streamclone: use backgroundfilecloser (issue4889)

Closing files that have been appended to is slow on Windows/NTFS.
CloseHandle() calls on this platform often take 1-10ms - and that's
on my i7-6700K Skylake processor with a modern and fast SSD. Contrast
with other I/O operations, such as writing data, which take <100us.

This means that creating/appending thousands of files can add
significant overhead. For example, cloning mozilla-central creates
~232,000 revlog files. Assuming 1ms per CloseHandle(), that yields
232s (3:52) of wall time waiting for file closes!

The impact of this overhead can be measured most directly when applying
stream clone bundles. Applying these files is effectively uncompressing
a tar archive (read: it's very fast).

Using a RAM disk (read: no I/O wait), the difference in wall time for a
`hg debugapplystreamclonebundle` for a ~1731 MB mozilla-central bundle
between Windows and Linux from the same machine is drastic:

Linux:    ~12.8s (128MB/s)
Windows: ~352.0s (4.7MB/s)

Windows is ~27.5x slower. Yikes!

After this patch:

Linux:    ~12.8s (128MB/s)
Windows: ~102.1s (16.1MB/s)

Windows is now ~3.4x faster. Unfortunately, it is still ~8x slower than
Linux. Profiling reveals a few hot code paths that could likely be
improved. But those are for other patches.

This patch introduces test-clone-uncompressed.t because existing tests
of `clone --uncompressed` are scattered about and adding a variation for
background thread closing to e.g. test-http.t doesn't feel correct.

tests/test-clone-uncompressed.t

@@ -0,0 +1,48 b''
+#require serve
+
+  $ hg init server
+  $ cd server
+  $ touch foo
+  $ hg -q commit -A -m initial
+  >>> for i in range(1024):
+  ...     with open(str(i), 'wb') as fh:
+  ...         fh.write(str(i))
+  $ hg -q commit -A -m 'add a lot of files'
+  $ hg serve -p $HGPORT -d --pid-file=hg.pid
+  $ cat hg.pid >> $DAEMON_PIDS
+  $ cd ..
+
+Basic clone
+
+  $ hg clone --uncompressed -U http://localhost:$HGPORT clone1
+  streaming all changes
+  1027 files to transfer, 96.3 KB of data
+  transferred 96.3 KB in * seconds (*/sec) (glob)
+  searching for changes
+  no changes found
+
+Clone with background file closing enabled
+
+  $ hg --debug --config worker.backgroundclose=true --config worker.backgroundcloseminfilecount=1 clone --uncompressed -U http://localhost:$HGPORT clone-background | grep -v adding
+  using http://localhost:$HGPORT/
+  sending capabilities command
+  sending branchmap command
+  streaming all changes
+  sending stream_out command
+  1027 files to transfer, 96.3 KB of data
+  starting 4 threads for background file closing
+  transferred 96.3 KB in * seconds (*/sec) (glob)
+  query 1; heads
+  sending batch command
+  searching for changes
+  all remote heads known locally
+  no changes found
+  sending getbundle command
+  bundle2-input-bundle: with-transaction
+  bundle2-input-part: "listkeys" (params: 1 mandatory) supported
+  bundle2-input-part: "listkeys" (params: 1 mandatory) supported
+  bundle2-input-bundle: 1 parts total
+  checking for updated bookmarks
+  preparing listkeys for "phases"
+  sending listkeys command
+  received listkey for "phases": 58 bytes

mercurial/streamclone.py

@@ -298,7 +298,7 b' def consumev1(repo, fp, filecount, bytec'
         start = time.time()
 
         with repo.transaction('clone'):
-            if True:
+            with repo.svfs.backgroundclosing(repo.ui, expectedcount=filecount):
                 for i in xrange(filecount):
                     # XXX doesn't support '\n' or '\r' in filenames
                     l = fp.readline()
@@ -312,7 +312,8 b' def consumev1(repo, fp, filecount, bytec'
                         repo.ui.debug('adding %s (%s)\n' %
                                       (name, util.bytecount(size)))
                     # for backwards compat, name was partially encoded
-                    with repo.svfs(store.decodedir(name), 'w') as ofp:
+                    path = store.decodedir(name)
+                    with repo.svfs(path, 'w', backgroundclose=True) as ofp:
                         for chunk in util.filechunkiter(fp, limit=size):
                             handled_bytes += len(chunk)
                             repo.ui.progress(_('clone'), handled_bytes,