sql: Have MetaTable expose a "delete the table" method

sql::MetaTable works by manipulating a caller-provided database to store
additional version information in a new metadata table. Some clients
using sql::MetaTable might start out with their databases unversioned
and switch to maintaining version metadata at a later time. In this
case, it can help to be able to test the no-metadata-maintained to
metadata-maintained upgrade.  However, once a database begins using a
metadata table, it might be difficult to emulate the state of the
database prior to using a metadata table. Clearing out the initialized
metadata table can achieve this effect, but the name of the metadata
table is an implementation detail, so it's not possible to just delete
the table directly from test code. To get around this issue, this CL
adds an sql::MetaTable::DeleteTableForTesting method to do just that.

As one example, the child CL crrev.com/c/2908133 uses this method to
verify its database correctly handles an upgrade from a state with no
version written to disk.

Bug: None
Change-Id: I5d440e11196638a04ad16beb4002ed5d3e25af19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2920848
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#894658}

sql/meta_table.cc

@@ -36,6 +36,12 @@ bool MetaTable::DoesTableExist(sql::Database* db) {
   return db->DoesTableExist("meta");
 }
 
+// static
+bool MetaTable::DeleteTableForTesting(sql::Database* db) {
+  DCHECK(db);
+  return db->Execute("DROP TABLE IF EXISTS meta");
+}
+
 // static
 bool MetaTable::GetMmapStatus(Database* db, int64_t* status) {
   const char* kMmapStatusSql = "SELECT value FROM meta WHERE key = ?";

sql/meta_table.h

@@ -37,6 +37,11 @@ class COMPONENT_EXPORT(SQL) MetaTable {
   // Returns true if the 'meta' table exists.
   static bool DoesTableExist(Database* db);
 
+  // Deletes the 'meta' table if it exists, returning false if an internal error
+  // occurred during the deletion and true otherwise (no matter whether the
+  // table existed).
+  static bool DeleteTableForTesting(Database* db);
+
   // If the current version of the database is less than or equal to
   // |deprecated_version|, raze the database. Must be called outside of a
   // transaction.

sql/meta_table_unittest.cc

@@ -42,6 +42,14 @@ TEST_F(SQLMetaTableTest, DoesTableExist) {
   EXPECT_TRUE(MetaTable::DoesTableExist(&db_));
 }
 
+TEST_F(SQLMetaTableTest, DeleteTableForTesting) {
+  MetaTable meta_table;
+  EXPECT_TRUE(meta_table.Init(&db_, 1, 1));
+
+  EXPECT_TRUE(MetaTable::DeleteTableForTesting(&db_));
+  EXPECT_FALSE(MetaTable::DoesTableExist(&db_));
+}
+
 TEST_F(SQLMetaTableTest, RazeIfDeprecated) {
   const int kDeprecatedVersion = 1;
   const int kVersion = 2;