copies-rust: make the comparison aware of the revision being current merged

This make no significant performance change in practice (all ±2%) in practice,
but it will help us to distinct between some semantically different cases later
on.

Differential Revision: https://phab.mercurial-scm.org/D9498

rust/hg-core/src/copy_tracing.rs

@@ -1,767 +1,795 b''
 use crate::utils::hg_path::HgPath;
 use crate::utils::hg_path::HgPathBuf;
 use crate::Revision;
 use crate::NULL_REVISION;
 
 use im_rc::ordmap::DiffItem;
 use im_rc::ordmap::Entry;
 use im_rc::ordmap::OrdMap;
 
 use std::cmp::Ordering;
 use std::collections::HashMap;
 use std::convert::TryInto;
 
 pub type PathCopies = HashMap<HgPathBuf, HgPathBuf>;
 
 type PathToken = usize;
 
 #[derive(Clone, Debug, PartialEq, Copy)]
 struct TimeStampedPathCopy {
     /// revision at which the copy information was added
     rev: Revision,
     /// the copy source, (Set to None in case of deletion of the associated
     /// key)
     path: Option<PathToken>,
 }
 
 /// maps CopyDestination to Copy Source (+ a "timestamp" for the operation)
 type TimeStampedPathCopies = OrdMap<PathToken, TimeStampedPathCopy>;
 
 /// hold parent 1, parent 2 and relevant files actions.
 pub type RevInfo<'a> = (Revision, Revision, ChangedFiles<'a>);
 
 /// represent the files affected by a changesets
 ///
 /// This hold a subset of mercurial.metadata.ChangingFiles as we do not need
 /// all the data categories tracked by it.
 /// This hold a subset of mercurial.metadata.ChangingFiles as we do not need
 /// all the data categories tracked by it.
 pub struct ChangedFiles<'a> {
     nb_items: u32,
     index: &'a [u8],
     data: &'a [u8],
 }
 
 /// Represent active changes that affect the copy tracing.
 enum Action<'a> {
     /// The parent ? children edge is removing a file
     ///
     /// (actually, this could be the edge from the other parent, but it does
     /// not matters)
     Removed(&'a HgPath),
     /// The parent ? children edge introduce copy information between (dest,
     /// source)
     Copied(&'a HgPath, &'a HgPath),
 }
 
 /// This express the possible "special" case we can get in a merge
 ///
 /// See mercurial/metadata.py for details on these values.
 #[derive(PartialEq)]
 enum MergeCase {
     /// Merged: file had history on both side that needed to be merged
     Merged,
     /// Salvaged: file was candidate for deletion, but survived the merge
     Salvaged,
     /// Normal: Not one of the two cases above
     Normal,
 }
 
 type FileChange<'a> = (u8, &'a HgPath, &'a HgPath);
 
 const EMPTY: &[u8] = b"";
 const COPY_MASK: u8 = 3;
 const P1_COPY: u8 = 2;
 const P2_COPY: u8 = 3;
 const ACTION_MASK: u8 = 28;
 const REMOVED: u8 = 12;
 const MERGED: u8 = 8;
 const SALVAGED: u8 = 16;
 
 impl<'a> ChangedFiles<'a> {
     const INDEX_START: usize = 4;
     const ENTRY_SIZE: u32 = 9;
     const FILENAME_START: u32 = 1;
     const COPY_SOURCE_START: u32 = 5;
 
     pub fn new(data: &'a [u8]) -> Self {
         assert!(
             data.len() >= 4,
             "data size ({}) is too small to contain the header (4)",
             data.len()
         );
         let nb_items_raw: [u8; 4] = (&data[0..=3])
             .try_into()
             .expect("failed to turn 4 bytes into 4 bytes");
         let nb_items = u32::from_be_bytes(nb_items_raw);
 
         let index_size = (nb_items * Self::ENTRY_SIZE) as usize;
         let index_end = Self::INDEX_START + index_size;
 
         assert!(
             data.len() >= index_end,
             "data size ({}) is too small to fit the index_data ({})",
             data.len(),
             index_end
         );
 
         let ret = ChangedFiles {
             nb_items,
             index: &data[Self::INDEX_START..index_end],
             data: &data[index_end..],
         };
         let max_data = ret.filename_end(nb_items - 1) as usize;
         assert!(
             ret.data.len() >= max_data,
             "data size ({}) is too small to fit all data ({})",
             data.len(),
             index_end + max_data
         );
         ret
     }
 
     pub fn new_empty() -> Self {
         ChangedFiles {
             nb_items: 0,
             index: EMPTY,
             data: EMPTY,
         }
     }
 
