changeset 20293:2f6b3900be64

cat: increase perf when catting single files

Special case the single file case in hg cat. This allows us to avoid
parsing the manifest, which shaves 15% off hg cat perf. This is worth
it, since automation often uses hg cat for retrieving single files.

--- a/mercurial/commands.py	Tue Jan 14 13:49:19 2014 -0800
+++ b/mercurial/commands.py	Tue Jan 14 13:38:16 2014 -0800
@@ -1168,14 +1168,28 @@
     ctx = scmutil.revsingle(repo, opts.get('rev'))
     err = 1
     m = scmutil.match(ctx, (file1,) + pats, opts)
-    for abs in ctx.walk(m):
+
+    def write(path):
         fp = cmdutil.makefileobj(repo, opts.get('output'), ctx.node(),
-                                 pathname=abs)
-        data = ctx[abs].data()
+                                 pathname=path)
+        data = ctx[path].data()
         if opts.get('decode'):
-            data = repo.wwritedata(abs, data)
+            data = repo.wwritedata(path, data)
         fp.write(data)
         fp.close()
+
+    # Automation often uses hg cat on single files, so special case it
+    # for performance to avoid the cost of parsing the manifest.
+    if len(m.files()) == 1 and not m.anypats():
+        file = m.files()[0]
+        mf = repo.manifest
+        mfnode = ctx._changeset[0]
+        if mf.find(mfnode, file)[0]:
+            write(file)
+            return 0
+
+    for abs in ctx.walk(m):
+        write(abs)
         err = 0
     return err