Add method to import custom template rendering functions (kube-burner…

…#727)

## Type of change

- [ ] Refactor
- [x] New feature
- [ ] Bug fix
- [ ] Optimization
- [ ] Documentation Update

## Description

New method to import custom rendering template functions from external
implementations.

## Related Tickets & Documents

- Related Issue #
- Closes #

---------

Signed-off-by: Raul Sevilla <[email protected]>

docs/wrappers/wrappers.md

@@ -0,0 +1,25 @@
+# Kube-burner wrappers
+
+It's possible to extend and take advantage of the kube-burner's features by developing wrappers. A kube-burner wrapper is basically a new binary that uses some of the exposed functions and interfaces of kube-burner.
+
+## Helper functions
+
+There're some helper functions meant to be consumed by wrappers in the `workloads` package. These functions provide some shortcuts to incorporate workloads in golang embedded filesystems.
+
+## Customizing template rendering
+
+It's possible to provide custom template rendering functions from a wrapper, this can be done by calling the function `AddRenderingFunction()` of the util package. For example:
+
+```golang
+util.AddRenderingFunction("isEven", func(n int) bool {
+    return n%2 == 0
+})
+```
+
+With the above code snippet we're provisioning a new template rendering function `isEven`, that basically returns true or false depending if the provided number is even. This function can be consumed in any of the kube-burner configuration files, for example
+
+```yaml
+jobs:
+  - name: job
+    podWait: {{ isEven 5 }}
+```

mkdocs.yml

@@ -17,6 +17,7 @@ nav:
     - contributing/pullrequest.md
     - contributing/release.md
   - Tests: contributing/tests.md
+- Wrappers: wrappers/index.md
 site_name: Kube-burner
 plugins:
   - search

pkg/util/template.go

@@ -34,19 +34,17 @@ const (
 	MissingKeyZero  templateOption = "missingkey=zero"
 )
 
-// RenderTemplate renders a go-template
-func RenderTemplate(original []byte, inputData interface{}, options templateOption) ([]byte, error) {
-	var rendered bytes.Buffer
-	funcMap := sprig.GenericFuncMap()
-	funcMap["Binomial"] = combin.Binomial
-	funcMap["IndexToCombination"] = combin.IndexToCombination
-	funcMap["GetSubnet24"] = func(subnetIdx int) string {
+var funcMap = sprig.GenericFuncMap()
+
+func init() {
+	AddRenderingFunction("Binomial", combin.Binomial)
+	AddRenderingFunction("IndexToCombination", combin.IndexToCombination)
+	funcMap["GetSubnet24"] = func(subnetIdx int) string { // TODO Document this function
 		return netip.AddrFrom4([4]byte{byte(subnetIdx>>16 + 1), byte(subnetIdx >> 8), byte(subnetIdx), 0}).String() + "/24"
 	}
 	// This function returns number of addresses requested per iteration from the list of total provided addresses
-	funcMap["GetIPAddress"] = func(Addresses string, iteration int, addressesPerIteration int) string {
+	funcMap["GetIPAddress"] = func(Addresses string, iteration int, addressesPerIteration int) string { // TODO Move this function to kube-burner-ocp
 		var retAddrs []string
-
 		addrSlice := strings.Split(Addresses, " ")
 		for i := 0; i < addressesPerIteration; i++ {
 			// For example, if iteration=6 and addressesPerIteration=2, return 12th address from list.
@@ -55,6 +53,16 @@ func RenderTemplate(original []byte, inputData interface{}, options templateOpti
 		}
 		return strings.Join(retAddrs, " ")
 	}
+}
+
+func AddRenderingFunction(name string, function any) {
+	log.Debugf("Importing template function: %s", name)
+	funcMap[name] = function
+}
+
+// RenderTemplate renders a go-template
+func RenderTemplate(original []byte, inputData interface{}, options templateOption) ([]byte, error) {
+	var rendered bytes.Buffer
 	t, err := template.New("").Option(string(options)).Funcs(funcMap).Parse(string(original))
 	if err != nil {
 		return nil, fmt.Errorf("parsing error: %s", err)