     /// internal function to return an individual entry at a given index
     fn entry(&'a self, idx: u32) -> FileChange<'a> {
         if idx >= self.nb_items {
             panic!(
                 "index for entry is higher that the number of file {} >= {}",
                 idx, self.nb_items
             )
         }
         let flags = self.flags(idx);
         let filename = self.filename(idx);
         let copy_idx = self.copy_idx(idx);
         let copy_source = self.filename(copy_idx);
         (flags, filename, copy_source)
     }
 
     /// internal function to return the filename of the entry at a given index
     fn filename(&self, idx: u32) -> &HgPath {
         let filename_start;
         if idx == 0 {
             filename_start = 0;
         } else {
             filename_start = self.filename_end(idx - 1)
         }
         let filename_end = self.filename_end(idx);
         let filename_start = filename_start as usize;
         let filename_end = filename_end as usize;
         HgPath::new(&self.data[filename_start..filename_end])
     }
 
     /// internal function to return the flag field of the entry at a given
     /// index
     fn flags(&self, idx: u32) -> u8 {
         let idx = idx as usize;
         self.index[idx * (Self::ENTRY_SIZE as usize)]
     }
 
     /// internal function to return the end of a filename part at a given index
     fn filename_end(&self, idx: u32) -> u32 {
         let start = (idx * Self::ENTRY_SIZE) + Self::FILENAME_START;
         let end = (idx * Self::ENTRY_SIZE) + Self::COPY_SOURCE_START;
         let start = start as usize;
         let end = end as usize;
         let raw = (&self.index[start..end])
             .try_into()
             .expect("failed to turn 4 bytes into 4 bytes");
         u32::from_be_bytes(raw)
     }
 
     /// internal function to return index of the copy source of the entry at a
     /// given index
     fn copy_idx(&self, idx: u32) -> u32 {
         let start = (idx * Self::ENTRY_SIZE) + Self::COPY_SOURCE_START;
         let end = (idx + 1) * Self::ENTRY_SIZE;
         let start = start as usize;
         let end = end as usize;
         let raw = (&self.index[start..end])
             .try_into()
             .expect("failed to turn 4 bytes into 4 bytes");
         u32::from_be_bytes(raw)
     }
 
     /// Return an iterator over all the `Action` in this instance.
     fn iter_actions(&self, parent: Parent) -> ActionsIterator {
         ActionsIterator {
             changes: &self,
             parent: parent,
             current: 0,
         }
     }
 
     /// return the MergeCase value associated with a filename
     fn get_merge_case(&self, path: &HgPath) -> MergeCase {
         if self.nb_items == 0 {
             return MergeCase::Normal;
         }
         let mut low_part = 0;
         let mut high_part = self.nb_items;
 
         while low_part < high_part {
             let cursor = (low_part + high_part - 1) / 2;
             let (flags, filename, _source) = self.entry(cursor);
             match path.cmp(filename) {
                 Ordering::Less => low_part = cursor + 1,
                 Ordering::Greater => high_part = cursor,
                 Ordering::Equal => {
                     return match flags & ACTION_MASK {
                         MERGED => MergeCase::Merged,
                         SALVAGED => MergeCase::Salvaged,
                         _ => MergeCase::Normal,
                     };
                 }
             }
         }
         MergeCase::Normal
     }
 }
 
 /// A struct responsible for answering "is X ancestors of Y" quickly
 ///
 /// The structure will delegate ancestors call to a callback, and cache the
 /// result.
 #[derive(Debug)]
 struct AncestorOracle<'a, A: Fn(Revision, Revision) -> bool> {
     inner: &'a A,
     pairs: HashMap<(Revision, Revision), bool>,
 }
 
 impl<'a, A: Fn(Revision, Revision) -> bool> AncestorOracle<'a, A> {
     fn new(func: &'a A) -> Self {
         Self {
             inner: func,
             pairs: HashMap::default(),
         }
     }
 
     fn record_overwrite(&mut self, anc: Revision, desc: Revision) {
         self.pairs.insert((anc, desc), true);
     }
 
