fix(runfiles): support bzlmod repo mappings

packages/runfiles/paths.ts

@@ -1,3 +1,7 @@
 // NB: on windows thanks to legacy 8-character path segments it might be like
 // c:/b/ojvxx6nx/execroot/build_~1/bazel-~1/x64_wi~1/bin/internal/npm_in~1/test
 export const BAZEL_OUT_REGEX = /(\/bazel-out\/|\/bazel-~1\/x64_wi~1\/)/;
+
+// The runfiles root symlink under which the repository mapping can be found.
+// https://cs.opensource.google/bazel/bazel/+/1b073ac0a719a09c9b2d1a52680517ab22dc971e:src/main/java/com/google/devtools/build/lib/analysis/Runfiles.java;l=424
+export const REPO_MAPPING_RLOCATION = "_repo_mapping";

packages/runfiles/runfiles.ts

@@ -1,7 +1,7 @@
 import * as fs from 'fs';
 import * as path from 'path';
 
-import {BAZEL_OUT_REGEX} from './paths';
+import {BAZEL_OUT_REGEX, REPO_MAPPING_RLOCATION} from './paths';
 
 /**
  * Class that provides methods for resolving Bazel runfiles.
@@ -17,6 +17,10 @@ export class Runfiles {
    * If the environment gives us enough hints, we can know the package path
    */
   package: string|undefined;
+  /**
+   * If the environment has repo mappings, we can use them to resolve repo relative paths.
+   */
+  repoMappings: Map<string, string>|undefined;
 
   constructor(private _env: typeof process.env) {
     // If Bazel sets a variable pointing to a runfiles manifest,
@@ -29,8 +33,10 @@ export class Runfiles {
       this.manifest = this.loadRunfilesManifest(_env['RUNFILES_MANIFEST_FILE']!);
     } else if (!!_env['RUNFILES_DIR']) {
       this.runfilesDir = path.resolve(_env['RUNFILES_DIR']!);
+      this.repoMappings = this.loadRepoMapping(this.runfilesDir);
     } else if (!!_env['RUNFILES']) {
       this.runfilesDir = path.resolve(_env['RUNFILES']!);
+      this.repoMappings = this.loadRepoMapping(this.runfilesDir);
     } else {
       throw new Error(
           'Every node program run under Bazel must have a $RUNFILES_DIR, $RUNFILES or $RUNFILES_MANIFEST_FILE environment variable');
@@ -116,6 +122,25 @@ export class Runfiles {
     return runfilesEntries;
   }
 
+  loadRepoMapping(runfilesDir: string): Map<string, string>|undefined {
+    const repoMappingPath = path.join(runfilesDir, REPO_MAPPING_RLOCATION);
+
+    if (fs.existsSync(repoMappingPath)) {
+      const repoMappings = new Map<string, string>()
+      const mappings = fs.readFileSync(repoMappingPath, {encoding: 'utf-8'});
+      for (const line of mappings.split('\n')) {
+        if (!line) continue;
+        const [from, repoName, repoPath] = line.split(',');
+
+        // TODO: from !== ''?
+        if (from === '') {
+          repoMappings.set(repoName, repoPath);
+        }
+      }
+      return repoMappings;
+    }
+  }
+
   /** Resolves the given module path. */
   resolve(modulePath: string) {
     // Normalize path by converting to forward slashes and removing all trailing
@@ -189,6 +214,15 @@ export class Runfiles {
         }
       }
     }
+    if (this.repoMappings && this.repoMappings.has(moduleBase)) {
+      const mappedRepo = this.repoMappings.get(moduleBase)
+      if (mappedRepo !== moduleBase) {
+        const maybe = this._resolve(mappedRepo, moduleTail)
+        if (maybe !== undefined) {
+          return maybe;
+        }
+      }
+    }
     if (this.runfilesDir) {
       const maybe = path.join(this.runfilesDir, moduleBase, moduleTail || '');
       if (fs.existsSync(maybe)) {