commands: config option to control bundle compression level

Currently, bundle compression uses the default compression level
for the active compression engine. The default compression level
is tuned as a compromise between speed and size.

Some scenarios may call for a different compression level. For
example, with clone bundles, bundles are generated once and used
several times. Since the cost to generate is paid infrequently,
server operators may wish to trade extra CPU time for better
compression ratios.

This patch introduces an experimental and undocumented config
option to control the bundle compression level. As the inline
comment says, this approach is a bit hacky. I'd prefer for
the compression level to be encoded in the bundle spec. e.g.
"zstd-v2;complevel=15." However, given that the 4.1 freeze is
imminent, I'm not comfortable implementing this user-facing
change without much time to test and consider the implications.
So, we're going with the quick and dirty solution for now.

Having this option in the 4.1 release will enable Mozilla to
easily produce and test zlib and zstd bundles with non-default
compression levels in production. This will help drive future
development of the feature and zstd integration with Mercurial.

mercurial/commands.py

@@ -1383,7 +1383,17 @@ def bundle(ui, repo, fname, dest=None, **opts):
         assert cgversion == '02'
         bversion = 'HG20'
 
-    bundle2.writebundle(ui, cg, fname, bversion, compression=bcompression)
+    # TODO compression options should be derived from bundlespec parsing.
+    # This is a temporary hack to allow adjusting bundle compression
+    # level without a) formalizing the bundlespec changes to declare it
+    # b) introducing a command flag.
+    compopts = {}
+    complevel = ui.configint('experimental', 'bundlecomplevel')
+    if complevel is not None:
+        compopts['level'] = complevel
+
+    bundle2.writebundle(ui, cg, fname, bversion, compression=bcompression,
+                        compopts=compopts)
 
 @command('cat',
     [('o', 'output', '',

tests/test-bundle-type.t

@@ -110,6 +110,37 @@ test bundle types
   c35a0f9217e65d1fdb90c936ffa7dbe679f83ddf
   gzip-v1
   
+
+Compression level can be adjusted for bundle2 bundles
+
+  $ hg init test-complevel
+  $ cd test-complevel
+
+  $ cat > file0 << EOF
+  > this is a file
+  > with some text
+  > and some more text
+  > and other content
+  > EOF
+  $ cat > file1 << EOF
+  > this is another file
+  > with some other content
+  > and repeated, repeated, repeated, repeated content
+  > EOF
+  $ hg -q commit -A -m initial
+
+  $ hg bundle -a -t gzip-v2 gzip-v2.hg
+  1 changesets found
+  $ f --size gzip-v2.hg
+  gzip-v2.hg: size=427
+
+  $ hg --config experimental.bundlecomplevel=1 bundle -a -t gzip-v2 gzip-v2-level1.hg
+  1 changesets found
+  $ f --size gzip-v2-level1.hg
+  gzip-v2-level1.hg: size=435
+
+  $ cd ..
+
 #if zstd
 
   $ for t in "zstd" "zstd-v2"; do