Use CR instead of NUL character for changeset collisions

README.md

@@ -271,6 +271,20 @@ The default protocol is https, and the port can be omitted.
 
 - `hg::tags:` becomes `hg://:tags`
 
+Compatibility:
+--------------
+
+As of version 0.7, some corner cases in Mercurial repositories will generate
+different git commits than with prior versions of git-cinnabar. This means
+a fresh clone might have different git SHA-1s than existing clones, but this
+doesn't impact the use of existing clones with newer versions of git-cinnabar.
+
+Most repositories should remain non-affected by the change.
+
+You can set the `cinnabar.compat` git configuration to `0.6` to keep the
+previous behavior.
+
+
 Experimental features:
 ----------------------
 

src/main.rs

@@ -4705,6 +4705,31 @@ pub fn check_enabled(checks: Checks) -> bool {
     CHECKS.contains(checks)
 }
 
+bitflags! {
+    #[derive(Debug)]
+    pub struct Compat: i32 {
+        const CHANGESET_CONFLICT_NUL = 0x1;
+
+        const CINNABAR_0_6 = Compat::CHANGESET_CONFLICT_NUL.bits();
+    }
+}
+
+static COMPAT: Lazy<Compat> =
+    Lazy::new(
+        || match get_config("compat").as_ref().map(|x| x.as_bytes()) {
+            Some(b"0.6") => Compat::CINNABAR_0_6,
+            Some(x) => {
+                warn!(target: "root", "Ignoring invalid value for compat: {}", x.as_bstr());
+                Compat::empty()
+            }
+            None => Compat::empty(),
+        },
+    );
+
+pub fn has_compat(c: Compat) -> bool {
+    COMPAT.contains(c)
+}
+
 pub struct Experiments {
     merge: bool,
     similarity: CString,

src/store.rs

@@ -55,7 +55,7 @@ use crate::util::{
     Transpose,
 };
 use crate::xdiff::{apply, textdiff, PatchInfo};
-use crate::{check_enabled, Checks};
+use crate::{check_enabled, has_compat, Checks, Compat};
 
 pub const REFS_PREFIX: &str = "refs/cinnabar/";
 pub const REPLACE_REFS_PREFIX: &str = "refs/cinnabar/replace/";
@@ -1526,7 +1526,11 @@ fn store_changeset(
             // we find a commit that doesn't map to another changeset.
             match GitChangesetId::from_unchecked(commit_id).to_hg(store) {
                 Some(existing_hg_id) if existing_hg_id != changeset_id => {
-                    raw_commit.push(b'\0');
+                    if has_compat(Compat::CHANGESET_CONFLICT_NUL) {
+                        raw_commit.push(b'\0');
+                    } else {
+                        raw_commit.push(b'\n');
+                    }
                 }
                 _ => {
                     break commit_id;

tests/conflicts.t

@@ -60,15 +60,14 @@ changesets properly.
   author nobody <> 1 +0000
   committer nobody <> 1 +0000
   
-  b\x00 (no-eol) (esc)
+  b
 
   $ git clone -q --mirror repo-git repo-mirror
   $ git -C repo-mirror cinnabar rollback
   $ git -C repo-mirror rev-list --count --all
   3
   $ git -C repo-mirror gc --prune=now
   $ git -C repo-mirror fsck
-  warning in commit 36376d9e996376d02d00f72a8c7903cef9d6ea45: nulInCommit: NUL byte in the commit object body
 
 Check that this round-trips properly.
 
@@ -90,6 +89,52 @@ Check that this round-trips properly.
   o  f92470d7f6966a39dfbced6a525fe81ebf5c37b9 default a
 
 
+Trying again with git-cinnabar 0.6 compat.
+
+  $ git -c cinnabar.compat=0.6 -c fetch.prune=true clone -q hg::$REPO repo-git-0.6
+  $ git -C repo-git-0.6 cat-file -p $(git -C repo-git-0.6 cinnabar hg2git 636e60525868096cbdc961870493510558f41d2f)
+  tree 3683f870be446c7cc05ffaef9fa06415276e1828
+  parent 8b86a58578d5270969543e287634e3a2f122a338
+  author nobody <> 1 +0000
+  committer nobody <> 1 +0000
+
+  b (no-eol)
+  $ git -C repo-git-0.6 cat-file -p $(git -C repo-git-0.6 cinnabar hg2git 97b815fb8d45129120112766f8c69db8e93fbe8f)
+  tree 3683f870be446c7cc05ffaef9fa06415276e1828
+  parent 8b86a58578d5270969543e287634e3a2f122a338
+  author nobody <> 1 +0000
+  committer nobody <> 1 +0000
+
+  b\x00 (no-eol) (esc)
+
+  $ git clone -q --mirror repo-git-0.6 repo-mirror-0.6
+  $ git -C repo-mirror-0.6 cinnabar rollback
+  $ git -C repo-mirror-0.6 rev-list --count --all
+  3
+  $ git -C repo-mirror-0.6 gc --prune=now
+  $ git -C repo-mirror-0.6 fsck
+  warning in commit 36376d9e996376d02d00f72a8c7903cef9d6ea45: nulInCommit: NUL byte in the commit object body
+
+Check that this round-trips properly.
+
+  $ hg init ${REPO}2-0.6
+  $ git -C repo-git-0.6 push hg::${REPO}2-0.6 "refs/remotes/origin/branches/foo/tip:refs/heads/branches/foo/tip" "refs/remotes/origin/branches/default/tip:refs/heads/branches/default/tip"
+  remote: adding changesets
+  remote: adding manifests
+  remote: adding file changes
+  remote: added 3 changesets with 2 changes to 2 files (+1 heads)
+  To hg::.*/conflicts.t/repo2-0.6 (re)
+   * [new branch]      origin/branches/foo/tip -> branches/foo/tip
+   * [new branch]      origin/branches/default/tip -> branches/default/tip
+
+  $ hg -R ${REPO}2-0.6 log -G --template '{node} {branch} {desc}'
+  o  97b815fb8d45129120112766f8c69db8e93fbe8f foo b
+  |
+  | o  636e60525868096cbdc961870493510558f41d2f default b
+  |/
+  o  f92470d7f6966a39dfbced6a525fe81ebf5c37b9 default a
+  
+
 The obvious consequence is that without initial metadata, pushing this to a
 mercurial repo will create a different changeset for the one in branch foo.
 TODO: But we don't support creating new branches anyway, so we can't really