settings: use fsync / fdatasync on Linux to sync contents

Avoid an OS-level sync by using fsync and fdatasync when they are available.

Related-to: #1305
Signed-off-by: David Aguilar <davvid@gmail.com>

cola/core.py

@@ -3,6 +3,7 @@
 The @interruptable functions retry when system calls are interrupted,
 e.g. when python raises an IOError or OSError with errno == EINTR.
 """
+import ctypes
 import functools
 import itertools
 import mimetypes
@@ -481,9 +482,28 @@ def _find_executable(executable, path=None):
     return executable
 
 
-def sync():
-    """Force writing of everything to disk. No-op on systems without os.sync()"""
-    if hasattr(os, 'sync'):
+def sync(path):
+    """Flush contents to disk. No-op on systems without os.sync() or libc.so.6"""
+    try:
+        libc = ctypes.CDLL('libc.so.6')
+    except OSError:
+        libc = None
+    synced = False
+    has_fdatasync = libc and hasattr(libc, 'fdatasync')
+    has_fsync = libc and hasattr(libc, 'fsync')
+    if has_fdatasync:
+        try:
+            with open(path, 'a', encoding='utf-8') as fd:
+                synced = libc.fdatasync(fd.fileno()) == 0
+        except (OSError, IOError):
+            pass
+    elif has_fsync:
+        try:
+            with open(path, 'a', encoding='utf-8') as fd:
+                synced = libc.fsync(fd.fileno()) == 0
+        except (OSError, IOError):
+            pass
+    if not synced and hasattr(os, 'sync'):
         os.sync()
 
 

cola/settings.py

@@ -191,7 +191,7 @@ def save(self, sync=True):
             return
         # Flush the data to disk.
         if sync:
-            core.sync()
+            core.sync(path)
         # Delete the .bak file.
         if core.exists(path_bak):
             remove_path(path_bak)

docs/git-cola.rst

@@ -857,9 +857,9 @@ Defaults to `false`.
 
 cola.sync
 ---------
-Set to `false` to disable calling `os.sync()` when saving settings.
-Defaults to `true`, which means that `os.sync()` is called when windows are closed
-and their settings are saved.
+Set to `false` to disable calling `os.sync()`  / `fdatasync(2)` / `fsync(2)` when saving
+settings. Defaults to `true`, which means that these functions are called when windows
+are closed and their settings are saved.
 
 cola.tabwidth
 -------------