Upgrade debug mode: Add detection for default checksum algorithm breaking change

.changes/next-release/enhancement-Migration-87248.json

@@ -0,0 +1,5 @@
+{
+  "type": "enhancement",
+  "category": "Migration",
+  "description": "Added support to upgrade debug mode for detecting breaking change introduced in AWS CLI v2: in v2, Cyclic Redundancy Check 64 (CRC64NVME) is used by default for ``aws s3`` commands that upload a file to S3, but Cyclic Redundancy Check 32 (CRC32) is used instead in v1. To use upgrade debug mode, specify the ``--v2-debug`` global parameter on a command."
+}

awscli/customizations/s3/subcommands.py

@@ -1144,6 +1144,24 @@ def run(self):
                     '#cliv2-migration-s3-copy-metadata.\n\n',
                     out_file=sys.stderr
                 )
+            elif (
+                operation_name == 'upload'
+                and self.parameters.get('checksum_algorithm') is None
+            ):
+                uni_print(
+                    '\nAWS CLI v2 UPGRADE WARNING: In AWS CLI v2, for '
+                    '`aws s3` commands that upload a file to an S3 bucket, '
+                    'Cyclic Redundancy Check 64 (CRC64NVME) will be used to '
+                    'compute object checksums by default and include it in '
+                    'the request. This is different from v1 behavior, where '
+                    'Cyclic Redundancy Check 32 (CRC32) checksums will '
+                    'instead be used. For guidance on retaining v1 behavior '
+                    'in AWS CLI v2, or for more details, see '
+                    'https://docs.aws.amazon.com/cli/latest/userguide/'
+                    'cliv2-migration-changes.html'
+                    '#cliv2-migration-checksums.\n\n',
+                    out_file=sys.stderr
+                )
 
         fgen_kwargs = {
             'client': self._source_client, 'operation_name': operation_name,

tests/functional/s3/test_cp_command.py

@@ -817,6 +817,33 @@ def test_download_with_checksum_mode_crc32c(self):
         self.assertEqual(self.operations_called[1][0].name, 'GetObject')
         self.assertEqual(self.operations_called[1][1]['ChecksumMode'], 'ENABLED')
 
+    def test_upload_with_no_checksum_param_v2_debug(self):
+        full_path = self.files.create_file('foo.txt', 'contents')
+        cmdline = f'{self.prefix} {full_path} s3://bucket/key.txt --v2-debug'
+        _, stderr, _ = self.run_cmd(cmdline, expected_rc=0)
+        self.assertIn(
+            'AWS CLI v2 UPGRADE WARNING: In AWS CLI v2, for `aws s3` '
+            'commands that upload a file to an S3 bucket, Cyclic Redundancy '
+            'Check 64 (CRC64NVME) will be used to compute object checksums by '
+            'default and include it in the request.',
+            stderr
+        )
+
+    def test_upload_with_crc32_checksum_v2_debug(self):
+        full_path = self.files.create_file('foo.txt', 'contents')
+        cmdline = (
+            f'{self.prefix} {full_path} s3://bucket/key.txt '
+            f'--checksum-algorithm CRC32 --v2-debug'
+        )
+        _, stderr, _ = self.run_cmd(cmdline, expected_rc=0)
+        self.assertNotIn(
+            'AWS CLI v2 UPGRADE WARNING: In AWS CLI v2, for `aws s3` '
+            'commands that upload a file to an S3 bucket, Cyclic Redundancy '
+            'Check 64 (CRC64NVME) will be used to compute object checksums by '
+            'default and include it in the request.',
+            stderr
+        )
+
 
 class TestStreamingCPCommand(BaseAWSCommandParamsTest):
     def test_streaming_upload(self):

tests/functional/s3/test_mv_command.py

@@ -165,6 +165,33 @@ def test_download_with_checksum_mode_crc32(self):
         self.assertEqual(self.operations_called[1][0].name, 'GetObject')
         self.assertEqual(self.operations_called[1][1]['ChecksumMode'], 'ENABLED')
 
+    def test_upload_with_no_checksum_param_v2_debug(self):
+        full_path = self.files.create_file('foo.txt', 'contents')
+        cmdline = f'{self.prefix} {full_path} s3://bucket/key.txt --v2-debug'
+        _, stderr, _ = self.run_cmd(cmdline, expected_rc=0)
+        self.assertIn(
+            'AWS CLI v2 UPGRADE WARNING: In AWS CLI v2, for `aws s3` '
+            'commands that upload a file to an S3 bucket, Cyclic Redundancy '
+            'Check 64 (CRC64NVME) will be used to compute object checksums by '
+            'default and include it in the request.',
+            stderr
+        )
+
+    def test_upload_with_crc32_checksum_v2_debug(self):
+        full_path = self.files.create_file('foo.txt', 'contents')
+        cmdline = (
+            f'{self.prefix} {full_path} s3://bucket/key.txt '
+            f'--checksum-algorithm CRC32 --v2-debug'
+        )
+        _, stderr, _ = self.run_cmd(cmdline, expected_rc=0)
+        self.assertNotIn(
+            'AWS CLI v2 UPGRADE WARNING: In AWS CLI v2, for `aws s3` '
+            'commands that upload a file to an S3 bucket, Cyclic Redundancy '
+            'Check 64 (CRC64NVME) will be used to compute object checksums by '
+            'default and include it in the request.',
+            stderr
+        )
+
 
 class TestMvCommandWithValidateSameS3Paths(BaseS3TransferCommandTest):