bitbake: bb/utils.py: add break_hardlinks helper

bb.utils.copyfile is called in a few places with identical src and dst
in order to create an st_nlinks==1 version of the file. That that even
works relies on an implementation detail of copyfile (namely, that it
creates a temporary file and then does a rename). Moreover, it's a waste
of time if the file already has st_nlinks==1.

So create a helper that optimizes away the copy in the st_nlinks==1
case. Of course, this helper relies on the same implementation detail,
but that's now contained within bb.utils itself.

To test that we do at least sometimes hit the no-copy path, I tested
locally with

     if sstat[stat.ST_NLINK] == 1:
+        bb.note("Woohoo, 2*%d bytes I/O avoided" % sstat[stat.ST_SIZE])
         return True

(and the obvious places in oe-core patched), and the do_package log files
are indeed filled with woohoo notes.

(Bitbake rev: 7ae93cf40ab91965147055100432961436bce46c)

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/utils.py

@@ -906,6 +906,23 @@ def copyfile(src, dest, newmtime = None, sstat = None):
         newmtime = sstat[stat.ST_MTIME]
     return newmtime
 
+def break_hardlinks(src, sstat = None):
+    """
+    Ensures src is the only hardlink to this file.  Other hardlinks,
+    if any, are not affected (other than in their st_nlink value, of
+    course).  Returns true on success and false on failure.
+
+    """
+    try:
+        if not sstat:
+            sstat = os.lstat(src)
+    except Exception as e:
+        logger.warning("break_hardlinks: stat of %s failed (%s)" % (src, e))
+        return False
+    if sstat[stat.ST_NLINK] == 1:
+        return True
+    return copyfile(src, src, sstat=sstat)
+
 def which(path, item, direction = 0, history = False, executable=False):
     """
     Locate `item` in the list of paths `path` (colon separated string like $PATH).