MidxPackList: getAllPlainPacks returns object-lookup order

For object lookup we want the most recent packs first, because we
usually talk more about recent object. In midx, we want oldest packs
first to choose the oldest copy of an object for duplicates.

MidxPackList replaces a midx in the pack list with its covered packs,
but it keep them in midx order, producing a list that is in no useful
order. e.g. [INSERT, midx(GC, COMPACT)] returns [INSERT, GC, COMPACT].

Introduced the covered packs in reverse order, so the list matches the
object lookup order. Then [INSERT, midx(GC, COMPACT)] becomes [INSERT,
COMPACT, GC].

Link: https://review.gerrithub.io/id/I96458e5585b00d31ae6f1085412a2f5b6a6a6964

Author: ifradeo

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/dfs/MidxPackListTest.java

@@ -44,7 +44,7 @@ public void getAllPlainPacks_onlyMidx() {
 		DfsPackFileMidx midx = packPool.midx("midx", null, "a", "b", "c");
 
 		MidxPackList packList = MidxPackList.create(midx);
-		assertEquals(List.of(a, b, c), packList.getAllPlainPacks());
+		assertEquals(List.of(c, b, a), packList.getAllPlainPacks());
 	}
 
 	@Test
@@ -56,7 +56,7 @@ public void getAllPlainPacks_midxPlusOne() {
 		DfsPackFile d = packPool.pack("d");
 
 		MidxPackList packList = MidxPackList.create(d, midx);
-		assertEquals(List.of(d, a, b, c), packList.getAllPlainPacks());
+		assertEquals(List.of(d, c, b, a), packList.getAllPlainPacks());
 	}
 
 	@Test
@@ -74,7 +74,7 @@ public void getAllPlainPacks_nestedMidx() {
 				"f");
 
 		MidxPackList packList = MidxPackList.create(midxTip);
-		assertEquals(List.of(e, f, c, d, a, b), packList.getAllPlainPacks());
+		assertEquals(List.of(f, e, d, c, b, a), packList.getAllPlainPacks());
 	}
 
 	@Test
@@ -304,9 +304,9 @@ public void getPlainPacksNotCoveredBy_midxChain() {
 				"f");
 
 		MidxPackList packList = MidxPackList.create(midxTip);
-		assertEquals(List.of(e, f, c, d),
+		assertEquals(List.of(f, e, d, c),
 				packList.getPlainPacksNotCoveredBy(midxBase));
-		assertEquals(List.of(e, f),
+		assertEquals(List.of(f, e),
 				packList.getPlainPacksNotCoveredBy(midxMiddle));
 		assertEquals(List.of(), packList.getPlainPacksNotCoveredBy(midxTip));
 	}

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/MidxPackList.java

@@ -53,6 +53,11 @@ private MidxPackList(List<DfsPackFile> packs) {
 
 	/**
 	 * Get all plain packs in the list, either top-level or inside midxs
+	 * <p>
+	 * Inside midx, the packs are in reverse lookup order. This code restores
+	 * their original order. i.e. the list with packs [INSERT, midx(COMPACT-2,
+	 * COMPACT-3), midx(GC, COMPACT-1)] becomes [INSERT, COMPACT-3, COMPACT-2,
+	 * COMPACT-1, GC].
 	 *
 	 * @return a list of all "real" packs in this pack list, either top level or
 	 *         inside midxs.
@@ -63,7 +68,11 @@ public List<DfsPackFile> getAllPlainPacks() {
 		while (!pending.isEmpty()) {
 			DfsPackFile pack = pending.poll();
 			if (pack instanceof DfsPackFileMidx midxPack) {
-				plainPacks.addAll(midxPack.getCoveredPacks());
+				// Midx order is the reverse of object lookup order
+				ArrayList<DfsPackFile> coveredPacks = new ArrayList<>(
+						midxPack.getCoveredPacks());
+				Collections.reverse(coveredPacks);
+				plainPacks.addAll(coveredPacks);
 				if (midxPack.getMultipackIndexBase() != null) {
 					pending.add(midxPack.getMultipackIndexBase());
 				}