chunker: use safe_fadvise

also: refactor safe_advise a bit, use has_posix_fadvise.

src/borg/chunker.pyx

@@ -13,6 +13,7 @@ from libc.stdlib cimport malloc, free
 from libc.string cimport memcpy, memmove
 
 from .constants import CH_DATA, CH_ALLOC, CH_HOLE, zeros
+from .platform import safe_fadvise
 
 # this will be True if Python's seek implementation supports data/holes seeking.
 # this does not imply that it will actually work on the filesystem,
@@ -47,11 +48,7 @@ def dread(offset, size, fd=None, fh=-1):
     use_fh = fh >= 0
     if use_fh:
         data = os.read(fh, size)
-        if hasattr(os, 'posix_fadvise'):
-            # UNIX only and, in case of block sizes that are not a multiple of the
-            # system's page size, better be used with a bug fixed linux kernel > 4.6.0,
-            # see comment/workaround in _chunker.c and borgbackup issue #907.
-            os.posix_fadvise(fh, offset, len(data), os.POSIX_FADV_DONTNEED)
+        safe_fadvise(fh, offset, len(data), "DONTNEED")
         return data
     else:
         return fd.read(size)

src/borg/platform/base.py

@@ -21,6 +21,7 @@ are correctly composed into the base functionality.
 API_VERSION = "1.2_05"
 
 fdatasync = getattr(os, "fdatasync", os.fsync)
+has_posix_fadvise = hasattr(os, "posix_fadvise")
 
 from .xattr import ENOATTR
 
@@ -114,9 +115,11 @@ def sync_dir(path):
 
 
 def safe_fadvise(fd, offset, len, advice):
-    if hasattr(os, "posix_fadvise"):
+    if has_posix_fadvise:
         advice = getattr(os, "POSIX_FADV_" + advice)
         try:
+            # UNIX only and, in case of block sizes that are not a multiple of the system's page size,
+            # better be used with a bug fixed linux kernel > 4.6.0, see borgbackup issue #907.
             os.posix_fadvise(fd, offset, len, advice)
         except OSError:
             # usually, posix_fadvise can't fail for us, but there seem to