From 6124d3dfd274ac24e5da0817ae172aca58c22d4e Mon Sep 17 00:00:00 2001
From: Dave Henderson <dhenderson@gmail.com>
Date: Thu, 3 May 2018 21:50:37 -0400
Subject: Adding datasourceReachable function

Signed-off-by: Dave Henderson <dhenderson@gmail.com>
---
 data/datasource.go                        | 11 +++++++++++
 data/datasource_test.go                   | 27 +++++++++++++++++++++++++++
 docs/content/functions/data.md            | 13 ++++++++++++-
 funcs/data.go                             |  1 +
 test/integration/datasources_file_test.go |  9 +++++++++
 5 files changed, 60 insertions(+), 1 deletion(-)

diff --git a/data/datasource.go b/data/datasource.go
index e4336fb5..6370bd5f 100644
--- a/data/datasource.go
+++ b/data/datasource.go
@@ -257,6 +257,17 @@ func (d *Data) Datasource(alias string, args ...string) (interface{}, error) {
 	return nil, errors.Errorf("Datasources of type %s not yet supported", source.Type)
 }
 
+// DatasourceReachable - Determines if the named datasource is reachable with
+// the given arguments. Reads from the datasource, and discards the returned data.
+func (d *Data) DatasourceReachable(alias string, args ...string) bool {
+	source, ok := d.Sources[alias]
+	if !ok {
+		return false
+	}
+	_, err := d.ReadSource(source, args...)
+	return err == nil
+}
+
 // Include -
 func (d *Data) Include(alias string, args ...string) (string, error) {
 	source, ok := d.Sources[alias]
diff --git a/data/datasource_test.go b/data/datasource_test.go
index dbf1f0af..ab71a060 100644
--- a/data/datasource_test.go
+++ b/data/datasource_test.go
@@ -170,6 +170,33 @@ func TestDatasource(t *testing.T) {
 	assert.Equal(t, "", actual)
 }
 
+func TestDatasourceReachable(t *testing.T) {
+	fname := "foo.json"
+	fs := memfs.Create()
+	_ = fs.Mkdir("/tmp", 0777)
+	f, _ := vfs.Create(fs, "/tmp/"+fname)
+	_, _ = f.Write([]byte("{}"))
+
+	sources := map[string]*Source{
+		"foo": {
+			Alias: "foo",
+			URL:   &url.URL{Scheme: "file", Path: "/tmp/" + fname},
+			Ext:   "json",
+			Type:  "application/json",
+			FS:    fs,
+		},
+		"bar": {
+			Alias: "bar",
+			URL:   &url.URL{Scheme: "file", Path: "/bogus"},
+			FS:    fs,
+		},
+	}
+	data := &Data{Sources: sources}
+
+	assert.True(t, data.DatasourceReachable("foo"))
+	assert.False(t, data.DatasourceReachable("bar"))
+}
+
 func TestDatasourceExists(t *testing.T) {
 	sources := map[string]*Source{
 		"foo": {Alias: "foo"},
diff --git a/docs/content/functions/data.md b/docs/content/functions/data.md
index 654e45af..277daafb 100644
--- a/docs/content/functions/data.md
+++ b/docs/content/functions/data.md
@@ -319,13 +319,24 @@ defined.
 
 Note: this does _not_ verify if the datasource is reachable.
 
-Useful when used in an `if`/`else` block
+Useful when used in an `if`/`else` block.
 
 ```console
 $ echo '{{if (datasourceExists "test")}}{{datasource "test"}}{{else}}no worries{{end}}' | gomplate
 no worries
 ```
 
+## `datasourceReachable`
+
+Tests whether or not a given datasource is defined and reachable, where the definition of "reachable" differs by datasource, but generally means the data is able to be read successfully.
+
+Useful when used in an `if`/`else` block.
+
+```console
+$ gomplate -i '{{if (datasourceReachable "test")}}{{datasource "test"}}{{else}}no worries{{end}}' -d test=https://bogus.example.com/wontwork.json
+no worries
+```
+
 ## `ds`
 
 Alias to [`datasource`](#datasource)
diff --git a/funcs/data.go b/funcs/data.go
index b2c2b7eb..4b1e0526 100644
--- a/funcs/data.go
+++ b/funcs/data.go
@@ -23,6 +23,7 @@ func AddDataFuncs(f map[string]interface{}, d *data.Data) {
 	f["datasource"] = d.Datasource
 	f["ds"] = d.Datasource
 	f["datasourceExists"] = d.DatasourceExists
+	f["datasourceReachable"] = d.DatasourceReachable
 	f["include"] = d.Include
 
 	f["data"] = DataNS
diff --git a/test/integration/datasources_file_test.go b/test/integration/datasources_file_test.go
index 1c8d1530..5703217f 100644
--- a/test/integration/datasources_file_test.go
+++ b/test/integration/datasources_file_test.go
@@ -52,6 +52,15 @@ func (s *FileDatasourcesSuite) TestFileDatasources(c *C) {
 	)
 	result.Assert(c, icmd.Expected{ExitCode: 0, Out: "bar"})
 
+	result = icmd.RunCommand(GomplateBin,
+		"-d", "config="+s.tmpDir.Join("config2.yml"),
+		"-i", `{{ if (datasourceReachable "bogus") }}bogus!{{ end -}}
+{{ if (datasourceReachable "config") -}}
+{{ (ds "config").foo -}}
+{{ end }}`,
+	)
+	result.Assert(c, icmd.Expected{ExitCode: 0, Out: "bar"})
+
 	result = icmd.RunCommand(GomplateBin,
 		"-d", "csv="+s.tmpDir.Join("foo.csv"),
 		"-i", `{{ index (index (ds "csv") 2) 1 }}`,
-- 
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