Allow disabling all fingerprint caches via environment variable (#31171)

packages/flutter_tools/lib/src/base/fingerprint.dart

@@ -12,9 +12,17 @@ import '../convert.dart' show json;
 import '../globals.dart';
 import '../version.dart';
 import 'file_system.dart';
+import 'platform.dart';
 
 typedef FingerprintPathFilter = bool Function(String path);
 
+/// Whether to completely disable build caching.
+///
+/// This is done by always returning false from fingerprinter invocations. This
+/// is safe to do generally, because fingerprinting is only a performance
+/// improvement.
+bool get _disableBuildCache => platform.environment['DISABLE_FLUTTER_BUILD_CACHE']?.toLowerCase() == 'true';
+
 /// A tool that can be used to compute, compare, and write [Fingerprint]s for a
 /// set of input files and associated build settings.
 ///
@@ -50,6 +58,9 @@ class Fingerprinter {
   }
 
   Future<bool> doesFingerprintMatch() async {
+    if (_disableBuildCache) {
+      return false;
+    }
     try {
       final File fingerprintFile = fs.file(fingerprintPath);
       if (!fingerprintFile.existsSync())

packages/flutter_tools/test/base/fingerprint_test.dart

@@ -5,6 +5,7 @@
 import 'dart:convert' show json;
 
 import 'package:file/memory.dart';
+import 'package:flutter_tools/src/base/platform.dart';
 import 'package:flutter_tools/src/build_info.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/base/fingerprint.dart';
@@ -192,6 +193,46 @@ void main() {
       expect(await fingerprinter.doesFingerprintMatch(), isTrue);
     }, overrides: contextOverrides);
 
+    final Platform mockPlatformDisabledCache = MockPlatform();
+    mockPlatformDisabledCache.environment['DISABLE_FLUTTER_BUILD_CACHE']  = 'true';
+    testUsingContext('can be disabled with an environment variable', () async {
+      await fs.file('a.dart').create();
+      await fs.file('b.dart').create();
+
+      final Fingerprinter fingerprinter = Fingerprinter(
+        fingerprintPath: 'out.fingerprint',
+        paths: <String>['a.dart', 'b.dart'],
+        properties: <String, String>{
+          'bar': 'baz',
+          'wobble': 'womble',
+        },
+      );
+      await fingerprinter.writeFingerprint();
+      expect(await fingerprinter.doesFingerprintMatch(), isFalse);
+    }, overrides: <Type, Generator>{
+      Platform: () => mockPlatformDisabledCache,
+    }..addAll(contextOverrides));
+
+    final Platform mockPlatformEnabledCache = MockPlatform();
+    mockPlatformEnabledCache.environment['DISABLE_FLUTTER_BUILD_CACHE']  = 'false';
+    testUsingContext('can be not-disabled with an environment variable', () async {
+      await fs.file('a.dart').create();
+      await fs.file('b.dart').create();
+
+      final Fingerprinter fingerprinter = Fingerprinter(
+        fingerprintPath: 'out.fingerprint',
+        paths: <String>['a.dart', 'b.dart'],
+        properties: <String, String>{
+          'bar': 'baz',
+          'wobble': 'womble',
+        },
+      );
+      await fingerprinter.writeFingerprint();
+      expect(await fingerprinter.doesFingerprintMatch(), isTrue);
+    }, overrides: <Type, Generator>{
+      Platform: () => mockPlatformEnabledCache,
+    }..addAll(contextOverrides));
+
     testUsingContext('fails to write fingerprint if inputs are missing', () async {
       final Fingerprinter fingerprinter = Fingerprinter(
         fingerprintPath: 'out.fingerprint',
@@ -471,3 +512,8 @@ void main() {
     }, overrides: contextOverrides);
   });
 }
+
+class MockPlatform extends Mock implements Platform {
+  @override
+  Map<String, String> environment = <String, String>{};
+}