platform: use F_FULLSYNC on macOS for SyncFile data durability

[mr-raj12]

Summary

• SyncFile.sync() calls platform.fdatasync(self.fd), which resolves to os.fdatasync or os.fsync
• On macOS, fsync() only flushes data to the drive's write cache, not to persistent storage
• True durability on macOS requires fcntl(fd, F_FULLSYNC)
• This was already acknowledged with a TODO comment at src/borg/platform/base.py:154: TODO: Use F_FULLSYNC on macOS.
• Without F_FULLSYNC, Borg's data durability guarantees on macOS are weaker than intended - a power loss could result in data that SyncFile believed was safely persisted being lost

Proposed fix

Detect macOS (sys.platform == "darwin") and use fcntl.fcntl(fd, fcntl.F_FULLSYNC) in the fdatasync path:

import sys

if sys.platform == "darwin":
    import fcntl
    fcntl.fcntl(fd, fcntl.F_FULLSYNC)
else:
    os.fsync(fd)

This could either live in platform/darwin.py as a platform-specific override of fdatasync, or inline in base.py with a platform check.

File	Change
src/borg/platform/base.py or src/borg/platform/darwin.py	Use fcntl.F_FULLSYNC on macOS instead of os.fsync;

[ThomasWaldmann]

Maybe we could have a platform specific own sync implementation / wrapper and use that everywhere needed.

[mr-raj12]

I have raised the #9592 , waiting for ci .
Also, i ran black so it has more number of line changes most of them beinf ' to " and some line adjustments.
Happy to revert those if you'd prefer the diff stays minimal and strictly on-topic

[ThomasWaldmann]

Minimal is better for 1.4-maint, iirc we don't use black there.

[mr-raj12]

Minimal is better for 1.4-maint, iirc we don't use black there.

Fixing