posix: give checkexec a fast path; keep the check files and test read only

Before, Mercurial would create a new temporary file every time, stat it, change
its exec mode, stat it again, and delete it. Most of this dance was done to
handle the rare and not-so-essential case of VFAT mounts on unix. The cost of
that was paid by the much more common and important case of using normal file
systems.

Instead, try to create and preserve .hg/cache/checkisexec and
.hg/cache/checknoexec with and without exec flag set. If the files exist and
have correct exec flags set, we can conclude that that file system supports the
exec flag. Best case, the whole exec check can thus be done with two stat
calls. Worst case, we delete the wrong files and check as usual. That will be
because temporary loss of exec bit or on file systems without support for the
exec bit. In that case we check as we did before, with the additional overhead
of one extra stat call.

It is possible that this different test algorithm in some cases on odd file
systems will give different behaviour. Again, I think it will be rare and
special cases and I think it is worth the risk.

test-clone.t happens to show the situation where checkisexec is left behind
from the old style check, while checknoexec only will be created next time a
exec check will be performed.

mercurial/posix.py

@@ -161,18 +161,55 b' def checkexec(path):'
     try:
         EXECFLAGS = stat.S_IXUSR | stat.S_IXGRP | stat.S_IXOTH
         cachedir = os.path.join(path, '.hg', 'cache')
-        if not os.path.isdir(cachedir):
-            cachedir = path
-        fh, fn = tempfile.mkstemp(dir=cachedir, prefix='hg-checkexec-')
+        if os.path.isdir(cachedir):
+            checkisexec = os.path.join(cachedir, 'checkisexec')
+            checknoexec = os.path.join(cachedir, 'checknoexec')
+
+            try:
+                m = os.stat(checkisexec).st_mode
+            except OSError as e:
+                if e.errno != errno.ENOENT:
+                    raise
+                # checkisexec does not exist - fall through ...
+            else:
+                # checkisexec exists, check if it actually is exec
+                if m & EXECFLAGS != 0:
+                    # ensure checkisexec exists, check it isn't exec
+                    try:
+                        m = os.stat(checknoexec).st_mode
+                    except OSError as e:
+                        if e.errno != errno.ENOENT:
+                            raise
+                        file(checknoexec, 'w').close() # might fail
+                        m = os.stat(checknoexec).st_mode
+                    if m & EXECFLAGS == 0:
+                        # check-exec is exec and check-no-exec is not exec
+                        return True
+                    # checknoexec exists but is exec - delete it
+                    os.unlink(checknoexec)
+                # checkisexec exists but is not exec - delete it
+                os.unlink(checkisexec)
+
+            # check using one file, leave it as checkisexec
+            checkdir = cachedir
+        else:
+            # check directly in path and don't leave checkisexec behind
+            checkdir = path
+            checkisexec = None
+        fh, fn = tempfile.mkstemp(dir=checkdir, prefix='hg-checkexec-')
         try:
             os.close(fh)
             m = os.stat(fn).st_mode
-            if m & EXECFLAGS:
-                return False
-            os.chmod(fn, m & 0o777 | EXECFLAGS)
-            return os.stat(fn).st_mode & EXECFLAGS
+            if m & EXECFLAGS == 0:
+                os.chmod(fn, m & 0o777 | EXECFLAGS)
+                if os.stat(fn).st_mode & EXECFLAGS != 0:
+                    if checkisexec is not None:
+                        os.rename(fn, checkisexec)
+                        fn = None
+                    return True
         finally:
-            os.unlink(fn)
+            if fn is not None:
+                os.unlink(fn)
     except (IOError, OSError):
         # we don't care, the user probably won't be able to commit anyway
         return False

tests/test-clone.t

@@ -31,6 +31,8 b' Trigger branchcache creation:'
   default                       10:a7949464abda
   $ ls .hg/cache
   branch2-served
+  checkisexec
+  checknoexec
   rbc-names-v1
   rbc-revs-v1
 
@@ -45,6 +47,7 b' Ensure branchcache got copied over:'
 
   $ ls .hg/cache
   branch2-served
+  checkisexec
 
   $ cat a
   a

tests/test-hardlinks.t

@@ -211,6 +211,8 b' r4 has hardlinks in the working dir (not'
   2 r4/.hg/00changelog.i
   2 r4/.hg/branch
   2 r4/.hg/cache/branch2-served
+  2 r4/.hg/cache/checkisexec
+  2 r4/.hg/cache/checknoexec
   2 r4/.hg/cache/rbc-names-v1
   2 r4/.hg/cache/rbc-revs-v1
   2 r4/.hg/dirstate
@@ -247,6 +249,8 b' Update back to revision 11 in r4 should '
   2 r4/.hg/00changelog.i
   1 r4/.hg/branch
   2 r4/.hg/cache/branch2-served
+  2 r4/.hg/cache/checkisexec
+  2 r4/.hg/cache/checknoexec
   2 r4/.hg/cache/rbc-names-v1
   2 r4/.hg/cache/rbc-revs-v1
   1 r4/.hg/dirstate

tests/test-tags.t

@@ -672,6 +672,7 b' Missing tags2* files means the cache was'
 
   $ ls tagsclient/.hg/cache
   branch2-served
+  checkisexec
   hgtagsfnodes1
   rbc-names-v1
   rbc-revs-v1
@@ -696,6 +697,7 b' Running hg tags should produce tags2* fi'
 
   $ ls tagsclient/.hg/cache
   branch2-served
+  checkisexec
   hgtagsfnodes1
   rbc-names-v1
   rbc-revs-v1