fix: chain upstream plugin sourcemaps whose sources contain full paths

When a bundler plugin (e.g. a Vite plugin using MagicString.generateMap({ source: id })
where id is the absolute module path) passes a sourcemap to compileOptions.sourcemap,
the source entry is a full or relative path (e.g. /project/src/App.svelte) rather than
just the basename. The previous fix (PR #18518) only handled empty sources[0] (from
generateMap() without a source option), but this case was still failing because a strict
string comparison against the basename didn't match, causing @jridgewell/remapping to
silently drop the upstream chain and produce wrong devtools/stack-trace positions.

Fixes #18778

.changeset/sourcemap-chain-basename.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly chain upstream plugin sourcemaps whose `sources` contain full paths (e.g. from `MagicString.generateMap({ source: id })`) by normalizing source entries to the file basename before remapping

packages/svelte/src/compiler/preprocess/index.js

@@ -12,6 +12,24 @@ import {
 import { decode_map } from './decode_sourcemap.js';
 import { replace_in_code, slice_source } from './replace_in_code.js';
 
+/**
+ * Normalize the `sources` array of a preprocessor source map so that any entry referring to
+ * the file being compiled (by basename match — e.g. a full or relative path) is replaced with
+ * `file_basename`. This mirrors the normalization done in `apply_preprocessor_sourcemap` for
+ * upstream plugin maps passed via `compileOptions.sourcemap`.
+ * @param {DecodedSourceMap | undefined} decoded_map
+ * @param {string} file_basename
+ */
+function normalize_preprocessor_sources(decoded_map, file_basename) {
+	if (!decoded_map?.sources) return;
+	for (let i = 0; i < decoded_map.sources.length; i++) {
+		const source = decoded_map.sources[i];
+		if (!source || get_basename(source) === file_basename) {
+			decoded_map.sources[i] = file_basename;
+		}
+	}
+}
+
 /**
  * Represents intermediate states of the preprocessing.
  * Implements the Source interface.
@@ -115,6 +133,7 @@ function processed_content_to_code(processed, location, file_basename) {
 		decoded_map = decode_map(processed);
 		// decoded map may not have sources for empty maps like `{ mappings: '' }`
 		if (decoded_map?.sources) {
+			normalize_preprocessor_sources(decoded_map, file_basename);
 			// offset only segments pointing at original component source
 			const source_index = decoded_map.sources.indexOf(file_basename);
 			if (source_index !== -1) {
@@ -311,14 +330,19 @@ async function process_markup(process, source) {
 		filename: source.filename
 	});
 	if (processed) {
+		/** @type {any} */
+		let map = undefined;
+		if (processed.map) {
+			// TODO: can we use decode_sourcemap?
+			map = typeof processed.map === 'string' ? JSON.parse(processed.map) : processed.map;
+			if (source.file_basename && map.sources) {
+				map = { ...map, sources: map.sources.slice() };
+				normalize_preprocessor_sources(map, source.file_basename);
+			}
+		}
 		return {
 			string: processed.code,
-			map: processed.map
-				? // TODO: can we use decode_sourcemap?
-					typeof processed.map === 'string'
-					? JSON.parse(processed.map)
-					: processed.map
-				: undefined,
+			map,
 			dependencies: processed.dependencies
 		};
 	} else {

packages/svelte/src/compiler/utils/mapped_code.js

@@ -311,12 +311,24 @@ function apply_preprocessor_sourcemap(filename, svelte_map, preprocessor_map_inp
 		typeof preprocessor_map_input === 'string'
 			? JSON.parse(preprocessor_map_input)
 			: preprocessor_map_input;
-	// A preprocessor map with a missing/empty `sources[0]` (e.g. from a MagicString transform
-	// created without a `source` option) can't be matched against `filename` during combination,
-	// which silently drops the chain instead of erroring. Normalize it to `filename` first, the
-	// same way Vite treats an empty `sources[0]` as referring to the file being transformed.
-	if (preprocessor_map.sources?.length === 1 && !preprocessor_map.sources[0]) {
-		preprocessor_map.sources = [filename];
+	// Normalize source entries in the incoming map so they match the filename (basename) that
+	// svelte uses in its own map, otherwise `@jridgewell/remapping` can't chain the two maps.
+	// - A missing/empty `sources[i]` (e.g. from `new MagicString(code).generateMap()` without a
+	//   `source` option) is treated as "this file", matching Vite's behavior in pluginContainer.
+	// - A full path or relative path whose basename equals `filename` (e.g. from
+	//   `generateMap({ source: id })` where `id` is an absolute Vite module id like
+	//   `/project/src/App.svelte`) also refers to this file, but a strict string comparison
+	//   against the basename fails. Normalize it to `filename` so the chain matches.
+	if (preprocessor_map.sources) {
+		preprocessor_map.sources = preprocessor_map.sources.map((source) => {
+			if (!source) return filename;
+			if (source === filename) return source;
+			// If the source's basename matches `filename`, treat it as referring to this file.
+			// This handles full paths, relative paths with directories, etc. that upstream
+			// tools (Vite, MagicString with `{source: id}`) commonly produce.
+			if (get_basename(source) === filename) return filename;
+			return source;
+		});
 	}
 	const result_map = combine_sourcemaps(filename, [svelte_map, preprocessor_map]);
 	// Svelte expects a SourceMap which includes toUrl and toString. Instead of wrapping our output in a class,

packages/svelte/tests/sourcemaps/samples/sourcemap-fullpath-source/_config.js

@@ -0,0 +1,23 @@
+import * as fs from 'node:fs';
+import MagicString from 'magic-string';
+import { test } from '../../test';
+
+// Simulates a bundler plugin (e.g. a Vite plugin using `magic-string`) that transforms
+// the Svelte source *before* it reaches `compile()` and calls `generateMap({ source: id })`
+// where `id` is an absolute/fully-qualified module id (as Vite does). The source entry in
+// the resulting map is a full path (e.g. `/project/src/App.svelte`) rather than a basename,
+// which previously failed to match during map chaining (see #18778) and caused the upstream
+// mappings to be silently dropped, producing wrong devtools/stack-trace positions.
+const input = fs.readFileSync(new URL('./input.svelte', import.meta.url), 'utf-8');
+const src = new MagicString(input);
+// Perform a no-op transform but generate the map with a full absolute path as `source`
+// (basename is still `input.svelte`), reproducing the scenario from the bug report where
+// the reporter tried `m.generateMap({source: id})`, `m.generateMap({file: id})`, etc.
+const simulated_vite_id = '/fake/project/path/src/input.svelte';
+
+export default test({
+	compileOptions: {
+		sourcemap: src.generateMap({ source: simulated_vite_id, hires: true })
+	},
+	client: [{ str: 'let doubled' }]
+});

packages/svelte/tests/sourcemaps/samples/sourcemap-fullpath-source/input.svelte

@@ -0,0 +1,6 @@
+<script>
+	let count = 0;
+	let doubled = count * 2;
+</script>
+
+<button>clicks: {count}</button>