Allow getting a path prefix from a string (#4132)

Will be required for selective subtree pruning since hierarchy merging starts at the category directory.

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

- [x] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Issue(s)

* #4117

#### Test Plan

- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E

src/pkg/path/path.go

@@ -185,11 +185,7 @@ func BuildPrefix(
 	}, nil
 }
 
-// FromDataLayerPath parses the escaped path p, validates the elements in p
-// match a resource-specific path format, and returns a Path struct for that
-// resource-specific type. If p does not match any resource-specific paths or
-// is malformed returns an error.
-func FromDataLayerPath(p string, isItem bool) (Path, error) {
+func pathFromDataLayerPath(p string, isItem bool, minElements int) (Path, error) {
 	p = TrimTrailingSlash(p)
 	// If p was just the path separator then it will be empty now.
 	if len(p) == 0 {
@@ -202,15 +198,21 @@ func FromDataLayerPath(p string, isItem bool) (Path, error) {
 		return nil, clues.Stack(errParsingPath, err).With("path_string", p)
 	}
 
-	if len(pb.elements) < 5 {
-		return nil, clues.New("path has too few segments").With("path_string", p)
+	if len(pb.elements) < minElements {
+		return nil, clues.New(
+			"missing required tenant, service, category, protected resource ID, or non-prefix segment").
+			With("path_string", pb)
 	}
 
 	service, category, err := validateServiceAndCategoryStrings(
 		pb.elements[1],
 		pb.elements[3])
 	if err != nil {
-		return nil, clues.Stack(errParsingPath, err).With("path_string", p)
+		return nil, clues.Stack(errParsingPath, err).With("path_string", pb)
+	}
+
+	if err := verifyInputValues(pb.elements[0], pb.elements[2]); err != nil {
+		return nil, clues.Stack(err).With("path_string", pb)
 	}
 
 	return &dataLayerResourcePath{
@@ -221,6 +223,20 @@ func FromDataLayerPath(p string, isItem bool) (Path, error) {
 	}, nil
 }
 
+func PrefixOrPathFromDataLayerPath(p string, isItem bool) (Path, error) {
+	res, err := pathFromDataLayerPath(p, isItem, 4)
+	return res, clues.Stack(err).OrNil()
+}
+
+// FromDataLayerPath parses the escaped path p, validates the elements in p
+// match a resource-specific path format, and returns a Path struct for that
+// resource-specific type. If p does not match any resource-specific paths or
+// is malformed returns an error.
+func FromDataLayerPath(p string, isItem bool) (Path, error) {
+	res, err := pathFromDataLayerPath(p, isItem, 5)
+	return res, clues.Stack(err).OrNil()
+}
+
 // TrimTrailingSlash takes an escaped path element and returns an escaped path
 // element with the trailing path separator character(s) removed if they were not
 // escaped. If there were no trailing path separator character(s) or the separator(s)

src/pkg/path/path_test.go

@@ -426,6 +426,26 @@ func (suite *PathUnitSuite) TestFromDataLayerPath() {
 	}
 }
 
+func (suite *PathUnitSuite) TestPrefixOrPathFromDataLayerPath() {
+	t := suite.T()
+	input := fmt.Sprintf(
+		"%s/%s/%s/%s",
+		"tenant",
+		ExchangeService.String(),
+		"user",
+		EmailCategory.String())
+
+	// Check that we can make a valid prefix path.
+	p, err := PrefixOrPathFromDataLayerPath(input, false)
+	assert.NoError(t, err, clues.ToCore(err))
+	assert.Equal(t, input, p.String())
+
+	// Check we can't make a regular path with the same input since it doesn't
+	// have enough segments.
+	_, err = FromDataLayerPath(input, false)
+	assert.Error(t, err)
+}
+
 func (suite *PathUnitSuite) TestBuildPrefix() {
 	table := []struct {
 		name      string