PLT-7407: Back-end plugin mechanism (#7177)

* begin backend plugin wip

* flesh out rpcplugin. everything done except for minor supervisor stubs

* done with basic plugin infrastructure

* simplify tests

* remove unused test lines

plugin/pluginenv/search_path_test.go

@@ -0,0 +1,62 @@
+package pluginenv
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/mattermost/platform/plugin"
+)
+
+func TestScanSearchPath(t *testing.T) {
+	dir := initTmpDir(t, map[string]string{
+		".foo/plugin.json": `{"id": "foo"}`,
+		"foo/bar":          "asdf",
+		"foo/plugin.json":  `{"id": "foo"}`,
+		"bar/zxc":          "qwer",
+		"baz/plugin.yaml":  "id: baz",
+		"bad/plugin.json":  "asd",
+		"qwe":              "asd",
+	})
+	defer os.RemoveAll(dir)
+
+	plugins, err := ScanSearchPath(dir)
+	require.NoError(t, err)
+	assert.Len(t, plugins, 3)
+	assert.Contains(t, plugins, &plugin.BundleInfo{
+		Path:         filepath.Join(dir, "foo"),
+		ManifestPath: filepath.Join(dir, "foo", "plugin.json"),
+		Manifest: &plugin.Manifest{
+			Id: "foo",
+		},
+	})
+	assert.Contains(t, plugins, &plugin.BundleInfo{
+		Path:         filepath.Join(dir, "baz"),
+		ManifestPath: filepath.Join(dir, "baz", "plugin.yaml"),
+		Manifest: &plugin.Manifest{
+			Id: "baz",
+		},
+	})
+	foundError := false
+	for _, x := range plugins {
+		if x.ManifestError != nil {
+			assert.Equal(t, x.Path, filepath.Join(dir, "bad"))
+			assert.Equal(t, x.ManifestPath, filepath.Join(dir, "bad", "plugin.json"))
+			syntexError, ok := x.ManifestError.(*json.SyntaxError)
+			assert.True(t, ok)
+			assert.EqualValues(t, 1, syntexError.Offset)
+			foundError = true
+		}
+	}
+	assert.True(t, foundError)
+}
+
+func TestScanSearchPath_Error(t *testing.T) {
+	plugins, err := ScanSearchPath("not a valid path!")
+	assert.Nil(t, plugins)
+	assert.Error(t, err)
+}