Separate Lockfile into new class

3 files changed, 55 insertions, 54 deletions

diff --git a/jimbrella/admin_log.py b/jimbrella/admin_log.py
index 811d722..4cc9b16 100644
--- a/jimbrella/admin_log.py
+++ b/jimbrella/admin_log.py
@@ -1,6 +1,7 @@
 import csv
 import os
 from datetime import datetime
+from .lockfile import Lockfile
 
 
 class AdminLog:
@@ -33,25 +34,7 @@ class AdminLog:
 
     def __init__(self, path: str):
         self.path = path
-
-    def _lock(self) -> int:
-        """Create a lockfile for admin log. Operates in the sameway as `Database._lock`."""
-        try:
-            f = open(self.path + ".lock", "x")
-            f.close()
-            return 1
-        except FileExistsError:
-            return 0
-
-    def _unlock(self) -> None:
-        """Remove lockfile created by `_lock`.
-
-        If there is no lockfile, simply ignore.
-        """
-        try:
-            os.remove(self.path + ".lock")
-        except FileNotFoundError:
-            pass
+        self.lockfile = Lockfile(self.path)
 
     def _read(self) -> list:
         """Deserialize admin log."""
@@ -102,7 +85,7 @@ class AdminLog:
         `logs` is a list of log entries, to be appended at the end of the log file.
         """
         # wait until database is locked for this write
-        while not self._lock():
+        while not self.lockfile.lock():
             continue
 
         with open(self.path, "a") as f:  # append only
@@ -112,9 +95,21 @@ class AdminLog:
                 line = [event, entry["date"].isoformat()]
                 info = []
                 if event in ("TAKEAWAY", "GIVEBACK", "OVERDUE"):
-                    info = [entry["serial"], entry["tenant_name"], entry["tenant_id"], entry["tenant_phone"], entry["tenant_email"]]
+                    info = [
+                        entry["serial"],
+                        entry["tenant_name"],
+                        entry["tenant_id"],
+                        entry["tenant_phone"],
+                        entry["tenant_email"],
+                    ]
                 elif event == "ADMIN_MODIFY_DB":
-                    info = [entry["admin_name"], entry["serial"], entry["column"], entry["past_value"], entry["new_value"]]
+                    info = [
+                        entry["admin_name"],
+                        entry["serial"],
+                        entry["column"],
+                        entry["past_value"],
+                        entry["new_value"],
+                    ]
 
                 line.extend(info)
                 line.append(entry["note"])
@@ -122,4 +117,4 @@ class AdminLog:
 
             f.close()
 
-        self._unlock()
+        self.lockfile.unlock()
diff --git a/jimbrella/database.py b/jimbrella/database.py
index 37f6c82..fd8f35c 100644
--- a/jimbrella/database.py
+++ b/jimbrella/database.py
@@ -1,6 +1,7 @@
 import csv
 import os
 from datetime import datetime, timedelta
+from .lockfile import Lockfile
 from .utils import human_datetime, human_timedelta
 from .config import DUE_HOURS
 from .exceptions import *
@@ -43,33 +44,7 @@ class Database:
                         | jForm answer sheet. is blank otherwise.
         """
         self.path = path
-
-    def _lock(self) -> int:
-        """Create a lockfile for database.
-
-        If the database is called "db.csv", the lockfile, located in the same
-        directory, would be called "db.csv.lock".
-
-        Returns 1 if the lockfile is successfully created, 0 if the lockfile
-        already exists (strong hint of unrecovered crash or potential race
-        condition - caller of this method should abort whatever it is doing)
-        """
-        try:
-            f = open(self.path + ".lock", "x")
-            f.close()
-            return 1
-        except FileExistsError:
-            return 0
-
-    def _unlock(self) -> None:
-        """Remove lockfile created by _lock.
-
-        If there is no lockfile, simply ignore.
-        """
-        try:
-            os.remove(self.path + ".lock")
-        except FileNotFoundError:
-            pass
+        self.lockfile = Lockfile(self.path)
 
     def _read(self) -> list:
         """Deserialize database."""
@@ -106,9 +81,7 @@ class Database:
             backup = f.read()
             f.close()
 
-        # wait until database is locked for this write
-        while not self._lock():
-            continue
+        self.lockfile.lock()
 
         f = open(self.path, "w")
         try:
@@ -140,7 +113,7 @@ class Database:
             f.close()
             raise e
 
-        self._unlock()
+        self.lockfile.unlock()
 
     def _update(self, update: dict) -> list:
         """Update status of one umbrella, and return the entire updated database."""
diff --git a/jimbrella/lockfile.py b/jimbrella/lockfile.py
new file mode 100644
index 0000000..b898142
--- /dev/null
+++ b/jimbrella/lockfile.py
@@ -0,0 +1,33 @@
+import os
+
+
+class Lockfile:
+    """Prevent unwanted concurrency for file I/O.
+
+    For a file named "<file>", create or remove a lockfile named "<file>.lock".
+
+    When a process is modifying the file, call `lock(). When the modification
+    is done, call `unlock()`.
+    """
+
+    def __init__(self, filepath):
+        self.filepath = filepath
+        self.lockpath = filepath + ".lock"
+
+    def lock(self):
+        """Continue attempting to lock until locked."""
+        locked = False
+        while not locked:
+            try:
+                f = open(self.lockpath, "x")
+                f.close()
+                locked = True
+            except FileExistsError:
+                continue
+
+    def unlock(self):
+        """Remove lockfile."""
+        try:
+            os.remove(self.lockpath)
+        except FileNotFoundError:
+            return