[PATCH 5 of 5] posix: give checklink a fast path that cache the check file and is read only

[Mads Kiilerich]

# HG changeset patch
# User Mads Kiilerich <madski at unity3d.com>
# Date 1421194526 -3600
#      Wed Jan 14 01:15:26 2015 +0100
# Node ID 73b671fbed41d82a5dd46e485c61ddb8afe42faf
# Parent  5409e0c5e6c0764e802360a3912f7719885ba2b5
posix: give checklink a fast path that cache the check file and is read only

util.checklink would create a symlink and remove it again. That would sometimes
happen multiple times. Write operations are relatively expensive and give disk
tear and noise for applications monitoring file system activity.

Instead of creating a symlink and deleting it again, just create it once and
leave it in .hg/cache/check-link . If the file exists, just verify that
os.islink reports true. We will assume that this check is as good as symlink
creation not failing.

Note: The symlink left in .hg/cache has to resolve to a file - otherwise 'make
dist' will fail ...

test-symlink-os-yes-fs-no.py does some monkey patching to simulate a platform
without symlink support. The slightly different testing method requires
additional monkeying.

diff --git a/mercurial/posix.py b/mercurial/posix.py
--- a/mercurial/posix.py
+++ b/mercurial/posix.py
@@ -220,6 +220,10 @@ def checklink(path):
     # file already exists
     while True:
         cachedir = os.path.join(path, '.hg', 'cache')
+        checklink = os.path.join(cachedir, 'checklink')
+        # try fast path, read only
+        if os.path.islink(checklink):
+            return True
         if os.path.isdir(cachedir):
             checkdir = cachedir
         else:
@@ -231,7 +235,13 @@ def checklink(path):
                                              prefix='hg-checklink-')
             try:
                 os.symlink(os.path.basename(fd.name), name)
-                os.unlink(name)
+                if cachedir is None:
+                    os.unlink(name)
+                else:
+                    try:
+                        os.rename(name, checklink)
+                    except OSError:
+                        os.unlink(name)
                 return True
             except OSError as inst:
                 # link creation might race, try again
diff --git a/tests/test-clone.t b/tests/test-clone.t
--- a/tests/test-clone.t
+++ b/tests/test-clone.t
@@ -32,6 +32,7 @@ Trigger branchcache creation:
   $ ls .hg/cache
   branch2-served
   checkisexec
+  checklink
   checknoexec
   rbc-names-v1
   rbc-revs-v1
@@ -48,6 +49,7 @@ Ensure branchcache got copied over:
   $ ls .hg/cache
   branch2-served
   checkisexec
+  checklink
 
   $ cat a
   a
diff --git a/tests/test-symlink-os-yes-fs-no.py b/tests/test-symlink-os-yes-fs-no.py
--- a/tests/test-symlink-os-yes-fs-no.py
+++ b/tests/test-symlink-os-yes-fs-no.py
@@ -35,6 +35,9 @@ commands.status(u, repo)
 def symlink_failure(src, dst):
     raise OSError(1, "Operation not permitted")
 os.symlink = symlink_failure
+def islink_failure(path):
+    return False
+os.path.islink = islink_failure
 
 # dereference links as if a Samba server has exported this to a
 # Windows client
diff --git a/tests/test-tags.t b/tests/test-tags.t
--- a/tests/test-tags.t
+++ b/tests/test-tags.t
@@ -673,6 +673,7 @@ Missing tags2* files means the cache was
   $ ls tagsclient/.hg/cache
   branch2-served
   checkisexec
+  checklink
   hgtagsfnodes1
   rbc-names-v1
   rbc-revs-v1
@@ -698,6 +699,7 @@ Running hg tags should produce tags2* fi
   $ ls tagsclient/.hg/cache
   branch2-served
   checkisexec
+  checklink
   hgtagsfnodes1
   rbc-names-v1
   rbc-revs-v1