fix(builtin): account for racy deletion of symlink in linker (#2662)

On Windows, the linker has been prone to race conditions with respect to
symlinking packages into the `node_modules` tree, causing ENOENT errors
that causes the target to fail. Since the intention of `symlinkWithUnlink`
and `deleteDirectory` is to unlink the directory anyway, we can simply
ignore ENOENT errors for the directory that would be about to be unlinked.

internal/linker/index.js

@@ -50,6 +50,30 @@ function gracefulLstat(path) {
         }
     });
 }
+function gracefulReadlink(path) {
+    try {
+        return fs.readlinkSync(path);
+    }
+    catch (e) {
+        if (e.code === 'ENOENT') {
+            return null;
+        }
+        throw e;
+    }
+}
+function gracefulReaddir(path) {
+    return __awaiter(this, void 0, void 0, function* () {
+        try {
+            return yield fs.promises.readdir(path);
+        }
+        catch (e) {
+            if (e.code === 'ENOENT') {
+                return [];
+            }
+            throw e;
+        }
+    });
+}
 function unlink(moduleName) {
     return __awaiter(this, void 0, void 0, function* () {
         const stat = yield gracefulLstat(moduleName);
@@ -69,11 +93,12 @@ function unlink(moduleName) {
 function deleteDirectory(p) {
     return __awaiter(this, void 0, void 0, function* () {
         log_verbose("Deleting children of", p);
-        for (let entry of yield fs.promises.readdir(p)) {
+        for (let entry of yield gracefulReaddir(p)) {
             const childPath = path.join(p, entry);
             const stat = yield gracefulLstat(childPath);
             if (stat === null) {
-                throw Error(`File does not exist, but is listed as directory entry: ${childPath}`);
+                log_verbose(`File does not exist, but is listed as directory entry: ${childPath}`);
+                continue;
             }
             if (stat.isDirectory()) {
                 yield deleteDirectory(childPath);
@@ -277,11 +302,17 @@ function main(args, runfiles) {
                     stats = yield gracefulLstat(p);
                 }
                 if (runfiles.manifest && execroot && stats !== null && stats.isSymbolicLink()) {
-                    const symlinkPath = fs.readlinkSync(p).replace(/\\/g, '/');
-                    if (path.relative(symlinkPath, target) != '' &&
-                        !path.relative(execroot, symlinkPath).startsWith('..')) {
-                        log_verbose(`Out-of-date symlink for ${p} to ${symlinkPath} detected. Target should be ${target}. Unlinking.`);
-                        yield unlink(p);
+                    const symlinkPathRaw = gracefulReadlink(p);
+                    if (symlinkPathRaw !== null) {
+                        const symlinkPath = symlinkPathRaw.replace(/\\/g, '/');
+                        if (path.relative(symlinkPath, target) != '' &&
+                            !path.relative(execroot, symlinkPath).startsWith('..')) {
+                            log_verbose(`Out-of-date symlink for ${p} to ${symlinkPath} detected. Target should be ${target}. Unlinking.`);
+                            yield unlink(p);
+                        }
+                        else {
+                            log_verbose(`The symlink at ${p} no longer exists, so no need to unlink it.`);
+                        }
                     }
                 }
                 return symlink(target, p);

internal/linker/link_node_modules.ts

@@ -59,6 +59,36 @@ async function gracefulLstat(path: string): Promise<fs.Stats|null> {
   }
 }
 
+/**
+ * Resolves a symlink to its linked path for a given path. Returns `null` if the path
+ * does not exist on disk.
+ */
+function gracefulReadlink(path: string): string|null {
+  try {
+    return fs.readlinkSync(path);
+  } catch (e) {
+    if (e.code === 'ENOENT') {
+      return null;
+    }
+    throw e;
+  }
+}
+
+/**
+ * Lists the names of files and directories that exist in the given path. Returns an empty
+ * array if the path does not exist on disk.
+ */
+async function gracefulReaddir(path: string): Promise<string[]> {
+  try {
+    return await fs.promises.readdir(path);
+  } catch (e) {
+    if (e.code === 'ENOENT') {
+      return [];
+    }
+    throw e;
+  }
+}
+
 /**
  * Deletes the given module name from the current working directory (i.e. symlink root).
  * If the module name resolves to a directory, the directory is deleted. Otherwise the
@@ -81,11 +111,12 @@ async function unlink(moduleName: string) {
 /** Asynchronously deletes a given directory (with contents). */
 async function deleteDirectory(p: string) {
   log_verbose("Deleting children of", p);
-  for (let entry of await fs.promises.readdir(p)) {
+  for (let entry of await gracefulReaddir(p)) {
     const childPath = path.join(p, entry);
     const stat = await gracefulLstat(childPath);
     if (stat === null) {
-      throw Error(`File does not exist, but is listed as directory entry: ${childPath}`);
+      log_verbose(`File does not exist, but is listed as directory entry: ${childPath}`);
+      continue;
     }
     if (stat.isDirectory()) {
       await deleteDirectory(childPath);
@@ -413,18 +444,25 @@ export async function main(args: string[], runfiles: Runfiles) {
     // then this is guaranteed to be not an artifact from a previous linker run. If not we need to
     // check.
     if (runfiles.manifest && execroot && stats !== null && stats.isSymbolicLink()) {
-      const symlinkPath = fs.readlinkSync(p).replace(/\\/g, '/');
-      if (path.relative(symlinkPath, target) != '' &&
-          !path.relative(execroot, symlinkPath).startsWith('..')) {
-        // Left-over out-of-date symlink from previous run. This can happen if switching between
-        // root configuration options such as `--noenable_runfiles` and/or
-        // `--spawn_strategy=standalone`. It can also happen if two different targets link the same
-        // module name to different targets in a non-sandboxed environment. The latter will lead to
-        // undeterministic behavior.
-        // TODO: can we detect the latter case and throw an apprioriate error?
-        log_verbose(`Out-of-date symlink for ${p} to ${symlinkPath} detected. Target should be ${
-            target}. Unlinking.`);
-        await unlink(p);
+      // Although `stats` suggests that the file exists as a symlink, it may have been deleted by
+      // another process. Only proceed unlinking if the file actually still exists.
+      const symlinkPathRaw = gracefulReadlink(p);
+      if (symlinkPathRaw !== null) {
+        const symlinkPath = symlinkPathRaw.replace(/\\/g, '/');
+        if (path.relative(symlinkPath, target) != '' &&
+            !path.relative(execroot, symlinkPath).startsWith('..')) {
+          // Left-over out-of-date symlink from previous run. This can happen if switching between
+          // root configuration options such as `--noenable_runfiles` and/or
+          // `--spawn_strategy=standalone`. It can also happen if two different targets link the
+          // same module name to different targets in a non-sandboxed environment. The latter will
+          // lead to undeterministic behavior.
+          // TODO: can we detect the latter case and throw an apprioriate error?
+          log_verbose(`Out-of-date symlink for ${p} to ${symlinkPath} detected. Target should be ${
+              target}. Unlinking.`);
+          await unlink(p);
+        } else {
+          log_verbose(`The symlink at ${p} no longer exists, so no need to unlink it.`);
+        }
       }
     }
     return symlink(target, p);