feat(templater) Add a template function to insert surrounding repo's Git hash

A template function has been added that allows one to template the
Git hash of the surrounding repo. This is useful to be able to inspect the
deployment revision of an object in Kubernetes.

2 files changed, 13 insertions, 0 deletions

diff --git a/docs/templates.md b/docs/templates.md
index 11488f573a59..6c44035d0967 100644
--- a/docs/templates.md
+++ b/docs/templates.md
@@ -75,6 +75,7 @@ Some template functions come from Go's standard library and are listed in the
 available in kontemplate, as well as three custom functions:
 
 * `json`: Encodes any supplied data structure as JSON.
+* `gitHEAD`: Retrieves the commit hash at Git `HEAD`.
 * `passLookup`: Looks up the supplied key in [pass][].
 * `insertFile`: Insert the contents of the given file in the resource
   set folder as a string.
@@ -96,6 +97,9 @@ certKeyPath: my-website/cert-key
 
 {{ .certKeyPath | passLookup }}
 -> Returns content of 'my-website/cert-key' from pass
+
+{{ gitHEAD }}
+-> Returns the Git commit hash at HEAD.
 ```
 
 ## Conditionals & ranges
diff --git a/templater/templater.go b/templater/templater.go
index bfd2af1f61bf..89cc1cd732b5 100644
--- a/templater/templater.go
+++ b/templater/templater.go
@@ -15,6 +15,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	"path"
 	"strings"
 	"text/template"
@@ -169,6 +170,14 @@ func templateFuncs(rs *context.ResourceSet) template.FuncMap {
 		return string(b)
 	}
 	m["passLookup"] = GetFromPass
+	m["gitHEAD"] = func() (string, error) {
+			out, err := exec.Command("sh", "-c", "git rev-parse HEAD").Output()
+			if err != nil {
+				return "", err
+			}
+			output := strings.TrimSpace(string(out))
+			return output, nil
+		}
 	m["lookupIPAddr"] = GetIPsFromDNS
 	m["insertFile"] = func(file string) (string, error) {
 		data, err := ioutil.ReadFile(path.Join(rs.Path, file))