fix(sdk): refuse an unreadable SDK clearly, and fix the runtime fallback

An SDK directory that exists but cannot be read passed discovery, and the
analyzer then died with a raw FileSystemException stack trace. This
happens for real under macOS privacy protection: a Flutter SDK under
~/Documents stats fine but every open fails, and every app whose
package_config.json names that flutterRoot crashed:

  Unhandled exception:
  FileSystemException(path=.../dart-sdk/lib/_internal/allowed_experiments.json; ...)

The chosen SDK is now probed before the analysis collection is built, and
an unreadable one is a PipeException naming the path, where it was
discovered from, and the --sdk remedy.

The last-resort fallback to the running Dart SDK took the executable's
directory (<sdk>/bin) as the SDK root, which never contains
lib/libraries.json, so it could never succeed. sdkFromExecutable now
maps <sdk>/bin/dart to <sdk>.

lib/src/analysis.dart

@@ -55,6 +55,7 @@ class Analyzer {
       explicit: sdkOverride,
       flutterRootHint: project.flutterRoot,
     );
+    requireReadableSdk(resolution);
     final collection = AnalysisContextCollection(
       includedPaths: [project.root],
       resourceProvider: PhysicalResourceProvider.INSTANCE,

lib/src/sdk.dart

@@ -45,6 +45,44 @@ String? _normalizedDirectory(String? path) {
   return _isDirectory(path) ? p.normalize(p.absolute(path)) : null;
 }
 
+/// The SDK root holding a `<sdk>/bin/dart` executable, or null when the
+/// executable does not sit inside a Dart SDK.
+///
+/// The executable's own directory is `<sdk>/bin`, which never contains
+/// `lib/libraries.json`; checking it meant this fallback could never succeed.
+String? sdkFromExecutable(String executable) {
+  final root = p.dirname(p.dirname(p.normalize(executable)));
+  return _isDartSdk(root) ? root : null;
+}
+
+/// Fails with an actionable error when the chosen SDK cannot be read.
+///
+/// Existence is not enough: macOS privacy protection (for example a Flutter SDK
+/// under ~/Documents) lets `stat` succeed while every open fails, so discovery
+/// accepted the SDK and the analyzer later died with a raw FileSystemException.
+void requireReadableSdk(SdkResolution resolution) {
+  final sdk = resolution.sdkPath;
+  if (sdk == null) return;
+  for (final probe in [
+    p.join(sdk, 'version'),
+    p.join(sdk, 'lib', 'libraries.json'),
+  ]) {
+    final file = File(probe);
+    if (!file.existsSync()) continue;
+    try {
+      file.openSync().closeSync();
+    } on FileSystemException catch (error) {
+      throw PipeException(
+        'the Dart SDK at $sdk (from ${resolution.source}) cannot be read: '
+        '${error.osError?.message ?? error.message}',
+        remedy:
+            'grant this process access to that directory, or pass '
+            '--sdk <path to a readable dart-sdk>',
+      );
+    }
+  }
+}
+
 /// Given a Flutter root, return its bundled Dart SDK if one is present.
 String? dartSdkInFlutterRoot(String flutterRoot) {
   final candidate = p.join(flutterRoot, 'bin', 'cache', 'dart-sdk');
@@ -144,8 +182,8 @@ SdkResolution resolveSdk({String? explicit, String? flutterRootHint}) {
     );
   }
 
-  final runtimeSdk = p.dirname(p.normalize(Platform.resolvedExecutable));
-  if (_isDartSdk(runtimeSdk)) {
+  final runtimeSdk = sdkFromExecutable(Platform.resolvedExecutable);
+  if (runtimeSdk != null) {
     return SdkResolution(
       sdkPath: runtimeSdk,
       flutterRoot: null,

test/sdk_test.dart

@@ -0,0 +1,61 @@
+import 'dart:io';
+
+import 'package:pipe/pipe.dart';
+import 'package:pipe/src/sdk.dart';
+import 'package:test/test.dart';
+
+/// A directory laid out like a Dart SDK: `bin/dart`, `lib/libraries.json`,
+/// `version`.
+Directory _fakeSdk(String label) {
+  final sdk = Directory.systemTemp.createTempSync('pipe-$label');
+  File('${sdk.path}/version').writeAsStringSync('3.13.0\n');
+  File('${sdk.path}/lib/libraries.json')
+    ..createSync(recursive: true)
+    ..writeAsStringSync('{}');
+  File('${sdk.path}/bin/dart').createSync(recursive: true);
+  return sdk;
+}
+
+void main() {
+  test('an SDK that exists but cannot be read is a clear error', () {
+    // macOS privacy protection (a Flutter SDK under ~/Documents) lets stat
+    // succeed while every open fails, so discovery accepted the SDK and the
+    // analyzer later died with a raw FileSystemException stack trace.
+    final sdk = _fakeSdk('unreadable-sdk');
+    final version = File('${sdk.path}/version');
+    Process.runSync('chmod', ['000', version.path]);
+    try {
+      version.readAsStringSync();
+      markTestSkipped('chmod 000 does not restrict this process');
+      return;
+    } on FileSystemException {
+      // Expected: the restriction is real, so the check below is meaningful.
+    }
+
+    final resolution = resolveSdk(explicit: sdk.path);
+    expect(
+      () => requireReadableSdk(resolution),
+      throwsA(
+        isA<PipeException>()
+            .having((e) => e.message, 'message', contains(sdk.path))
+            .having((e) => e.remedy, 'remedy', contains('--sdk')),
+      ),
+    );
+  });
+
+  test('a readable SDK passes the check', () {
+    final sdk = _fakeSdk('readable-sdk');
+    expect(
+      () => requireReadableSdk(resolveSdk(explicit: sdk.path)),
+      returnsNormally,
+    );
+  });
+
+  test('the running dart binary maps to its SDK root, not to bin/', () {
+    // dirname(<sdk>/bin/dart) is <sdk>/bin, which never contains
+    // lib/libraries.json, so this fallback could never succeed.
+    final sdk = _fakeSdk('runtime-sdk');
+    expect(sdkFromExecutable('${sdk.path}/bin/dart'), sdk.path);
+    expect(sdkFromExecutable('${sdk.path}/version'), isNull);
+  });
+}