Merge pull request #2476 from mtsgrd/fix/false-conflict-empty-insertion-overlap

fix: coalesce split Myers hunks to prevent false merge conflicts

Author: Byron

gix-merge/src/blob/builtin_driver/text/utils.rs

@@ -479,7 +479,9 @@ pub fn collect_hunks(
     side: Side,
     mut hunks: Vec<Hunk>,
 ) -> Vec<Hunk> {
-    hunks.extend(imara_diff::Diff::compute(algorithm, input).hunks().map(|hunk| Hunk {
+    let mut diff = imara_diff::Diff::compute(algorithm, input);
+    diff.postprocess_lines(input);
+    hunks.extend(diff.hunks().map(|hunk| Hunk {
         before: hunk.before,
         after: hunk.after,
         side,

gix-merge/tests/merge/blob/builtin_driver.rs

@@ -369,6 +369,73 @@ mod text {
         }
     }
 
+    mod false_conflict {
+        use gix_merge::blob::{builtin_driver, builtin_driver::text::Conflict, Resolution};
+        use imara_diff::InternedInput;
+
+        /// Minimal reproduction: Myers produces a false conflict where git merge-file resolves cleanly.
+        ///
+        /// base:   alpha_x / (blank) / bravo_x / charlie_x / (blank)
+        /// ours:   (blank) / (blank) / bravo_x / charlie_x
+        /// theirs: alpha_x / (blank) / charlie_x / (blank)
+        ///
+        /// base→ours:  alpha_x deleted (replaced by blank), trailing blank removed
+        /// base→theirs: bravo_x deleted
+        ///
+        /// These are non-overlapping changes that git merges cleanly.
+        /// See https://github.com/GitoxideLabs/gitoxide/issues/2475
+        #[test]
+        fn myers_false_conflict_with_blank_line_ambiguity() {
+            let base = b"alpha_x\n\nbravo_x\ncharlie_x\n\n";
+            let ours = b"\n\nbravo_x\ncharlie_x\n";
+            let theirs = b"alpha_x\n\ncharlie_x\n\n";
+
+            let labels = builtin_driver::text::Labels {
+                ancestor: Some("base".into()),
+                current: Some("ours".into()),
+                other: Some("theirs".into()),
+            };
+
+            // Histogram resolves cleanly.
+            {
+                let options = builtin_driver::text::Options {
+                    diff_algorithm: imara_diff::Algorithm::Histogram,
+                    conflict: Conflict::Keep {
+                        style: builtin_driver::text::ConflictStyle::Merge,
+                        marker_size: 7.try_into().unwrap(),
+                    },
+                };
+                let mut out = Vec::new();
+                let mut input = InternedInput::default();
+                let res = builtin_driver::text(&mut out, &mut input, labels, ours, base, theirs, options);
+                assert_eq!(res, Resolution::Complete, "Histogram should resolve cleanly");
+            }
+
+            // Myers should also resolve cleanly (it used to produce a false conflict because
+            // imara-diff's Myers splits the ours change into two hunks — a deletion at base[0]
+            // and an empty insertion at base[2] — and the insertion collided with theirs'
+            // deletion at base[2]).
+            {
+                let options = builtin_driver::text::Options {
+                    diff_algorithm: imara_diff::Algorithm::Myers,
+                    conflict: Conflict::Keep {
+                        style: builtin_driver::text::ConflictStyle::Merge,
+                        marker_size: 7.try_into().unwrap(),
+                    },
+                };
+                let mut out = Vec::new();
+                let mut input = InternedInput::default();
+                let res = builtin_driver::text(&mut out, &mut input, labels, ours, base, theirs, options);
+                assert_eq!(
+                    res,
+                    Resolution::Complete,
+                    "Myers should resolve cleanly (git merge-file does). Output:\n{}",
+                    String::from_utf8_lossy(&out)
+                );
+            }
+        }
+    }
+
     mod baseline {
         use std::path::Path;