fix: Python local buildpack build

pkg/builders/buildpacks/builder.go

@@ -120,10 +120,6 @@ var DefaultLifecycleImage = "docker.io/buildpacksio/lifecycle:553c041"
 
 // Build the Function at path.
 func (b *Builder) Build(ctx context.Context, f fn.Function, platforms []fn.Platform) (err error) {
-	if f.Runtime == "python" {
-		return fmt.Errorf("python is not currently supported with pack builder (use host or s2i builder instead")
-	}
-
 	if len(platforms) != 0 {
 		return errors.New("the pack builder does not support specifying target platforms directly")
 	}
@@ -186,6 +182,10 @@ func (b *Builder) Build(ctx context.Context, f fn.Function, platforms []fn.Platf
 		opts.ContainerConfig.Network = "host"
 	}
 
+	if _, ok := opts.Env["BPE_DEFAULT_LISTEN_ADDRESS"]; !ok {
+		opts.Env["BPE_DEFAULT_LISTEN_ADDRESS"] = "[::]:8080"
+	}
+
 	var bindings = make([]string, 0, len(f.Build.Mounts))
 	for _, m := range f.Build.Mounts {
 		bindings = append(bindings, fmt.Sprintf("%s:%s", m.Source, m.Destination))
@@ -215,6 +215,10 @@ func (b *Builder) Build(ctx context.Context, f fn.Function, platforms []fn.Platf
 			return fmt.Errorf("podman 4.3 is not supported, use podman 4.2 or 4.4")
 		}
 
+		if f.Runtime == "python" {
+			cli = pyScaffoldInjector{cli}
+		}
+
 		// Client with a logger which is enabled if in Verbose mode and a dockerClient that supports SSH docker daemon connection.
 		if impl, err = pack.NewClient(pack.WithLogger(b.logger), pack.WithDockerClient(cli)); err != nil {
 			return fmt.Errorf("cannot create pack client: %w", err)

pkg/builders/buildpacks/scaffolding_injector.go

@@ -0,0 +1,163 @@
+package buildpacks
+
+import (
+	"archive/tar"
+	"context"
+	"errors"
+	"io"
+	"runtime"
+	"strings"
+
+	"github.com/docker/docker/api/types/container"
+	"github.com/docker/docker/client"
+)
+
+// Hack implementation of DockerClient that overrides CopyToContainer method.
+// The CopyToContainer method hijacks the uploaded project stream and injects function scaffolding to it.
+// It basically moves content of /workspace to /workspace/fn and then setup scaffolding code directly in /workspace.
+type pyScaffoldInjector struct {
+	client.CommonAPIClient
+}
+
+func (s pyScaffoldInjector) CopyToContainer(ctx context.Context, ctr, p string, r io.Reader, opts container.CopyToContainerOptions) error {
+
+	if pc, _, _, ok := runtime.Caller(1); ok &&
+		!strings.Contains(runtime.FuncForPC(pc).Name(), "build.copyDir") {
+		// We are not called by "project dir copy" so we do simple direct forward call.
+		return s.CommonAPIClient.CopyToContainer(ctx, ctr, p, r, opts)
+	}
+
+	pr, pw := io.Pipe()
+	go func() {
+		var err error
+		defer func() {
+			_ = pw.CloseWithError(err)
+		}()
+		tr := tar.NewReader(r)
+		tw := tar.NewWriter(pw)
+
+		for {
+			var hdr *tar.Header
+			hdr, err = tr.Next()
+
+			if err != nil {
+				if errors.Is(err, io.EOF) {
+					err = nil
+					break
+				}
+				return
+			}
+
+			if strings.HasPrefix(hdr.Name, "/workspace/") {
+				hdr.Name = strings.Replace(hdr.Name, "/workspace/", "/workspace/fn/", 1)
+			}
+
+			err = tw.WriteHeader(hdr)
+			if err != nil {
+				return
+			}
+			_, err = io.Copy(tw, tr)
+			if err != nil {
+				return
+			}
+		}
+		err = writePythonScaffolding(tw)
+		if err != nil {
+			return
+		}
+		err = tw.Close()
+	}()
+
+	return s.CommonAPIClient.CopyToContainer(ctx, ctr, p, pr, opts)
+}
+
+func writePythonScaffolding(tw *tar.Writer) error {
+	for _, f := range []struct {
+		path    string
+		content string
+	}{
+		{
+			path:    "pyproject.toml",
+			content: pyprojectToml,
+		},
+		{
+			path:    "service/main.py",
+			content: serviceMain,
+		},
+		{
+			path:    "service/__init__.py",
+			content: "",
+		},
+	} {
+		err := tw.WriteHeader(&tar.Header{
+			Name: "/workspace/" + f.path,
+			Size: int64(len(f.content)),
+			Mode: 0644,
+		})
+		if err != nil {
+			return err
+		}
+		_, err = tw.Write([]byte(f.content))
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+const pyprojectToml = `[project]
+name = "service"
+description = "an autogenerated service which runs a Function"
+version = "0.1.0"
+requires-python = ">=3.9"
+license = "MIT"
+dependencies = [
+    "func-python",
+    "function @ ./fn"
+]
+authors = [
+  { name="The Knative Authors", email="[email protected]"},
+]
+
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[tool.hatch.metadata]
+allow-direct-references = true
+
+[tool.poetry.dependencies]
+python = ">=3.9,<4.0"
+
+[tool.poetry.scripts]
+script = "service.main:main"
+`
+
+const serviceMain = `"""
+This code is glue between a user's Function and the middleware which will
+expose it as a network service.  This code is written on-demand when a
+Function is being built, deployed or run.  This will be included in the
+final container.
+"""
+import logging
+from func_python.cloudevent import serve
+
+logging.basicConfig(level=logging.INFO)
+
+try:
+    from function import new as handler  # type: ignore[import]
+except ImportError:
+    try:
+        from function import handle as handler  # type: ignore[import]
+    except ImportError:
+        logging.error("Function must export either 'new' or 'handle'")
+        raise
+
+
+def main():
+    logging.info("Functions middleware invoking user function")
+    serve(handler)
+
+if __name__ == "__main__":
+    main()
+`