[PATCH V1] util: move checknlink away from the dependency of a hard-coded filename

[ttung at fb.com]

# HG changeset patch
# User Tony Tung <tonytung at merly.org>
# Date 1471973931 25200
#      Tue Aug 23 10:38:51 2016 -0700
# Node ID 800bdaf50847ed85c913d8d8701d7c6738ee082c
# Parent  2efc5a05d80a6d4253767f2ce0c2fb062ba83cb6
util: move checknlink away from the dependency of a hard-coded filename

This somewhat insulates us against bugs in checknlink when a stale file
left behind could result in checknlink always returning False.

diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -1305,34 +1305,46 @@
 def checknlink(testfile):
     '''check whether hardlink count reporting works properly'''
 
-    # testfile may be open, so we need a separate file for checking to
-    # work around issue2543 (or testfile may get lost on Samba shares)
-    f1 = testfile + ".hgtmp1"
-    if os.path.lexists(f1):
-        return False
+    f1path, f2path, f1handle, f2handle = None, None, None, None
+
     try:
-        posixfile(f1, 'w').close()
-    except IOError:
+        dirpath, filename = os.path.split(testfile)
+
+        # testfile may be open, so we need a separate file for checking to
+        # work around issue2543 (or testfile may get lost on Samba shares)
+        f1handle, f1path = tempfile.mkstemp(prefix=filename, dir=dirpath)
+        os.close(f1handle)
+        f1handle = None
+
+        f2handle, f2path = tempfile.mkstemp(prefix=filename, dir=dirpath)
+        os.close(f2handle)
+        f2handle = None
+
+        # there's a small race condition that another file can jam itself in
+        # between the time we remove f2 and the time we create the hard link.
+        # in the unlikely scenario that happens, we'll treat it as nlink being
+        # unreliable.
         try:
-            os.unlink(f1)
+            os.unlink(f2path)
+            oslink(f1path, f2path)
+            # nlinks() may behave differently for files on Windows shares if
+            # the file is open.
+            f2handle = posixfile(f2path)
+            return nlinks(f2path) > 1
         except OSError:
-            pass
-        return False
-
-    f2 = testfile + ".hgtmp2"
-    fd = None
-    try:
-        oslink(f1, f2)
-        # nlinks() may behave differently for files on Windows shares if
-        # the file is open.
-        fd = posixfile(f2)
-        return nlinks(f2) > 1
-    except OSError:
-        return False
+            return False
     finally:
-        if fd is not None:
-            fd.close()
-        for f in (f1, f2):
+        for f in (f1handle, f2handle):
+            try:
+                if f is None:
+                    continue
+                elif isinstance(f, int):
+                    os.close(f)
+                else:
+                    f.close()
+            except OSError:
+                pass
+        for f in (f1path, f2path):
             try:
                 os.unlink(f)
             except OSError: