Add per-config process locking

README.rst

@@ -47,6 +47,9 @@ To run serac::
 
     /path/to/venv/bin/serac CONFIG COMMAND [OPTIONS]
 
+It is safe to run Serac from a cron job; it will not allow multiple processes to work
+with the same config file at the same time.
+
 
 Commands
 --------
@@ -165,3 +168,22 @@ To run tests::
     cd serac/repo
     . ../venv/bin/activate
     pytest
+
+
+Changelog
+=========
+
+0.0.2, 2019-09-25
+-----------------
+
+Feature:
+
+* Add process locking
+
+
+0.0.1, 2019-09-23
+-----------------
+
+Feature:
+
+* Initial release

serac/commands.py

@@ -1,6 +1,7 @@
 """
 Commands
 """
+import fcntl
 import sys
 from datetime import datetime
 from pathlib import Path
@@ -46,12 +47,21 @@ def __repr__(self):  # pragma: no cover
     type=click.Path(exists=True, file_okay=True, dir_okay=False, resolve_path=True),
 )
 @click.pass_context
-def cli(ctx, config):
+def cli(ctx, config: str):
     try:
         ctx.obj["config"] = Config(config)
     except Exception as e:
         raise click.ClickException(f"Invalid config: {e}")
 
+    # Lock - only one process on a config at a time
+    ctx.obj["lock"] = open(config, "r")
+    try:
+        fcntl.flock(ctx.obj["lock"], fcntl.LOCK_EX | fcntl.LOCK_NB)
+    except IOError:
+        raise click.ClickException(
+            f"Config {config} is already in use by another process"
+        )
+
 
 @cli.command()
 @click.pass_context

serac/config.py

@@ -108,15 +108,15 @@ class Config:
     archive: ArchiveConfig
     index: IndexConfig
 
-    def __init__(self, path: Path = None) -> None:
-        if path:
-            self.load(path)
+    def __init__(self, filename: str = None) -> None:
+        if filename:
+            self.load(filename)
 
-    def load(self, path: Path) -> None:
+    def load(self, filename: str) -> None:
         parser = ConfigParser()
 
         # Let parsing errors go through unchanged
-        parser.read(path)
+        parser.read(filename)
 
         if sorted(parser.sections()) != sorted(self.sections):
             raise ValueError(

tests/test_config.py

@@ -18,7 +18,7 @@ def test_parser_source__valid(fs):
     )
     fs.create_dir("/path/to")
     fs.create_dir("/path/to/backup")
-    config = Config(path=Path("/sample.conf"))
+    config = Config(filename="/sample.conf")
 
     assert isinstance(config.source, SourceConfig)
     assert config.source.includes == ["/path/to/source", "/path/somewhere/else"]
@@ -48,7 +48,7 @@ def test_parser_archive__local(fs):
     )
     fs.create_dir("/path/to")
     fs.create_dir("/path/to/backup")
-    config = Config(path=Path("/sample.conf"))
+    config = Config(filename="/sample.conf")
 
     assert isinstance(config.archive, ArchiveConfig)
     assert isinstance(config.archive.storage, Local)
@@ -61,7 +61,7 @@ def test_parser_archive__s3(fs):
         "/sample.conf", contents=SAMPLE_CONFIG.format(storage=SAMPLE_STORAGE_S3)
     )
     fs.create_dir("/path/to")
-    config = Config(path=Path("/sample.conf"))
+    config = Config(filename="/sample.conf")
 
     assert isinstance(config.archive, ArchiveConfig)
     assert isinstance(config.archive.storage, S3)
@@ -107,7 +107,7 @@ def test_parser_index(fs):
     )
     fs.create_dir("/path/to")
     fs.create_dir("/path/to/backup")
-    config = Config(path=Path("/sample.conf"))
+    config = Config(filename="/sample.conf")
 
     assert isinstance(config.index, IndexConfig)
     assert config.index.path == Path("/path/to/index.sqlite")
@@ -153,7 +153,7 @@ def test_parser_config__sections_missing__raises_exception(fs):
     )
 
     with pytest.raises(ValueError) as e:
-        Config(path=Path("/sample.conf"))
+        Config(filename="/sample.conf")
     assert str(e.value) == (
         "Invalid config file; must contain source, archive and "
         f"index sections; instead found invalid"
@@ -172,7 +172,7 @@ def test_parser_config__archive_section_missing__raises_exception(fs):
     )
 
     with pytest.raises(ValueError) as e:
-        Config(path=Path("/sample.conf"))
+        Config(filename="/sample.conf")
     assert str(e.value) == (
         "Invalid config file; must contain source, archive and "
         f"index sections; instead found source, index"