Refactor Restore()

This commit refactors out the complexity of downloading ordered WAL
files in parallel to a type called `WALDownloader`. This makes it
easier to test the restore separately from the download.

testdata/Makefile

@@ -0,0 +1,8 @@
+.PHONY: default
+default:
+	make -C find-latest-generation/ok
+	make -C generation-time-bounds/ok
+	make -C generation-time-bounds/snapshots-only
+	make -C replica-client-time-bounds/ok
+	make -C snapshot-time-bounds/ok
+	make -C wal-time-bounds/ok

testdata/find-latest-generation/ok/Makefile

@@ -0,0 +1,7 @@
+.PHONY: default
+default:
+	TZ=UTC touch -t 200001010000 generations/0000000000000000/snapshots/00000000.snapshot.lz4
+	TZ=UTC touch -t 200001020000 generations/0000000000000000/snapshots/00000001.snapshot.lz4
+	TZ=UTC touch -t 200001010000 generations/0000000000000001/snapshots/00000000.snapshot.lz4
+	TZ=UTC touch -t 200001030000 generations/0000000000000001/snapshots/00000001.snapshot.lz4
+	TZ=UTC touch -t 200001010000 generations/0000000000000002/snapshots/00000000.snapshot.lz4

testdata/generation-time-bounds/ok/Makefile

@@ -0,0 +1,8 @@
+.PHONY: default
+default:
+	TZ=UTC touch -t 200001010000 generations/0000000000000000/snapshots/00000000.snapshot.lz4
+	TZ=UTC touch -t 200001020000 generations/0000000000000000/snapshots/00000001.snapshot.lz4
+	TZ=UTC touch -t 200001010000 generations/0000000000000000/wal/00000000/00000000.wal.lz4
+	TZ=UTC touch -t 200001020000 generations/0000000000000000/wal/00000000/00000001.wal.lz4
+	TZ=UTC touch -t 200001030000 generations/0000000000000000/wal/00000001/00000000.wal.lz4
+

testdata/generation-time-bounds/snapshots-only/Makefile

@@ -0,0 +1,5 @@
+.PHONY: default
+default:
+	TZ=UTC touch -t 200001010000 generations/0000000000000000/snapshots/00000000.snapshot.lz4
+	TZ=UTC touch -t 200001020000 generations/0000000000000000/snapshots/00000001.snapshot.lz4
+

testdata/max-snapshot-index/ok/Makefile

@@ -0,0 +1,6 @@
+.PHONY: default
+default:
+	TZ=UTC touch -t 200001020000 generations/0000000000000000/snapshots/00000000.snapshot.lz4
+	TZ=UTC touch -t 200001010000 generations/0000000000000001/snapshots/00000000.snapshot.lz4
+	TZ=UTC touch -t 200001030000 generations/0000000000000001/snapshots/00000001.snapshot.lz4
+	TZ=UTC touch -t 200001010000 generations/0000000000000002/snapshots/00000000.snapshot.lz4

testdata/replica-client-time-bounds/ok/Makefile

@@ -0,0 +1,6 @@
+.PHONY: default
+default:
+	TZ=UTC touch -t 200001020000 generations/0000000000000000/snapshots/00000000.snapshot.lz4
+	TZ=UTC touch -t 200001010000 generations/0000000000000001/snapshots/00000000.snapshot.lz4
+	TZ=UTC touch -t 200001030000 generations/0000000000000001/snapshots/00000001.snapshot.lz4
+	TZ=UTC touch -t 200001010000 generations/0000000000000002/snapshots/00000000.snapshot.lz4

