--- a/mercurial/vfs.py Mon Jun 24 13:14:05 2024 +0200
+++ b/mercurial/vfs.py Wed Jul 03 12:22:48 2024 +0200
@@ -189,6 +189,35 @@
def lstat(self, path: Optional[bytes] = None):
return os.lstat(self.join(path))
+ def is_mmap_safe(self, path: Optional[bytes] = None) -> bool:
+ """return True if it is safe to read a file content as mmap
+
+ This focus on the file system aspect of such safety, the application
+ logic around that file is not taken into account, so caller need to
+ make sure the file won't be truncated in a way that will create SIGBUS
+ on access.
+
+
+ The initial motivation for this logic is that if mmap is used on NFS
+ and somebody deletes the mapped file (e.g. by renaming on top of it),
+ then you get SIGBUS, which can be pretty disruptive: we get core dump
+ reports, and the process terminates without writing to the blackbox.
+
+ Instead in this situation we prefer to read the file normally.
+ The risk of ESTALE in the middle of the read remains, but it's
+ smaller because we read sooner and the error should be reported
+ just as any other error.
+
+ Note that python standard library does not offer the necessary function
+ to detect the file stem bits. So this detection rely on compiled bits
+ and is not available in pure python.
+ """
+ # XXX Since we already assume a vfs to address a consistent file system
+ # in other location, we could determine the fstype once for the root
+ # and cache that value.
+ fstype = util.getfstype(self.join(path))
+ return fstype is not None and fstype != b'nfs'
+
def listdir(self, path: Optional[bytes] = None):
return os.listdir(self.join(path))