     /// returns `true` if `anc` is an ancestors of `desc`, `false` otherwise
     fn is_overwrite(&mut self, anc: Revision, desc: Revision) -> bool {
         if anc > desc {
             false
         } else if anc == desc {
             true
         } else {
             if let Some(b) = self.pairs.get(&(anc, desc)) {
                 *b
             } else {
                 let b = (self.inner)(anc, desc);
                 self.pairs.insert((anc, desc), b);
                 b
             }
         }
     }
 }
 
 struct ActionsIterator<'a> {
     changes: &'a ChangedFiles<'a>,
     parent: Parent,
     current: u32,
 }
 
 impl<'a> Iterator for ActionsIterator<'a> {
     type Item = Action<'a>;
 
     fn next(&mut self) -> Option<Action<'a>> {
         let copy_flag = match self.parent {
             Parent::FirstParent => P1_COPY,
             Parent::SecondParent => P2_COPY,
         };
         while self.current < self.changes.nb_items {
             let (flags, file, source) = self.changes.entry(self.current);
             self.current += 1;
             if (flags & ACTION_MASK) == REMOVED {
                 return Some(Action::Removed(file));
             }
             let copy = flags & COPY_MASK;
             if copy == copy_flag {
                 return Some(Action::Copied(file, source));
             }
         }
         return None;
     }
 }
 
 /// A small struct whose purpose is to ensure lifetime of bytes referenced in
 /// ChangedFiles
 ///
 /// It is passed to the RevInfoMaker callback who can assign any necessary
 /// content to the `data` attribute. The copy tracing code is responsible for
 /// keeping the DataHolder alive at least as long as the ChangedFiles object.
 pub struct DataHolder<D> {
     /// RevInfoMaker callback should assign data referenced by the
     /// ChangedFiles struct it return to this attribute. The DataHolder
     /// lifetime will be at least as long as the ChangedFiles one.
     pub data: Option<D>,
 }
 
 pub type RevInfoMaker<'a, D> =
     Box<dyn for<'r> Fn(Revision, &'r mut DataHolder<D>) -> RevInfo<'r> + 'a>;
 
 /// enum used to carry information about the parent → child currently processed
 #[derive(Copy, Clone, Debug)]
 enum Parent {
     /// The `p1(x) → x` edge
     FirstParent,
     /// The `p2(x) → x` edge
     SecondParent,
 }
 
 /// A small "tokenizer" responsible of turning full HgPath into lighter
 /// PathToken
 ///
 /// Dealing with small object, like integer is much faster, so HgPath input are
 /// turned into integer "PathToken" and converted back in the end.
 #[derive(Clone, Debug, Default)]
 struct TwoWayPathMap {
     token: HashMap<HgPathBuf, PathToken>,
     path: Vec<HgPathBuf>,
 }
 
 impl TwoWayPathMap {
     fn tokenize(&mut self, path: &HgPath) -> PathToken {
         match self.token.get(path) {
             Some(a) => *a,
             None => {
                 let a = self.token.len();
                 let buf = path.to_owned();
                 self.path.push(buf.clone());
                 self.token.insert(buf, a);
                 a
             }
         }
     }
 
     fn untokenize(&self, token: PathToken) -> &HgPathBuf {
         assert!(token < self.path.len(), format!("Unknown token: {}", token));
         &self.path[token]
     }
 }
 
 /// Same as mercurial.copies._combine_changeset_copies, but in Rust.
 ///
 /// Arguments are:
 ///
 /// revs: all revisions to be considered
 /// children: a {parent ? [childrens]} mapping
 /// target_rev: the final revision we are combining copies to
 /// rev_info(rev): callback to get revision information:
 ///   * first parent
 ///   * second parent
 ///   * ChangedFiles
 /// isancestors(low_rev, high_rev): callback to check if a revision is an
 ///                                 ancestor of another
 pub fn combine_changeset_copies<A: Fn(Revision, Revision) -> bool, D>(
     revs: Vec<Revision>,
     mut children_count: HashMap<Revision, usize>,
     target_rev: Revision,
     rev_info: RevInfoMaker<D>,
     is_ancestor: &A,
 ) -> PathCopies {
     let mut all_copies = HashMap::new();
     let mut oracle = AncestorOracle::new(is_ancestor);
 
     let mut path_map = TwoWayPathMap::default();
 
     for rev in revs {
         let mut d: DataHolder<D> = DataHolder { data: None };
         let (p1, p2, changes) = rev_info(rev, &mut d);
 
