozPZFVghXaHOwFs4luLUK2k=
 github.com/hashicorp/golang-lru/v2 v2.0.7/go.mod h1:QeFd9opnmA6QUJc5vARoKUSoFhyfM2/ZepoAG6RGpeM=
 github.com/hexops/gotextdiff v1.0.3 h1:gitA9+qJrrTCsiCl7+kh75nPqQt1cx4ZkudSTLoUqJM=
@@ -154,6 +158,8 @@ github.com/pkg/errors v0.9.1 h1:FEBLx1zS214owpjy7qsBeixbURkuhQAwrK5UwLGTwt4=
 github.com/pkg/errors v0.9.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/posener/complete v1.2.3 h1:NP0eAhjcjImqslEwo/1hq7gpajME0fTLTezBKDqfXqo=
+github.com/posener/complete v1.2.3/go.mod h1:WZIdtGGp+qx0sLrYKtIRAruyNpv6hFCicSgv7Sy7s/s=
 github.com/power-devops/perfstat v0.0.0-20210106213030-5aafc221ea8c/go.mod h1:OmDBASR4679mdNQnz2pUhc2G8CO2JrUAVFDRBDP/hJE=
 github.com/power-devops/perfstat v0.0.0-20221212215047-62379fc7944b h1:0LFwY6Q3gMACTjAbMZBjXAqTOzOwFaj2Ld6cjeQ7Rig=
 github.com/power-devops/perfstat v0.0.0-20221212215047-62379fc7944b/go.mod h1:OmDBASR4679mdNQnz2pUhc2G8CO2JrUAVFDRBDP/hJE=
@@ -171,6 +177,8 @@ github.com/quic-go/quic-go v0.40.1 h1:X3AGzUNFs0jVuO3esAGnTfvdgvL4fq655WaOi1snv1
 github.com/quic-go/quic-go v0.40.1/go.mod h1:PeN7kuVJ4xZbxSv/4OX6S1USOX8MJvydwpTx31vx60c=
 github.com/rcrowley/go-metrics v0.0.0-20201227073835-cf1acfcdf475 h1:N/ElC8H3+5XpJzTSTfLsJV/mx9Q9g7kxmchpfZyxgzM=
 github.com/rcrowley/go-metrics v0.0.0-20201227073835-cf1acfcdf475/go.mod h1:bCqnVzQkZxMG4s8nGwiZ5l3QUCyqpo9Y+/ZMZ9VjZe4=
+github.com/riywo/loginshell v0.0.0-20200815045211-7d26008be1ab h1:ZjX6I48eZSFetPb41dHudEyVr5v953N15TsNZXlkcWY=
+github.com/riywo/loginshell v0.0.0-20200815045211-7d26008be1ab/go.mod h1:/PfPXh0EntGc3QAAyUaviy4S9tzy4Zp0e2ilq4voC6E=
 github.com/russross/blackfriday/v2 v2.1.0 h1:JIOH55/0cWyOuilr9/qlrm0BSXldqnqwMsf35Ld67mk=
 github.com/russross/blackfriday/v2 v2.1.0/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=
 github.com/sclevine/spec v1.4.0 h1:z/Q9idDcay5m5irkZ28M7PtQM4aOISzOpj4bUPkDee8=
@@ -181,6 +189,7 @@ github.com/shoenig/test v0.6.4/go.mod h1:byHiCGXqrVaflBLAMq/srcZIHynQPQgeyvkvXnj
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/objx v0.4.0/go.mod h1:YvHI0jy2hoMjB+UWwv71VJQ9isScKT/TqJzVSSt89Yw=
 github.com/stretchr/objx v0.5.0/go.mod h1:Yh+to48EsGEfYuaHDzXPcE3xhTkx73EhmCGUpEOglKo=
+github.com/stretchr/testify v1.4.0/go.mod h1:j7eGeouHqKxXV5pUuKE4zz7dFj8WfuZ+81PSLYec5m4=
 github.com/stretchr/testify v1.5.1/go.mod h1:5W2xD1RspED5o8YsWQXVCued0rvSQ+mT+I5cxcmMvtA=
 github.com/stretchr/testify v1.6.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
 github.com/stretchr/testify v1.7.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
@@ -201,6 +210,8 @@ github.com/urfave/cli v1.22.14 h1:ebbhrRiGK2i4naQJr+1Xj92HXZCrK7MsyTS/ob3HnAk=
 github.com/urfave/cli v1.22.14/go.mod h1:X0eDS6pD6Exaclxm99NJ3FiCDRED7vIHpx2mDOHLvkA=
 github.com/vitrun/qart v0.0.0-20160531060029-bf64b92db6b0 h1:okhMind4q9H1OxF44gNegWkiP4H/gsTFLalHFa4OOUI=
 github.com/vitrun/qart v0.0.0-20160531060029-bf64b92db6b0/go.mod h1:TTbGUfE+cXXceWtbTHq6lqcTvYPBKLNejBEbnUsQJtU=
+github.com/willabides/kongplete v0.3.0 h1:8dJZ0r2a2YnSdYCQk9TjQDKzLrj1zUvIOPIG3bOV75c=
+github.com/willabides/kongplete v0.3.0/go.mod h1:VPdrG6LY+tP0LMkSBuTgIQ8c6+P8wvIDHVJzDdDh9Fw=
 github.com/yuin/goldmark v1.1.27/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=
 github.com/yuin/goldmark v1.2.1/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=
 github.com/yuin/goldmark v1.4.13/go.mod h1:6yULJ656Px+3vBD8DxQVa3kxgyrAnzto9xy5taEt/CY=