From a8bbf7cea0e858fb66a1d18b5a1ed6f36ca44441 Mon Sep 17 00:00:00 2001
From: Sam Woodard <sam.h.woodard@gmail.com>
Date: Fri, 5 Aug 2022 08:41:36 -0700
Subject: [PATCH] feat(AIP-122): resource patterns must start with lowercase
 letter (#995)

---
 docs/rules/0122/resource-collection-identifiers.md | 14 +++++++++++++-
 rules/aip0122/resource_collection_identifiers.go   | 11 +++++++++++
 .../resource_collection_identifiers_test.go        |  2 ++
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/docs/rules/0122/resource-collection-identifiers.md b/docs/rules/0122/resource-collection-identifiers.md
index ea9c08517..8eaac6664 100644
--- a/docs/rules/0122/resource-collection-identifiers.md
+++ b/docs/rules/0122/resource-collection-identifiers.md
@@ -18,7 +18,7 @@ in [AIP-122][].
 
 This rule scans messages with a `google.api.resource` annotation, and validates
 the format of `pattern` collection identifiers, specifically that they are in
-lowerCamelCase form.
+lowerCamelCase form and must start with a lowercase letter.
 
 ## Examples
 
@@ -36,6 +36,18 @@ message Book {
 }
 ```
 
+```proto
+// Incorrect.
+message Book {
+  option (google.api.resource) = {
+    type: "library.googleapis.com/Book"
+    // Collection identifiers must begin with a lower-cased letter.
+    pattern: "/publishers/{publisher}/Books/{book}"
+  };
+  string name = 1;
+}
+```
+
 **Correct** code for this rule:
 
 ```proto
diff --git a/rules/aip0122/resource_collection_identifiers.go b/rules/aip0122/resource_collection_identifiers.go
index 2de9eafce..32655544b 100644
--- a/rules/aip0122/resource_collection_identifiers.go
+++ b/rules/aip0122/resource_collection_identifiers.go
@@ -15,6 +15,7 @@
 package aip0122
 
 import (
+	"regexp"
 	"strings"
 	"unicode"
 
@@ -24,6 +25,8 @@ import (
 	"github.com/jhump/protoreflect/desc"
 )
 
+var firstCharRegexp = regexp.MustCompile(`^[a-z]`)
+
 var resourceCollectionIdentifiers = &lint.MessageRule{
 	Name: lint.NewRuleName(122, "resource-collection-identifiers"),
 	OnlyIf: func(m *desc.MessageDescriptor) bool {
@@ -33,6 +36,14 @@ var resourceCollectionIdentifiers = &lint.MessageRule{
 		var problems []lint.Problem
 		resource := utils.GetResource(m)
 		for _, p := range resource.GetPattern() {
+			if !firstCharRegexp.MatchString(p) {
+				return append(problems, lint.Problem{
+					Message:    "Resource patterns must start with a lowercase letter.",
+					Descriptor: m,
+					Location:   locations.MessageResource(m),
+				})
+			}
+
 			segs := strings.Split(p, "/")
 			for _, seg := range segs {
 				// Get first rune of each pattern segment.
diff --git a/rules/aip0122/resource_collection_identifiers_test.go b/rules/aip0122/resource_collection_identifiers_test.go
index 586773a75..9dc285129 100644
--- a/rules/aip0122/resource_collection_identifiers_test.go
+++ b/rules/aip0122/resource_collection_identifiers_test.go
@@ -28,6 +28,8 @@ func TestResourceCollectionIdentifiers(t *testing.T) {
 	}{
 		{"Valid", "author/{author}/books/{book}", testutils.Problems{}},
 		{"InvalidUpperCase", "author/{author}/Books/{book}", testutils.Problems{{Message: "lowerCamelCase"}}},
+		{"InvalidStartsWithSlash", "/author/{author}/Books/{book}", testutils.Problems{{Message: "lowercase letter"}}},
+		{"InvalidStartsWithCapitalLetter", "Author/{author}/Books/{book}", testutils.Problems{{Message: "lowercase letter"}}},
 	} {
 		t.Run(test.name, func(t *testing.T) {
 			f := testutils.ParseProto3Tmpl(t, `