Add support to run pipelines using a local backend (#709)

This adds support for #559. I tested using [this .woodpecker.yml](https://git.exozy.me/Ta180m/Hello-world/src/branch/main/.woodpecker.yml) on my self-hosted [Woodpecker instance](https://ci.exozy.me/Ta180m/Hello-world). I was also able to get this to build [Hugo websites](https://ci.exozy.me/Ta180m/howtuwu/build/1). It's currently very simplistic but works!

close #559

docs/docs/20-usage/20-pipeline-syntax.md

@@ -164,7 +164,9 @@ pipeline:
 
 ### `image`
 
-Woodpecker uses Docker images for the build environment, for plugins and for service containers. The image field is exposed in the container blocks in the Yaml:
+With the `docker` backend, Woodpecker uses Docker images for the build environment, for plugins and for service containers. The image field is exposed in the container blocks in the Yaml:
+
+When using the `local` backend, the `image` entry is used to specify the shell, such as Bash or Fish, that is used to run the commands.
 
 ```diff
  pipeline:
@@ -406,7 +408,9 @@ For more details check the [matrix build docs](/docs/usage/matrix-builds/).
 
 ### `clone`
 
-Woodpecker automatically configures a default clone step if not explicitly defined. You can manually configure the clone step in your pipeline for customization:
+Woodpecker automatically configures a default clone step if not explicitly defined. When using the `local` backend, the [plugin-git](https://github.com/woodpecker-ci/plugin-git) binary must be on your `$PATH` for the default clone step to work. If not, you can still write a manual clone step.
+
+You can manually configure the clone step in your pipeline for customization:
 
 ```diff
 +clone:

docs/docs/30-administration/15-agent-config.md

@@ -153,4 +153,4 @@ Configures if the gRPC server certificate should be verified, only valid when `W
 ### `WOODPECKER_BACKEND`
 > Default: `auto-detect`
 
-Configures the backend engine to run pipelines on. Possible values are `auto-detect` or `docker`.
+Configures the backend engine to run pipelines on. Possible values are `auto-detect`, `docker`, or `local`.

pipeline/backend/backend.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 
 	"github.com/woodpecker-ci/woodpecker/pipeline/backend/docker"
+	"github.com/woodpecker-ci/woodpecker/pipeline/backend/local"
 	"github.com/woodpecker-ci/woodpecker/pipeline/backend/types"
 )
 
@@ -12,6 +13,7 @@ var engines map[string]types.Engine
 func init() {
 	loadedEngines := []types.Engine{
 		docker.New(),
+		local.New(),
 		// kubernetes.New(), // TODO: disabled for now as kubernetes backend has not been implemented yet
 	}
 

pipeline/backend/local/local.go

@@ -0,0 +1,111 @@
+package local
+
+import (
+	"context"
+	"encoding/base64"
+	"io"
+	"os"
+	"os/exec"
+	"strings"
+
+	"github.com/woodpecker-ci/woodpecker/pipeline/backend/types"
+	"github.com/woodpecker-ci/woodpecker/server"
+)
+
+type local struct {
+	cmd    *exec.Cmd
+	output io.ReadCloser
+}
+
+// make sure local implements Engine
+var _ types.Engine = &local{}
+
+// New returns a new local Engine.
+func New() types.Engine {
+	return &local{}
+}
+
+func (e *local) Name() string {
+	return "local"
+}
+
+func (e *local) IsAvailable() bool {
+	return true
+}
+
+func (e *local) Load() error {
+	return nil
+}
+
+// Setup the pipeline environment.
+func (e *local) Setup(ctx context.Context, proc *types.Config) error {
+	return nil
+}
+
+// Exec the pipeline step.
+func (e *local) Exec(ctx context.Context, proc *types.Step) error {
+	// Get environment variables
+	Command := []string{}
+	for a, b := range proc.Environment {
+		if a != "HOME" && a != "SHELL" { // Don't override $HOME and $SHELL
+			Command = append(Command, a+"="+b)
+		}
+	}
+
+	// Get default clone image
+	defaultCloneImage := "docker.io/woodpeckerci/plugin-git:latest"
+	if len(server.Config.Pipeline.DefaultCloneImage) > 0 {
+		defaultCloneImage = server.Config.Pipeline.DefaultCloneImage
+	}
+
+	if proc.Image == defaultCloneImage {
+		// Default clone step
+		Command = append(Command, "CI_WORKSPACE=/tmp/woodpecker/"+proc.Environment["CI_REPO"])
+		Command = append(Command, "plugin-git")
+	} else {
+		// Use "image name" as run command
+		Command = append(Command, proc.Image[18:len(proc.Image)-7])
+		Command = append(Command, "-c")
+
+		// Decode script and delete initial lines
+		// Deleting the initial lines removes netrc support but adds compatibility for more shells like fish
+		Script, _ := base64.RawStdEncoding.DecodeString(proc.Environment["CI_SCRIPT"])
+		Command = append(Command, string(Script)[strings.Index(string(Script), "\n\n")+2:])
+	}
+
+	// Prepare command
+	e.cmd = exec.CommandContext(ctx, "/bin/env", Command...)
+
+	// Prepare working directory
+	if proc.Image == defaultCloneImage {
+		e.cmd.Dir = "/tmp/woodpecker/" + proc.Environment["CI_REPO_OWNER"]
+	} else {
+		e.cmd.Dir = "/tmp/woodpecker/" + proc.Environment["CI_REPO"]
+	}
+	_ = os.MkdirAll(e.cmd.Dir, 0o700)
+
+	// Get output and redirect Stderr to Stdout
+	e.output, _ = e.cmd.StdoutPipe()
+	e.cmd.Stderr = e.cmd.Stdout
+
+	return e.cmd.Start()
+}
+
+// Wait for the pipeline step to complete and returns
+// the completion results.
+func (e *local) Wait(context.Context, *types.Step) (*types.State, error) {
+	return &types.State{
+		Exited: true,
+	}, e.cmd.Wait()
+}
+
+// Tail the pipeline step logs.
+func (e *local) Tail(context.Context, *types.Step) (io.ReadCloser, error) {
+	return e.output, nil
+}
+
+// Destroy the pipeline environment.
+func (e *local) Destroy(context.Context, *types.Config) error {
+	os.RemoveAll(e.cmd.Dir)
+	return nil
+}