         // We will chain the copies information accumulated for the parent with
         // the individual copies information the curent revision.  Creating a
         // new TimeStampedPath for each `rev` → `children` vertex.
         let mut copies: Option<TimeStampedPathCopies> = None;
         if p1 != NULL_REVISION {
             // Retrieve data computed in a previous iteration
             let parent_copies = get_and_clean_parent_copies(
                 &mut all_copies,
                 &mut children_count,
                 p1,
             );
             if let Some(parent_copies) = parent_copies {
                 // combine it with data for that revision
                 let vertex_copies = add_from_changes(
                     &mut path_map,
                     &mut oracle,
                     &parent_copies,
                     &changes,
                     Parent::FirstParent,
                     rev,
                 );
                 // keep that data around for potential later combination
                 copies = Some(vertex_copies);
             }
         }
         if p2 != NULL_REVISION {
             // Retrieve data computed in a previous iteration
             let parent_copies = get_and_clean_parent_copies(
                 &mut all_copies,
                 &mut children_count,
                 p2,
             );
             if let Some(parent_copies) = parent_copies {
                 // combine it with data for that revision
                 let vertex_copies = add_from_changes(
                     &mut path_map,
                     &mut oracle,
                     &parent_copies,
                     &changes,
                     Parent::SecondParent,
                     rev,
                 );
 
                 copies = match copies {
                     None => Some(vertex_copies),
                     // Merge has two parents needs to combines their copy
                     // information.
                     //
                     // If we got data from both parents, We need to combine
                     // them.
                     Some(copies) => Some(merge_copies_dict(
                         &path_map,
+                        rev,
                         vertex_copies,
                         copies,
                         &changes,
                         &mut oracle,
                     )),
                 };
             }
         }
         match copies {
             Some(copies) => {
                 all_copies.insert(rev, copies);
             }
             _ => {}
         }
     }
 
     // Drop internal information (like the timestamp) and return the final
     // mapping.
     let tt_result = all_copies
         .remove(&target_rev)
         .expect("target revision was not processed");
     let mut result = PathCopies::default();
     for (dest, tt_source) in tt_result {
         if let Some(path) = tt_source.path {
             let path_dest = path_map.untokenize(dest).to_owned();
             let path_path = path_map.untokenize(path).to_owned();
             result.insert(path_dest, path_path);
         }
     }
     result
 }
 
 /// fetch previous computed information
 ///
 /// If no other children are expected to need this information, we drop it from
 /// the cache.
 ///
 /// If parent is not part of the set we are expected to walk, return None.
 fn get_and_clean_parent_copies(
     all_copies: &mut HashMap<Revision, TimeStampedPathCopies>,
     children_count: &mut HashMap<Revision, usize>,
     parent_rev: Revision,
 ) -> Option<TimeStampedPathCopies> {
     let count = children_count.get_mut(&parent_rev)?;
     *count -= 1;
     if *count == 0 {
         match all_copies.remove(&parent_rev) {
             Some(c) => Some(c),
             None => Some(TimeStampedPathCopies::default()),
         }
     } else {
         match all_copies.get(&parent_rev) {
             Some(c) => Some(c.clone()),
             None => Some(TimeStampedPathCopies::default()),
         }
     }
 }
 
 /// Combine ChangedFiles with some existing PathCopies information and return
 /// the result
 fn add_from_changes<A: Fn(Revision, Revision) -> bool>(
     path_map: &mut TwoWayPathMap,
     oracle: &mut AncestorOracle<A>,
     base_copies: &TimeStampedPathCopies,
     changes: &ChangedFiles,
     parent: Parent,
     current_rev: Revision,
 ) -> TimeStampedPathCopies {
     let mut copies = base_copies.clone();
     for action in changes.iter_actions(parent) {
         match action {
             Action::Copied(path_dest, path_source) => {
                 let dest = path_map.tokenize(path_dest);
                 let source = path_map.tokenize(path_source);
                 let entry;
                 if let Some(v) = base_copies.get(&source) {
                     entry = match &v.path {
                         Some(path) => Some((*(path)).to_owned()),
                         None => Some(source.to_owned()),
                     }
                 } else {
                     entry = Some(source.to_owned());
                 }
                 // Each new entry is introduced by the children, we
                 // record this information as we will need it to take
                 // the right decision when merging conflicting copy
                 // information. See merge_copies_dict for details.
                 match copies.entry(dest) {
                     Entry::Vacant(slot) => {
                         let ttpc = TimeStampedPathCopy {
                             rev: current_rev,
                             path: entry,
                         };
                         slot.insert(ttpc);
                     }
                     Entry::Occupied(mut slot) => {
                         let mut ttpc = slot.get_mut();
                         oracle.record_overwrite(ttpc.rev, current_rev);
                         ttpc.rev = current_rev;
                         ttpc.path = entry;
                     }
                 }
             }
             Action::Removed(deleted_path) => {
                 // We must drop copy information for removed file.
                 //
                 // We need to explicitly record them as dropped to
                 // propagate this information when merging two
                 // TimeStampedPathCopies object.
                 let deleted = path_map.tokenize(deleted_path);
                 copies.entry(deleted).and_modify(|old| {
                     oracle.record_overwrite(old.rev, current_rev);
                     old.rev = current_rev;
                     old.path = None;
                 });
             }
         }
     }
     copies
 }
 
