[PATCH V3] 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 1472148645 25200
#      Thu Aug 25 11:10:45 2016 -0700
# Node ID 625f3bb4833b47c4d35754fced91f37b077f7866
# 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
@@ -1305,38 +1305,53 @@
 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
+    f1 = {}
+    f2 = {}
+
     try:
-        posixfile(f1, 'w').close()
-    except IOError:
-        try:
-            os.unlink(f1)
-        except OSError:
-            pass
-        return False
-
-    f2 = testfile + ".hgtmp2"
-    fd = None
-    try:
-        oslink(f1, f2)
+        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)
+        f1['fd'], f1['path'] = tempfile.mkstemp(prefix=filename, dir=dirpath)
+        os.close(f1['fd'])
+        del f1['fd']
+
+        f2['fd'], f2['path'] = tempfile.mkstemp(prefix=filename, dir=dirpath)
+        os.close(f2['fd'])
+        del f2['fd']
+
+        # 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.
+        os.unlink(f2['path'])
+        oslink(f1['path'], f2['path'])
         # nlinks() may behave differently for files on Windows shares if
         # the file is open.
-        fd = posixfile(f2)
-        return nlinks(f2) > 1
-    except OSError:
+        f2['handle'] = posixfile(f2['path'])
+        return nlinks(f2['path']) > 1
+    except EnvironmentError:
         return False
     finally:
-        if fd is not None:
-            fd.close()
-        for f in (f1, f2):
-            try:
-                os.unlink(f)
-            except OSError:
-                pass
+        for fi in (f1, f2):
+            if 'fd' in fi:
+                try:
+                    os.close(fi['fd'])
+                except EnvironmentError:
+                    pass
+
+            if 'handle' in fi:
+                try:
+                    fi['handle'].close()
+                except EnvironmentError:
+                    pass
+
+            if 'path' in fi:
+                try:
+                    os.unlink(fi['path'])
+                except EnvironmentError:
+                    pass
 
 def endswithsep(path):
     '''Check path ends with os.sep or os.altsep.'''