testdata/restore/bad-permissions/README

@@ -0,0 +1,36 @@
+To reproduce this testdata, run sqlite3 and execute:
+
+	PRAGMA journal_mode = WAL;
+	CREATE TABLE t (x);
+	INSERT INTO t (x) VALUES (1);
+	INSERT INTO t (x) VALUES (2);
+
+	sl3 split -o generations/0000000000000000/wal/00000000 db-wal
+	cp db generations/0000000000000000/snapshots/00000000.snapshot
+	lz4 -c --rm generations/0000000000000000/snapshots/00000000.snapshot
+
+
+Then execute:
+
+	PRAGMA wal_checkpoint(TRUNCATE);
+	INSERT INTO t (x) VALUES (3);
+
+	sl3 split -o generations/0000000000000000/wal/00000001 db-wal
+
+
+Then execute:
+
+	PRAGMA wal_checkpoint(TRUNCATE);
+	INSERT INTO t (x) VALUES (4);
+	INSERT INTO t (x) VALUES (5);
+
+	sl3 split -o generations/0000000000000000/wal/00000002 db-wal
+
+
+Finally, obtain the final snapshot:
+
+	PRAGMA wal_checkpoint(TRUNCATE);
+
+	cp db 00000002.db
+	rm db*
+

testdata/restore/ok/README

@@ -0,0 +1,36 @@
+To reproduce this testdata, run sqlite3 and execute:
+
+	PRAGMA journal_mode = WAL;
+	CREATE TABLE t (x);
+	INSERT INTO t (x) VALUES (1);
+	INSERT INTO t (x) VALUES (2);
+
+	sl3 split -o generations/0000000000000000/wal/00000000 db-wal
+	cp db generations/0000000000000000/snapshots/00000000.snapshot
+	lz4 -c --rm generations/0000000000000000/snapshots/00000000.snapshot
+
+
+Then execute:
+
+	PRAGMA wal_checkpoint(TRUNCATE);
+	INSERT INTO t (x) VALUES (3);
+
+	sl3 split -o generations/0000000000000000/wal/00000001 db-wal
+
+
+Then execute:
+
+	PRAGMA wal_checkpoint(TRUNCATE);
+	INSERT INTO t (x) VALUES (4);
+	INSERT INTO t (x) VALUES (5);
+
+	sl3 split -o generations/0000000000000000/wal/00000002 db-wal
+
+
+Finally, obtain the final snapshot:
+
+	PRAGMA wal_checkpoint(TRUNCATE);
+
+	cp db 00000002.db
+	rm db*
+

testdata/snapshot-time-bounds/ok/Makefile

@@ -0,0 +1,6 @@
+.PHONY: default
+default:
+	TZ=UTC touch -t 200001010000 generations/0000000000000000/snapshots/00000000.snapshot.lz4
+	TZ=UTC touch -t 200001020000 generations/0000000000000000/snapshots/00000001.snapshot.lz4
+	TZ=UTC touch -t 200001030000 generations/0000000000000000/snapshots/00000002.snapshot.lz4
+

testdata/wal-downloader/ok/README

@@ -0,0 +1,40 @@
+To reproduce this testdata, run sqlite3 and execute:
+
+	PRAGMA journal_mode = WAL;
+	CREATE TABLE t (x);
+	INSERT INTO t (x) VALUES (1);
+	INSERT INTO t (x) VALUES (2);
+
+And copy & split the WAL into segments:
+
+	sl3 split -o generations/0000000000000000/wal/00000000 db-wal
+	cp db-wal 00000000.wal
+
+
+Then execute:
+
+	PRAGMA wal_checkpoint(TRUNCATE);
+	INSERT INTO t (x) VALUES (3);
+
+And split again:
+
+	sl3 split -o generations/0000000000000000/wal/00000001 db-wal
+	cp db-wal 00000001.wal
+
+
+Then execute:
+
+	PRAGMA wal_checkpoint(TRUNCATE);
+	INSERT INTO t (x) VALUES (4);
+	INSERT INTO t (x) VALUES (5);
+
+And split again:
+
+	sl3 split -o generations/0000000000000000/wal/00000002 db-wal
+	cp db-wal 00000002.wal
+
+
+Finally, remove the original database files:
+
+	rm db*
+

testdata/wal-downloader/one/README

@@ -0,0 +1,17 @@
+To reproduce this testdata, run sqlite3 and execute:
+
+	PRAGMA journal_mode = WAL;
+	CREATE TABLE t (x);
+	INSERT INTO t (x) VALUES (1);
+	INSERT INTO t (x) VALUES (2);
+
+And copy & split the WAL into segments:
+
+	sl3 split -o generations/0000000000000000/wal/00000000 db-wal
+	cp db-wal 00000000.wal
+
+
+Finally, remove the original database files:
+
+	rm db*
+