revlog: support none compression

revlog files had uncompressed revlog entries support since forever, but
it wasn't selectable or exposed explicitly. It is occassionally useful
for performance testing as it avoids the latency of zlib or zstd. It
also has the nice side effect of providing a non-default compression
engine out-of-the-box.

Differential Revision: https://phab.mercurial-scm.org/D9572

mercurial/utils/compression.py

@@ -617,8 +617,10 b' class _noopengine(compressionengine):'
     def wireprotosupport(self):
         return compewireprotosupport(b'none', 0, 10)
 
-    # We don't implement revlogheader because it is handled specially
-    # in the revlog class.
+    # revlog special cases the uncompressed case, but implementing
+    # revlogheader allows forcing uncompressed storage.
+    def revlogheader(self):
+        return b'\0'
 
     def compressstream(self, it, opts=None):
         return it

tests/test-repo-compengines.t

@@ -43,6 +43,27 b' A requirement specifying an unknown comp'
 
   $ cd ..
 
+Specifying a new format.compression on an existing repo won't introduce data
+with that engine or a requirement
+
+  $ cd default
+  $ touch bar
+  $ hg --config format.revlog-compression=none -q commit -A -m 'add bar with a lot of repeated repeated repeated text'
+
+  $ cat .hg/requires
+  dotencode
+  fncache
+  generaldelta
+  revlogv1
+  sparserevlog
+  store
+  testonly-simplestore (reposimplestore !)
+
+  $ hg debugrevlog -c | grep 0x78
+      0x78 (x)  :   2 (100.00%)
+      0x78 (x)  : 199 (100.00%)
+  $ cd ..
+
 #if zstd
 
   $ hg --config format.revlog-compression=zstd init zstd
@@ -66,25 +87,6 b' A requirement specifying an unknown comp'
 
   $ cd ..
 
-Specifying a new format.compression on an existing repo won't introduce data
-with that engine or a requirement
-
-  $ cd default
-  $ touch bar
-  $ hg --config format.revlog-compression=zstd -q commit -A -m 'add bar with a lot of repeated repeated repeated text'
-
-  $ cat .hg/requires
-  dotencode
-  fncache
-  generaldelta
-  revlogv1
-  sparserevlog
-  store
-  testonly-simplestore (reposimplestore !)
-
-  $ hg debugrevlog -c | grep 0x78
-      0x78 (x)  :   2 (100.00%)
-      0x78 (x)  : 199 (100.00%)
 
 #endif
 
@@ -116,10 +118,12 b' checking zlib options'
   > done
 
   $ $RUNTESTDIR/f -s */.hg/store/data/*
-  default/.hg/store/data/foo.i: size=64 (pure !)
+  default/.hg/store/data/bar.i: size=64
+  default/.hg/store/data/foo.i: size=64
   zlib-level-1/.hg/store/data/a.i: size=4146
   zlib-level-9/.hg/store/data/a.i: size=4138
   zlib-level-default/.hg/store/data/a.i: size=4138
+  zstd/.hg/store/data/foo.i: size=64 (zstd !)
 
 Test error cases
 
@@ -144,6 +148,41 b' Test error cases'
   abort: invalid value for `storage.revlog.zlib.level` config: 42
   [255]
 
+checking details of none compression
+====================================
+
+  $ hg init none-compression --config format.revlog-compression=none
+
+  $ commitone() {
+  >    repo=$1
+  >    cp $RUNTESTDIR/bundles/issue4438-r1.hg $repo/a
+  >    hg -R $repo add $repo/a
+  >    hg -R $repo commit -m some-commit
+  > }
+
+  $ commitone none-compression
+
+  $ hg log -R none-compression
+  changeset:   0:68b53da39cd8
+  tag:         tip
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     some-commit
+  
+
+  $ cat none-compression/.hg/requires
+  dotencode
+  exp-compression-none
+  fncache
+  generaldelta
+  revlogv1
+  sparserevlog
+  store
+  testonly-simplestore (reposimplestore !)
+
+  $ $RUNTESTDIR/f -s none-compression/.hg/store/data/*
+  none-compression/.hg/store/data/a.i: size=4216
+
 #if zstd
 
 checking zstd options