blockstore: allow initial SaveBlock() at any height

Followup from #4588. Allow the first `SaveBlock()` call in an empty block store to be at any height, to start from a truncated block history. Subsequent `SaveBlock()` calls must be for contiguous blocks.

______

For contributor use:

- [x] Wrote tests
- [ ] ~Updated CHANGELOG_PENDING.md~
- [x] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
- [x] Updated relevant documentation (`docs/`) and code comments
- [x] Re-reviewed `Files changed` in the Github PR explorer

store/store.go

@@ -275,7 +275,7 @@ func (bs *BlockStore) SaveBlock(block *types.Block, blockParts *types.PartSet, s
 	height := block.Height
 	hash := block.Hash()
 
-	if g, w := height, bs.Height()+1; g != w {
+	if g, w := height, bs.Height()+1; bs.Base() > 0 && g != w {
 		panic(fmt.Sprintf("BlockStore can only save contiguous blocks. Wanted %v, got %v", w, g))
 	}
 	if !blockParts.IsComplete() {
@@ -306,8 +306,8 @@ func (bs *BlockStore) SaveBlock(block *types.Block, blockParts *types.PartSet, s
 	// Done!
 	bs.mtx.Lock()
 	bs.height = height
-	if bs.base == 0 && height == 1 {
-		bs.base = 1
+	if bs.base == 0 {
+		bs.base = height
 	}
 	bs.mtx.Unlock()
 
@@ -319,9 +319,6 @@ func (bs *BlockStore) SaveBlock(block *types.Block, blockParts *types.PartSet, s
 }
 
 func (bs *BlockStore) saveBlockPart(height int64, index int, part *types.Part) {
-	if height != bs.Height()+1 {
-		panic(fmt.Sprintf("BlockStore can only save contiguous blocks. Wanted %v, got %v", bs.Height()+1, height))
-	}
 	partBytes := cdc.MustMarshalBinaryBare(part)
 	bs.db.Set(calcBlockPartKey(height, index), partBytes)
 }

store/store_test.go

@@ -187,8 +187,6 @@ func TestBlockStoreSaveLoadBlock(t *testing.T) {
 		ChainID: "block_test",
 		Time:    tmtime.Now(),
 	}
-	header2 := header1
-	header2.Height = 4
 
 	// End of setup, test data
 
@@ -218,9 +216,12 @@ func TestBlockStoreSaveLoadBlock(t *testing.T) {
 		},
 
 		{
-			block:     newBlock(header2, commitAtH10),
-			parts:     uncontiguousPartSet,
-			wantPanic: "only save contiguous blocks", // and incomplete and uncontiguous parts
+			block: newBlock( // New block at height 5 in empty block store is fine
+				types.Header{Height: 5, ChainID: "block_test", Time: tmtime.Now()},
+				makeTestCommit(5, tmtime.Now()),
+			),
+			parts:      validPartSet,
+			seenCommit: makeTestCommit(5, tmtime.Now()),
 		},
 
 		{