fix(bundler): escape backslashes in metafile paths and memoize getter

Two fixes for the metafile feature:

1. Properly JSON-escape paths in MetafileBuilder to handle Windows
   backslashes (e.g., C:\Users\... becomes C:\Users\...). Without
   this, JSON.parse fails with "Invalid escape character U".

2. Memoize the lazy metafile getter by replacing it with the parsed
   value after first access. This prevents assertion failures when
   the metafile property is accessed multiple times.

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: dylan-conway

src/bun.js/bindings/BundlerMetafile.cpp

@@ -39,6 +39,9 @@ JSC_DEFINE_CUSTOM_GETTER(bundlerMetafileLazyGetter, (JSGlobalObject * globalObje
     JSValue parsedValue = JSC::JSONParseWithException(globalObject, view);
     RETURN_IF_EXCEPTION(scope, {});
 
+    // Replace the lazy getter with the parsed value (memoize for subsequent accesses)
+    thisObject->putDirect(vm, property, parsedValue, 0);
+
     // Clear the raw JSON string so it can be GC'd
     thisObject->putDirect(vm, privateName, jsUndefined(), 0);
 

src/bundler/linker_context/MetafileBuilder.zig

@@ -214,9 +214,9 @@ pub fn generate(
         }
         first_input = false;
 
-        j.pushStatic("\n    \"");
-        j.pushStatic(path);
-        j.push(std.fmt.allocPrint(allocator, "\": {{\n      \"bytes\": {d}", .{source.contents.len}) catch return error.OutOfMemory, allocator);
+        j.pushStatic("\n    ");
+        j.push(try std.fmt.allocPrint(allocator, "{f}", .{bun.fmt.formatJSONStringUTF8(path, .{})}), allocator);
+        j.push(try std.fmt.allocPrint(allocator, ": {{\n      \"bytes\": {d}", .{source.contents.len}), allocator);
 
         // Write imports
         j.pushStatic(",\n      \"imports\": [");
@@ -231,19 +231,18 @@ pub fn generate(
                 }
                 first_import = false;
 
-                j.pushStatic("\n        {\n          \"path\": \"");
-                // Write path as-is - chunk references (unique_keys) will be resolved
+                j.pushStatic("\n        {\n          \"path\": ");
+                // Write path with JSON escaping - chunk references (unique_keys) will be resolved
                 // by breakOutputIntoPieces and code() below
-                j.pushStatic(record.path.text);
-                j.pushStatic("\",\n          \"kind\": \"");
+                j.push(try std.fmt.allocPrint(allocator, "{f}", .{bun.fmt.formatJSONStringUTF8(record.path.text, .{})}), allocator);
+                j.pushStatic(",\n          \"kind\": \"");
                 j.pushStatic(record.kind.label());
                 j.pushStatic("\"");
 
                 // Add "original" field if different from path
                 if (record.original_path.len > 0 and !std.mem.eql(u8, record.original_path, record.path.text)) {
-                    j.pushStatic(",\n          \"original\": \"");
-                    j.pushStatic(record.original_path);
-                    j.pushStatic("\"");
+                    j.pushStatic(",\n          \"original\": ");
+                    j.push(try std.fmt.allocPrint(allocator, "{f}", .{bun.fmt.formatJSONStringUTF8(record.original_path, .{})}), allocator);
                 }
 
                 // Add "external": true for external imports