feat: Add configuration for Python buildpack

This adds a new configuration `BP_FUNCTION` to the Python
buildpack. The check for `func.yaml` is now optional as long as
the configuration is defined.

Signed-off-by: Andrew Su <[email protected]>

buildpacks/python/README.md

@@ -17,19 +17,8 @@ The buildpack will do the following if detection passed:
 To get started you'll need to create a directory where your function will be defined.
 
 From within this directory we require a few files to properly detect this as a Python function:
-* <a name="func.yaml"></a>`func.yaml`: This is the configuration used to give the buildpack some configurations.
-  * The python module and function name can be modified here by defining some environment variables.
-    ```
-    envs:
-    - name: MODULE_NAME
-      value: my_module
-    - name: FUNCTION_NAME
-      value: my_func
-    ```
-    By defining the above instead of a `func.py` file like below, the file should now be `my_module.py` containing a function with the name `my_func`
-
 * `func.py`: This python module will be where we search for a function by default.
-  * If you want to use a different name for the file. See description for [`func.yaml`](#func.yaml).
+  * If you want to use a different name for the file. See [configuration](#configuration) or [`func.yaml`](#func.yaml).
   * This file should contain the function to invoke when we receive an event.
     * The function can handle http requests:
       ```
@@ -47,8 +36,20 @@ From within this directory we require a few files to properly detect this as a P
       ```
     * You can find more details about the different accepted parameters [below](#fp).
 
-* `requirements.txt`: This file is used for defining your dependencies. However if you have no dependencies, we're still expecting an empty file.
-  * TODO: Remove the expectation of file `requirements.txt`
+* <a name="func.yaml"></a>`func.yaml` (optional): This is the configuration used to configure your function.
+  * The python module and function name can be modified here by defining some environment variables in the `envs` section.
+    ```
+    envs:
+    - name: MODULE_NAME
+      value: my_module
+    - name: FUNCTION_NAME
+      value: my_func
+    ```
+    By defining the above, we will look for a `my_module.py` instead of `func.py` which contains a function with the name `my_func`.
+
+  **NOTE**: The environment variables here (namely `MODULE_NAME` and `FUNCTION_NAME` will be overriden by the values specified by `BP_FUNCTION`)
+
+* `requirements.txt`: This file is required by the Python dependency. It is used to define your function's dependencies. If you do not have any, you still need to provide an empty file.
 
 ## <a name="fp"></a> Accepted Function Parameters
 The function handles either HTTP or CloudEvents based on the parameter's name and type. Only the following arguments are accepted:
@@ -74,6 +75,13 @@ ghcr.io/vmware-tanzu/function-buildpacks-for-knative/functions-builder
 * [Buildpack CLI](https://buildpacks.io/docs/tools/pack/)
 
 ### <a name="usage"></a> Usage
+
+## <a name="configuration"></a> Configuration
+
+| Environment Variable | Description |
+| -------------------- | ----------- |
+| `$BP_FUNCTION` | Configure the function handler.  Defaults to `func.main`. |
+
 Build the function container with the Buildpack CLI
 ```
 pack build <your_image_name_and_tag> --builder ghcr.io/vmware-tanzu/function-buildpacks-for-knative/functions-builder:<version>

buildpacks/python/buildpack.toml

@@ -13,6 +13,12 @@ api = "0.6"
   include-files = ["README.md", "bin/build", "bin/detect", "bin/main", "buildpack.toml", "VERSION"]
   pre-package = "./build.sh"
 
+  [[metadata.configurations]]
+    build = true
+    default = "func.main"
+    description = "The function to run, specify in the form of `module.function_name`"
+    name = "BP_FUNCTION"
+
   [[metadata.dependencies]]
     id      = "invoker"
     name    = "Python Invoker"

buildpacks/python/python/detect.go

@@ -7,40 +7,68 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"strings"
 
 	"github.com/buildpacks/libcnb"
+	"github.com/paketo-buildpacks/libpak"
 	"github.com/paketo-buildpacks/libpak/bard"
 	knfn "knative.dev/kn-plugin-func"
 )
 
 const (
 	EnvModuleName   = "MODULE_NAME"
 	EnvFunctionName = "FUNCTION_NAME"
-
-	ModuleNameDefault   = "func"
-	FunctionNameDefault = "main"
 )
 
 type Detect struct {
 	Logger bard.Logger
 }
 
-func (d Detect) Detect(context libcnb.DetectContext) (libcnb.DetectResult, error) {
-	result := libcnb.DetectResult{}
-	configFile := filepath.Join(context.Application.Path, knfn.ConfigFile)
+func (d Detect) getFuncYamlEnvs(appPath string) (map[string]string, bool) {
+	configFile := filepath.Join(appPath, knfn.ConfigFile)
 	_, err := os.Stat(configFile)
 	if err != nil {
-		d.logf(fmt.Sprintf("unable to find file '%s'", configFile))
-		return result, nil
+		d.Logger.Bodyf("'%s' not detected", knfn.ConfigFile)
+		return make(map[string]string), false
 	}
 
-	f, err := knfn.NewFunction(context.Application.Path)
+	f, err := knfn.NewFunction(appPath)
 	if err != nil {
-		return result, fmt.Errorf("parsing function config: %v", err)
+		d.Logger.Bodyf("unable to parse '%s': %v", knfn.ConfigFile, err)
+		return make(map[string]string), false
 	}
 
-	envs := envsToMap(f.Envs)
-	setDefaults(envs)
+	return envsToMap(f.Envs), true
+}
+
+func (d Detect) Detect(context libcnb.DetectContext) (libcnb.DetectResult, error) {
+	result := libcnb.DetectResult{}
+
+	envs, hasValidFuncYaml := d.getFuncYamlEnvs(context.Application.Path)
+
+	cr, err := libpak.NewConfigurationResolver(context.Buildpack, &d.Logger)
+	if err != nil {
+		return libcnb.DetectResult{}, err
+	}
+
+	functionHandler, funcFound := cr.Resolve("BP_FUNCTION")
+	funcParts := strings.Split(functionHandler, ".")
+	if funcFound {
+		if len(funcParts) != 2 || len(funcParts[0]) == 0 || len(funcParts[1]) == 0 { // We're expecting the format of module.func_name
+			return libcnb.DetectResult{}, fmt.Errorf("BP_FUNCTION detected but is invalid, it should be in the form of `module.function_name`")
+		}
+
+		envs[EnvModuleName] = funcParts[0]
+		envs[EnvFunctionName] = funcParts[1]
+	} else {
+		if _, found := envs[EnvModuleName]; !found {
+			envs[EnvModuleName] = funcParts[0]
+		}
+
+		if _, found := envs[EnvFunctionName]; !found {
+			envs[EnvFunctionName] = funcParts[1]
+		}
+	}
 
 	result.Plans = append(result.Plans, libcnb.BuildPlan{
 		Provides: []libcnb.BuildPlanProvide{
@@ -72,7 +100,7 @@ func (d Detect) Detect(context libcnb.DetectContext) (libcnb.DetectResult, error
 		},
 	})
 
-	result.Pass = true
+	result.Pass = hasValidFuncYaml || funcFound
 
 	return result, nil
 }
@@ -95,15 +123,3 @@ func envsToMap(envs knfn.Envs) map[string]string {
 
 	return result
 }
-
-func setDefaults(envs map[string]string) {
-	setDefault(envs, EnvModuleName, ModuleNameDefault)
-	setDefault(envs, EnvFunctionName, FunctionNameDefault)
-}
-
-func setDefault(m map[string]string, key string, def string) {
-	_, ok := m[key]
-	if !ok {
-		m[key] = def
-	}
-}