Allow applying a patch with conflicts

In some settings, we want to let users apply a patch that does
not cleanly apply and add conflict markers. In Gerrit, this is
useful when cherry picking (via Git patches) from one host to
another.

This commit takes a simple approach: If a hunk doesn't apply,
go to the pre-image line, treat all lines in pre-image length
as left side of the conflict and all context and newly added
lines as right side of the conflict.

Change-Id: I01411d7a32b3f3207097b26231909aae6b835650

7 files changed, 119 insertions, 2 deletions

diff --git a/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/ConflictOutOfBounds.patch b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/ConflictOutOfBounds.patch
new file mode 100644
index 0000000000..6e7448b95c
--- /dev/null
+++ b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/ConflictOutOfBounds.patch
@@ -0,0 +1,10 @@
+diff --git a/ConflictOutOfBounds b/ConflictOutOfBounds
+index 0000000..de98044
+--- a/ConflictOutOfBounds
++++ b/ConflictOutOfBounds
+@@ -25,4 +25,4 @@
+ line3
+-lineA
++lineB
+ line5
+ line6
diff --git a/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/ConflictOutOfBounds_PostImage b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/ConflictOutOfBounds_PostImage
new file mode 100644
index 0000000000..4e5d5b2d88
--- /dev/null
+++ b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/ConflictOutOfBounds_PostImage
@@ -0,0 +1,15 @@
+line1
+line2
+line3
+line4
+line5
+line6
+line7
+line8
+<<<<<<< HEAD
+=======
+line3
+lineB
+line5
+line6
+>>>>>>> PATCH
diff --git a/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/ConflictOutOfBounds_PreImage b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/ConflictOutOfBounds_PreImage
new file mode 100644
index 0000000000..f62562a216
--- /dev/null
+++ b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/ConflictOutOfBounds_PreImage
@@ -0,0 +1,8 @@
+line1
+line2
+line3
+line4
+line5
+line6
+line7
+line8
diff --git a/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/allowconflict.patch b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/allowconflict.patch
new file mode 100644
index 0000000000..a99e636382
--- /dev/null
+++ b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/allowconflict.patch
@@ -0,0 +1,10 @@
+diff --git a/allowconflict b/allowconflict
+index 0000000..de98044
+--- a/allowconflict
++++ b/allowconflict
+@@ -3,4 +3,4 @@
+ line3
+-lineA
++lineB
+ line5
+ line6
diff --git a/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/allowconflict_PostImage b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/allowconflict_PostImage
new file mode 100644
index 0000000000..a963b40dfe
--- /dev/null
+++ b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/allowconflict_PostImage
@@ -0,0 +1,15 @@
+line1
+line2
+<<<<<<< HEAD
+line3
+line4
+line5
+line6
+=======
+line3
+lineB
+line5
+line6
+>>>>>>> PATCH
+line7
+line8
diff --git a/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/allowconflict_PreImage b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/allowconflict_PreImage
new file mode 100644
index 0000000000..f62562a216
--- /dev/null
+++ b/org.eclipse.jgit.test/tst-rsrc/org/eclipse/jgit/diff/allowconflict_PreImage
@@ -0,0 +1,8 @@
+line1
+line2
+line3
+line4
+line5
+line6
+line7
+line8
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/patch/PatchApplierTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/patch/PatchApplierTest.java
index 2aac15bbb6..e4b0ffbc21 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/patch/PatchApplierTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/patch/PatchApplierTest.java
@@ -27,6 +27,7 @@ import java.nio.file.Files;
 
 import org.eclipse.jgit.annotations.Nullable;
 import org.eclipse.jgit.api.Git;
+import org.eclipse.jgit.api.errors.PatchFormatException;
 import org.eclipse.jgit.attributes.FilterCommand;
 import org.eclipse.jgit.attributes.FilterCommandFactory;
 import org.eclipse.jgit.attributes.FilterCommandRegistry;
@@ -48,8 +49,7 @@ import org.junit.runner.RunWith;
 import org.junit.runners.Suite;
 
 @RunWith(Suite.class)
-@Suite.SuiteClasses({
- 		PatchApplierTest.WithWorktree. class, //
+@Suite.SuiteClasses({ PatchApplierTest.WithWorktree.class, //
 		PatchApplierTest.InCore.class, //
 })
 public class PatchApplierTest {
@@ -128,6 +128,20 @@ public class PatchApplierTest {
 			}
 		}
 
+		protected Result applyPatchAllowConflicts() throws IOException {
+			InputStream patchStream = getTestResource(name + ".patch");
+			Patch patch = new Patch();
+			patch.parse(patchStream);
+			if (inCore) {
+				try (ObjectInserter oi = db.newObjectInserter()) {
+					return new PatchApplier(db, baseTip, oi).allowConflicts()
+							.applyPatch(patch);
+				}
+			}
+			return new PatchApplier(db).allowConflicts()
+					.applyPatch(patch);
+		}
+
 		protected static InputStream getTestResource(String patchFile) {
 			return PatchApplierTest.class.getClassLoader()
 					.getResourceAsStream("org/eclipse/jgit/diff/" + patchFile);
@@ -169,6 +183,13 @@ public class PatchApplierTest {
 			verifyContent(result, aName, exists);
 		}
 
+		void verifyChange(Result result, String aName, boolean exists,
+				int numConflicts) throws Exception {
+			assertEquals(numConflicts, result.getErrors().size());
+			assertEquals(1, result.getPaths().size());
+			verifyContent(result, aName, exists);
+		}
+
 		protected byte[] readBlob(ObjectId treeish, String path)
 				throws Exception {
 			try (TestRepository<?> tr = new TestRepository<>(db);
@@ -346,6 +367,36 @@ public class PatchApplierTest {
 		}
 
 		@Test
+		public void testConflictMarkers() throws Exception {
+			init("allowconflict", true, true);
+
+			Result result = applyPatchAllowConflicts();
+
+			assertEquals(result.getErrors().size(), 1);
+			PatchApplier.Result.Error error = result.getErrors().get(0);
+			error.hh = null; // We don't assert the hunk header as it is a
+								// complex object with lots of internal state.
+			assertEquals(error, new PatchApplier.Result.Error(
+					"cannot apply hunk", "allowconflict", null));
+			verifyChange(result, "allowconflict", true, 1);
+		}
+
+		@Test
+		public void testConflictMarkersOutOfBounds() throws Exception {
+			init("ConflictOutOfBounds", true, true);
+
+			Result result = applyPatchAllowConflicts();
+
+			assertEquals(result.getErrors().size(), 1);
+			PatchApplier.Result.Error error = result.getErrors().get(0);
+			error.hh = null; // We don't assert the hunk header as it is a
+								// complex object with lots of internal state.
+			assertEquals(error, new PatchApplier.Result.Error(
+					"cannot apply hunk", "ConflictOutOfBounds", null));
+			verifyChange(result, "ConflictOutOfBounds", true, 1);
+		}
+
+		@Test
 		public void testShiftUp() throws Exception {
 			init("ShiftUp");