posix: retry on symlink race in checklink

Multiple threads might attempt to check links with the same temporary
name. This would cause one side to get an EEXIST error and wrongly
fail the support check. Here, we simply retry if our temporary name
exists.

mercurial/posix.py

@@ -170,6 +170,7 b' def checklink(path):'
     """check whether the given path is on a symlink-capable filesystem"""
     # mktemp is not racy because symlink creation will fail if the
     # file already exists
+    while True:
     name = tempfile.mktemp(dir=path, prefix='hg-checklink-')
     try:
         fd = tempfile.NamedTemporaryFile(dir=path, prefix='hg-checklink-')
@@ -177,15 +178,18 b' def checklink(path):'
             os.symlink(os.path.basename(fd.name), name)
             os.unlink(name)
             return True
+            except OSError as inst:
+                # link creation might race, try again
+                if inst[0] == errno.EEXIST:
+                    continue
+                # sshfs might report failure while successfully creating the link
+                if inst[0] == errno.EIO and os.path.exists(name):
+                    os.unlink(name)
+                return False
         finally:
             fd.close()
     except AttributeError:
         return False
-    except OSError as inst:
-        # sshfs might report failure while successfully creating the link
-        if inst[0] == errno.EIO and os.path.exists(name):
-            os.unlink(name)
-        return False
 
 def checkosfilename(path):
     '''Check that the base-relative path is a valid filename on this platform.