revlog-compression: update the config to be a list

revlog-compression: update the config to be a list

format.revlog-compression is now a list of engine, the first supported one is to
be used. Doing this have several benefits:

1) this is fully backward compatible, config using a single entry will be read
as a single item list, not changing any behavior.

2) This open the way to use zstd by default without impacting platform were it
is not available. This will be done in a later changesets.

Using zstd provide a significant performance boost explained in : bb271ec2fbfb.
However zstd is not available in some cases, A notable example is the `--pure`
version of Mercurial which doesn't come with zstd support.

Differential Revision: https://phab.mercurial-scm.org/D8148

--- a/mercurial/configitems.py	Wed Feb 19 13:39:00 2020 +0530
+++ b/mercurial/configitems.py	Wed Feb 19 17:30:04 2020 +0100
@@ -759,7 +759,7 @@
 coreconfigitem(
     b'format',
     b'revlog-compression',
-    default=b'zlib',
+    default=lambda: [b'zlib'],
     alias=[(b'experimental', b'format.compression')],
 )
 coreconfigitem(

--- a/mercurial/helptext/config.txt	Wed Feb 19 13:39:00 2020 +0530
+++ b/mercurial/helptext/config.txt	Wed Feb 19 17:30:04 2020 +0100
@@ -888,7 +888,8 @@
     Compression algorithm used by revlog. Supported values are `zlib` and
     `zstd`. The `zlib` engine is the historical default of Mercurial. `zstd` is
     a newer format that is usually a net win over `zlib`, operating faster at
-    better compression rates. Use `zstd` to reduce CPU usage.
+    better compression rates. Use `zstd` to reduce CPU usage. Multiple values
+    can be specified, the first available one will be used.
 
     On some systems, the Mercurial installation may lack `zstd` support.
 

--- a/mercurial/localrepo.py	Wed Feb 19 13:39:00 2020 +0530
+++ b/mercurial/localrepo.py	Wed Feb 19 17:30:04 2020 +0100
@@ -3578,14 +3578,17 @@
             if ui.configbool(b'format', b'dotencode'):
                 requirements.add(b'dotencode')
 
-    compengine = ui.config(b'format', b'revlog-compression')
-    if compengine not in util.compengines:
+    compengines = ui.configlist(b'format', b'revlog-compression')
+    for compengine in compengines:
+        if compengine in util.compengines:
+            break
+    else:
         raise error.Abort(
             _(
-                b'compression engine %s defined by '
+                b'compression engines %s defined by '
                 b'format.revlog-compression not available'
             )
-            % compengine,
+            % b', '.join(b'"%s"' % e for e in compengines),
             hint=_(
                 b'run "hg debuginstall" to list available '
                 b'compression engines'
@@ -3593,7 +3596,7 @@
         )
 
     # zlib is the historical default and doesn't need an explicit requirement.
-    elif compengine == b'zstd':
+    if compengine == b'zstd':
         requirements.add(b'revlog-compression-zstd')
     elif compengine != b'zlib':
         requirements.add(b'exp-compression-%s' % compengine)

--- a/mercurial/upgrade.py	Wed Feb 19 13:39:00 2020 +0530
+++ b/mercurial/upgrade.py	Wed Feb 19 17:30:04 2020 +0100
@@ -449,7 +449,14 @@
 
     @classmethod
     def fromconfig(cls, repo):
-        return repo.ui.config(b'format', b'revlog-compression')
+        compengines = repo.ui.configlist(b'format', b'revlog-compression')
+        # return the first valid value as the selection code would do
+        for comp in compengines:
+            if comp in util.compengines:
+                return comp
+
+        # no valide compression found lets display it all for clarity
+        return b','.join(compengines)
 
 
 @registerformatvariant

--- a/tests/test-repo-compengines.t	Wed Feb 19 13:39:00 2020 +0530
+++ b/tests/test-repo-compengines.t	Wed Feb 19 17:30:04 2020 +0100
@@ -22,10 +22,15 @@
 Unknown compression engine to format.compression aborts
 
   $ hg --config format.revlog-compression=unknown init unknown
-  abort: compression engine unknown defined by format.revlog-compression not available
+  abort: compression engines "unknown" defined by format.revlog-compression not available
   (run "hg debuginstall" to list available compression engines)
   [255]
 
+unknown compression engine in a list with known one works fine
+
+  $ hg --config format.revlog-compression=zlib,unknown init zlib-before-unknow
+  $ hg --config format.revlog-compression=unknown,zlib init unknown-before-zlib
+
 A requirement specifying an unknown compression engine results in bail
 
   $ hg init unknownrequirement