 /// merge two copies-mapping together, minor and major
 ///
 /// In case of conflict, value from "major" will be picked, unless in some
 /// cases. See inline documentation for details.
 fn merge_copies_dict<A: Fn(Revision, Revision) -> bool>(
     path_map: &TwoWayPathMap,
+    current_merge: Revision,
     mut minor: TimeStampedPathCopies,
     mut major: TimeStampedPathCopies,
     changes: &ChangedFiles,
     oracle: &mut AncestorOracle<A>,
 ) -> TimeStampedPathCopies {
     // This closure exist as temporary help while multiple developper are
     // actively working on this code. Feel free to re-inline it once this
     // code is more settled.
     let mut cmp_value =
         |dest: &PathToken,
          src_minor: &TimeStampedPathCopy,
          src_major: &TimeStampedPathCopy| {
             compare_value(
-                path_map, changes, oracle, dest, src_minor, src_major,
+                path_map,
+                current_merge,
+                changes,
+                oracle,
+                dest,
+                src_minor,
+                src_major,
             )
         };
     if minor.is_empty() {
         major
     } else if major.is_empty() {
         minor
     } else if minor.len() * 2 < major.len() {
         // Lets says we are merging two TimeStampedPathCopies instance A and B.
         //
         // If A contains N items, the merge result will never contains more
         // than N values differents than the one in A
         //
         // If B contains M items, with M > N, the merge result will always
         // result in a minimum of M - N value differents than the on in
         // A
         //
         // As a result, if N < (M-N), we know that simply iterating over A will
         // yield less difference than iterating over the difference
         // between A and B.
         //
         // This help performance a lot in case were a tiny
         // TimeStampedPathCopies is merged with a much larger one.
         for (dest, src_minor) in minor {
             let src_major = major.get(&dest);
             match src_major {
                 None => major.insert(dest, src_minor),
                 Some(src_major) => {
                     match cmp_value(&dest, &src_minor, src_major) {
                         MergePick::Any | MergePick::Major => None,
                         MergePick::Minor => major.insert(dest, src_minor),
                     }
                 }
             };
         }
         major
     } else if major.len() * 2 < minor.len() {
         // This use the same rational than the previous block.
         // (Check previous block documentation for details.)
         for (dest, src_major) in major {
             let src_minor = minor.get(&dest);
             match src_minor {
                 None => minor.insert(dest, src_major),
                 Some(src_minor) => {
                     match cmp_value(&dest, src_minor, &src_major) {
                         MergePick::Any | MergePick::Minor => None,
                         MergePick::Major => minor.insert(dest, src_major),
                     }
                 }
             };
         }
         minor
     } else {
         let mut override_minor = Vec::new();
         let mut override_major = Vec::new();
 
         let mut to_major = |k: &PathToken, v: &TimeStampedPathCopy| {
             override_major.push((k.clone(), v.clone()))
         };
         let mut to_minor = |k: &PathToken, v: &TimeStampedPathCopy| {
             override_minor.push((k.clone(), v.clone()))
         };
 
