psl: fix newline boundary search in relation reformatting

When adding an @relation attribute to a forward relation field in the
relation reformatting code that deals with adding missing relation
fields and attributes, we searched for the last character before a
newline to check whether it is a carriage return, but we failed to
check whether the byte offset was a char boundary first. This caused
crashes.

closes prisma/prisma#14895

psl/psl-core/src/reformat.rs

@@ -332,6 +332,11 @@ fn references_argument(inline: walkers::InlineRelationWalker<'_>) -> String {
 /// not including the newline in field spans.
 fn before_newline(span_end: usize, original_schema: &str) -> usize {
     assert!(&original_schema[span_end - 1..span_end] == "\n");
+
+    if !original_schema.is_char_boundary(span_end - 2) {
+        return span_end - 1;
+    }
+
     match &original_schema[span_end - 2..span_end - 1] {
         "\r" => span_end - 2,
         _ => span_end - 1,

psl/psl/tests/reformat_tests.rs

@@ -28,6 +28,8 @@ fn run_reformat_test(test_file_path: &str) {
     }
 
     if reformat(&reformatted_text) != reformatted_text {
+        println!("=== reformatted ===\n{reformatted_text}");
+        println!("=== reformatted again ===\n{}", reformat(&reformatted_text));
         panic!("Reformatting this schema is not idempotent.");
     }
 }

psl/psl/tests/reformatter/regression_thai_trailing_comments.prisma

@@ -0,0 +1,23 @@
+// This is your Prisma schema file,
+// learn more about it in the docs: https://pris.ly/d/prisma-schema
+
+generator client {
+  provider = "prisma-client-js"
+  output   = "./generated/client"
+}
+
+datasource db {
+  provider = "sqlserver"
+  url = "***"
+}
+
+model somemodel {
+    frownyFaceId  String  
+    frownyFace frownyface // 囧
+
+    id Int @id
+}
+
+model frownyface {
+    somes somemodel[]
+}

psl/psl/tests/reformatter/regression_thai_trailing_comments.reformatted.prisma

@@ -0,0 +1,23 @@
+// This is your Prisma schema file,
+// learn more about it in the docs: https://pris.ly/d/prisma-schema
+
+generator client {
+  provider = "prisma-client-js"
+  output   = "./generated/client"
+}
+
+datasource db {
+  provider = "sqlserver"
+  url      = "***"
+}
+
+model somemodel {
+  frownyFaceId String
+  frownyFace   frownyface // 囧 @relation(fields: [], references: [])
+
+  id Int @id
+}
+
+model frownyface {
+  somes somemodel[]
+}