Database lock

Is a lockfile named "$DATABASE_NAME.lock". `Database._write()` prevents
race conditions by enforcing the lock, and waiting until a previous lock
is released.

1 files changed, 43 insertions, 2 deletions

diff --git a/jimbrella/database.py b/jimbrella/database.py
index e841650..3d835a8 100644
--- a/jimbrella/database.py
+++ b/jimbrella/database.py
@@ -1,4 +1,5 @@
 import csv
+import os
 from datetime import datetime
 
 
@@ -9,7 +10,7 @@ class Database:
         Currently, the data are represented in a csv file.
 
         Explanation of data types:
-        - id        | a unique numerical identity of unsigned integer type.   
+        - id        | a unique numerical identity of unsigned integer type.
         - uint      | whatever fits the regex /[0-9]+/. despite carrying a numerical value, it is
                     | internally represented as a string.
         - string    | any UTF-8 string.
@@ -37,6 +38,33 @@ class Database:
         """
         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
+
     def _read(self) -> list:
         """Deserialize database."""
         # Create database if it does not yet exist
@@ -67,10 +95,15 @@ class Database:
 
     def _write(self, umbrellas: list) -> None:
         """Serialize database. When a failure occurs, abort and recover data."""
+        # make backup in memory
         with open(self.path) as f:
             backup = f.read()
             f.close()
 
+        # wait until database is locked for this write
+        while not self._lock():
+            continue
+
         f = open(self.path, "w")
         try:
             writer = csv.writer(f)
@@ -84,7 +117,9 @@ class Database:
                         umb["tenant_id"],
                         umb["tenant_phone"],
                         umb["tenant_email"],
-                        umb["lent_at"].isoformat(timespec="seconds") if umb["lent_at"] else "",
+                        umb["lent_at"].isoformat(timespec="seconds")
+                        if umb["lent_at"]
+                        else "",
                     ]
                     for umb in umbrellas
                 ]
@@ -98,6 +133,8 @@ class Database:
             f.write(backup)
             f.close()
 
+        self._unlock()
+
     def _update(self, update: dict) -> list:
         """Update status of one umbrella, and return the entire updated database."""
         umbrellas = self._read()
@@ -116,6 +153,10 @@ class Database:
             if umb["serial"] == serial:
                 return umb
 
+    def read(self) -> list:
+        """Currently an alias for `_read()`."""
+        return self._read()
+
     def take_away(
         self, serial, tenant_name, tenant_id, tenant_phone="", tenant_email=""
     ) -> None: