view rust/hg-core/src/copy_tracing.rs @ 46472:98e39f04d60e

upgrade: implement partial upgrade for upgrading persistent-nodemap

Upgrading repositories to use persistent nodemap should be fast and easy as it
requires only two things:

1) Updating the requirements
2) Writing a persistent-nodemap on disk

For both of the steps above, we don't need to edit existing revlogs.

This patch makes upgrade only do the above mentioned two steps if we are
only upgarding to use persistent-nodemap feature.

Since `nodemap.persist_nodemap()` assumes that there exists a nodemap file for
the given revlog if we are trying to call it, this patch adds `force` argument
to create a file if does not exist which is true in our upgrade case.

The test changes demonstrate that we no longer write nodemap files for manifest
after upgrade which I think is desirable.

Differential Revision: https://phab.mercurial-scm.org/D9936

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,
                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.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.
            oracle.record_overwrite(src_minor.rev, src_major.rev);
            MergePick::Major
        }
    } else if src_minor.rev == current_merge {
        // The last value comes the current merge, this value -will- win
        // eventually.
        oracle.record_overwrite(src_major.rev, src_minor.rev);
        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
        }
    }
}