feat: add template function for returning the path to the pack's directory

This allows users to read the fileContents of files within the pack folder, making templating out larger files less inconvenient and allowing users to use syntax highting from their editors when working on said files.

Author: josegonzalez

CHANGELOG.md

@@ -1,5 +1,8 @@
 ## UNRELEASED
 
+IMPROVEMENTS:
+* template: Add template function for returning the path to the pack's directory [[GH-574](https://github.com/hashicorp/nomad-pack/pull/574)]
+
 BUG FIXES:
 * cli: Update references to `hello-world` pack in command help text to `hello_world` [[GH-523](https://github.com/hashicorp/nomad-pack/pull/523)]
 * docker: Added tzdata to container to fix panics running packs containing periodic jobs with timezones [[GH-564](https://github.com/hashicorp/nomad-pack/pull/564)]

docs/writing-packs.md

@@ -189,7 +189,8 @@ Custom Nomad-specific and debugging functions are also provided:
 - `nomadNamespace` takes a single string parameter of a namespace ID which will be read via `/v1/namespace/:namespace`.
 - `spewDump` dumps the entirety of the passed object as a string. The output includes the content types and values. This uses the `spew.SDump` function.
 - `spewPrintf` dumps the supplied arguments into a string according to the supplied format. This utilises the `spew.Printf` function.
-- `fileContents` takes an argument to a file of the local host, reads its contents and provides this as a string.
+- `fileContents` takes an argument to a file on the local host relative to the directory `nomad-pack` is invoked in, reads its contents and provides the output as a string.
+- `packPath` returns the path to the pack being invoked.
 
 A custom function within a template is called like any other:
 

internal/pkg/manager/manager.go

@@ -118,6 +118,7 @@ func (pm *PackManager) ProcessTemplates(renderAux bool, format bool, ignoreMissi
 
 	r := new(renderer.Renderer)
 	r.Client = pm.client
+	r.PackPath = pm.cfg.Path
 	pm.renderer = r
 
 	// should auxiliary files be rendered as well?

internal/pkg/renderer/funcs.go

@@ -51,6 +51,12 @@ func funcMap(r *Renderer) template.FuncMap {
 		f["nomadRegions"] = nomadRegions(r.Client)
 	}
 
+	if r != nil && r.PackPath != "" {
+		f["packPath"] = func(file string) (string, error) {
+			return r.PackPath, nil
+		}
+	}
+
 	// Add additional custom functions.
 	f["fileContents"] = fileContents
 	f["toStringList"] = toStringList