         // The diff function leverage detection of the identical subpart if
         // minor and major has some common ancestors. This make it very
         // fast is most case.
         //
         // In case where the two map are vastly different in size, the current
         // approach is still slowish because the iteration will iterate over
         // all the "exclusive" content of the larger on. This situation can be
         // frequent when the subgraph of revision we are processing has a lot
         // of roots. Each roots adding they own fully new map to the mix (and
         // likely a small map, if the path from the root to the "main path" is
         // small.
         //
         // We could do better by detecting such situation and processing them
         // differently.
         for d in minor.diff(&major) {
             match d {
                 DiffItem::Add(k, v) => to_minor(k, v),
                 DiffItem::Remove(k, v) => to_major(k, v),
                 DiffItem::Update { old, new } => {
                     let (dest, src_major) = new;
                     let (_, src_minor) = old;
                     match cmp_value(dest, src_minor, src_major) {
                         MergePick::Major => to_minor(dest, src_major),
                         MergePick::Minor => to_major(dest, src_minor),
                         // If the two entry are identical, no need to do
                         // anything (but diff should not have yield them)
                         MergePick::Any => unreachable!(),
                     }
                 }
             };
         }
 
         let updates;
         let mut result;
         if override_major.is_empty() {
             result = major
         } else if override_minor.is_empty() {
             result = minor
         } else {
             if override_minor.len() < override_major.len() {
                 updates = override_minor;
                 result = minor;
             } else {
                 updates = override_major;
                 result = major;
             }
             for (k, v) in updates {
                 result.insert(k, v);
             }
         }
         result
     }
 }
 
 /// represent the side that should prevail when merging two
 /// TimeStampedPathCopies
 enum MergePick {
     /// The "major" (p1) side prevails
     Major,
     /// The "minor" (p2) side prevails
     Minor,
     /// Any side could be used (because they are the same)
     Any,
 }
 
 /// decide which side prevails in case of conflicting values
 #[allow(clippy::if_same_then_else)]
 fn compare_value<A: Fn(Revision, Revision) -> bool>(
     path_map: &TwoWayPathMap,
+    current_merge: Revision,
     changes: &ChangedFiles,
     oracle: &mut AncestorOracle<A>,
     dest: &PathToken,
     src_minor: &TimeStampedPathCopy,
     src_major: &TimeStampedPathCopy,
 ) -> MergePick {
-    if src_major.path == src_minor.path {
+    if src_major.rev == current_merge {
+        if src_minor.rev == current_merge {
+            if src_major.path.is_none() {
+                // We cannot get different copy information for both p1 and p2
+                // from the same revision. Unless this was a
+                // deletion
+                MergePick::Any
+            } else {
+                unreachable!();
+            }
+        } else {
+            // The last value comes the current merge, this value -will- win
+            // eventually.
+            MergePick::Major
+        }
+    } else if src_minor.rev == current_merge {
+        // The last value comes the current merge, this value -will- win
+        // eventually.
+        MergePick::Minor
+    } else if src_major.path == src_minor.path {
         // we have the same value, but from other source;
         if src_major.rev == src_minor.rev {
             // If the two entry are identical, they are both valid
             MergePick::Any
         } else if oracle.is_overwrite(src_major.rev, src_minor.rev) {
             MergePick::Minor
         } else {
             MergePick::Major
         }
     } else if src_major.rev == src_minor.rev {
         // We cannot get copy information for both p1 and p2 in the
         // same rev. So this is the same value.
         unreachable!(
             "conflict information from p1 and p2 in the same revision"
         );
     } else {
         let dest_path = path_map.untokenize(*dest);
         let action = changes.get_merge_case(dest_path);
         if src_major.path.is_none() && action == MergeCase::Salvaged {
             // If the file is "deleted" in the major side but was
             // salvaged by the merge, we keep the minor side alive
             MergePick::Minor
         } else if src_minor.path.is_none() && action == MergeCase::Salvaged {
             // If the file is "deleted" in the minor side but was
             // salvaged by the merge, unconditionnaly preserve the
             // major side.
             MergePick::Major
         } else if action == MergeCase::Merged {
             // If the file was actively merged, copy information
             // from each side might conflict.  The major side will
             // win such conflict.
             MergePick::Major
         } else if oracle.is_overwrite(src_major.rev, src_minor.rev) {
             // If the minor side is strictly newer than the major
             // side, it should be kept.
             MergePick::Minor
         } else if src_major.path.is_some() {
             // without any special case, the "major" value win
             // other the "minor" one.
             MergePick::Major
         } else if oracle.is_overwrite(src_minor.rev, src_major.rev) {
             // the "major" rev is a direct ancestors of "minor",
             // any different value should
             // overwrite
             MergePick::Major
         } else {
             // major version is None (so the file was deleted on
             // that branch) and that branch is independant (neither
             // minor nor major is an ancestors of the other one.)
             // We preserve the new
             // information about the new file.
             MergePick::Minor
         }
     }
 }