Fix "Argument list too long" error in the "spot" check when checking 100k+ files (#866).

NEWS

@@ -2,6 +2,8 @@
  * #860: Fix interaction between environment variable interpolation in constants and shell escaping.
  * #863: When color output is disabled (explicitly or implicitly), don't prefix each log line with
    the log level.
+ * #866: Fix "Argument list too long" error in the "spot" check when checking hundreds of thousands
+   of files at once.
  * #874: Add the configured repository label as "repository_label" to the interpolated variables
    passed to before/after command hooks.
  * In the "spot" check, don't try to hash symlinked directories.

borgmatic/actions/check.py

@@ -387,6 +387,9 @@ def collect_spot_check_archive_paths(
     )
 
 
+SAMPLE_PATHS_SUBSET_COUNT = 10000
+
+
 def compare_spot_check_hashes(
     repository,
     archive,
@@ -419,18 +422,42 @@ def compare_spot_check_hashes(
         f'{log_label}: Sampling {sample_count} source paths (~{spot_check_config["data_sample_percentage"]}%) for spot check'
     )
 
+    source_sample_paths_iterator = iter(source_sample_paths)
+    source_hashes = {}
+    archive_hashes = {}
+
+    # Only hash a few thousand files at a time (a subset of the total paths) to avoid an "Argument
+    # list too long" shell error.
+    while True:
         # Hash each file in the sample paths (if it exists).
+        source_sample_paths_subset = tuple(
+            itertools.islice(source_sample_paths_iterator, SAMPLE_PATHS_SUBSET_COUNT)
+        )
+        if not source_sample_paths_subset:
+            break
+
         hash_output = borgmatic.execute.execute_command_and_capture_output(
             (spot_check_config.get('xxh64sum_command', 'xxh64sum'),)
-        + tuple(path for path in source_sample_paths if path in existing_source_sample_paths)
+            + tuple(
+                path for path in source_sample_paths_subset if path in existing_source_sample_paths
+            )
         )
 
-    source_hashes = dict(
+        source_hashes.update(
+            **dict(
                 (reversed(line.split('  ', 1)) for line in hash_output.splitlines()),
-        **{path: '' for path in source_sample_paths if path not in existing_source_sample_paths},
+                # Represent non-existent files as having empty hashes so the comparison below still works.
+                **{
+                    path: ''
+                    for path in source_sample_paths_subset
+                    if path not in existing_source_sample_paths
+                },
+            )
         )
 
-    archive_hashes = dict(
+        # Get the hash for each file in the archive.
+        archive_hashes.update(
+            **dict(
                 reversed(line.split(' ', 1))
                 for line in borgmatic.borg.list.capture_archive_listing(
                     repository['path'],
@@ -438,13 +465,14 @@ def compare_spot_check_hashes(
                     config,
                     local_borg_version,
                     global_arguments,
-            list_paths=source_sample_paths,
+                    list_paths=source_sample_paths_subset,
                     path_format='{xxh64} /{path}{NL}',  # noqa: FS003
                     local_path=local_path,
                     remote_path=remote_path,
                 )
                 if line
             )
+        )
 
     # Compare the source hashes with the archive hashes to see how many match.
     failing_paths = []

tests/unit/actions/test_check.py

@@ -770,6 +770,46 @@ def test_compare_spot_check_hashes_considers_non_existent_path_as_not_matching()
     ) == ('/bar',)
 
 
+def test_compare_spot_check_hashes_with_too_many_paths_feeds_them_to_commands_in_chunks():
+    flexmock(module).SAMPLE_PATHS_SUBSET_COUNT = 2
+    flexmock(module.random).should_receive('sample').replace_with(
+        lambda population, count: population[:count]
+    )
+    flexmock(module.os.path).should_receive('exists').and_return(True)
+    flexmock(module.borgmatic.execute).should_receive(
+        'execute_command_and_capture_output'
+    ).with_args(('xxh64sum', '/foo', '/bar')).and_return('hash1  /foo\nhash2  /bar')
+    flexmock(module.borgmatic.execute).should_receive(
+        'execute_command_and_capture_output'
+    ).with_args(('xxh64sum', '/baz', '/quux')).and_return('hash3  /baz\nhash4  /quux')
+    flexmock(module.borgmatic.borg.list).should_receive('capture_archive_listing').and_return(
+        ['hash1 /foo', 'hash2 /bar']
+    ).and_return(['hash3 /baz', 'nothash4 /quux'])
+
+    assert module.compare_spot_check_hashes(
+        repository={'path': 'repo'},
+        archive='archive',
+        config={
+            'checks': [
+                {
+                    'name': 'archives',
+                    'frequency': '2 weeks',
+                },
+                {
+                    'name': 'spot',
+                    'data_sample_percentage': 100,
+                },
+            ]
+        },
+        local_borg_version=flexmock(),
+        global_arguments=flexmock(),
+        local_path=flexmock(),
+        remote_path=flexmock(),
+        log_label='repo',
+        source_paths=('/foo', '/bar', '/baz', '/quux'),
+    ) == ('/quux',)
+
+
 def test_spot_check_without_spot_configuration_errors():
     with pytest.raises(ValueError):
         module.spot_check(