[sql] Disable memory-mapping under sql::Recovery.

Recovery may be used to recover files with I/O errors, which would not
interact well with memory-mapped I/O.

[AFAICT, SQLite doesn't use memory-mapped I/O on temporary files, but as
of yet I don't see why this might be.]

BUG=551110

Review URL: https://codereview.chromium.org/1414563010

Cr-Commit-Position: refs/heads/master@{#358083}

sql/connection.cc

@@ -1628,18 +1628,6 @@ bool Connection::OpenInternal(const std::string& file_name,
   // secure_delete.
   ignore_result(Execute("PRAGMA journal_mode = TRUNCATE"));
 
-  // Enable memory-mapped access.  This value will be capped by
-  // SQLITE_MAX_MMAP_SIZE, which could be different between 32-bit and 64-bit
-  // platforms.
-  mmap_enabled_ = false;
-  if (!mmap_disabled_)
-    ignore_result(Execute("PRAGMA mmap_size = 268435456"));  // 256MB.
-  {
-    Statement s(GetUniqueStatement("PRAGMA mmap_size"));
-    if (s.Step() && s.ColumnInt64(0) > 0)
-      mmap_enabled_ = true;
-  }
-
   const base::TimeDelta kBusyTimeout =
     base::TimeDelta::FromSeconds(kBusyTimeoutSeconds);
 
@@ -1668,6 +1656,25 @@ bool Connection::OpenInternal(const std::string& file_name,
     return false;
   }
 
+  // Enable memory-mapped access.  The explicit-disable case is because SQLite
+  // can be built to default-enable mmap.  This value will be capped by
+  // SQLITE_MAX_MMAP_SIZE, which could be different between 32-bit and 64-bit
+  // platforms.
+  if (mmap_disabled_) {
+    ignore_result(Execute("PRAGMA mmap_size = 0"));
+  } else {
+    ignore_result(Execute("PRAGMA mmap_size = 268435456"));  // 256MB.
+  }
+
+  // Determine if memory-mapping has actually been enabled.  The Execute() above
+  // can succeed without changing the amount mapped.
+  mmap_enabled_ = false;
+  {
+    Statement s(GetUniqueStatement("PRAGMA mmap_size"));
+    if (s.Step() && s.ColumnInt64(0) > 0)
+      mmap_enabled_ = true;
+  }
+
   return true;
 }
 

sql/recovery.cc

@@ -141,6 +141,9 @@ Recovery::Recovery(Connection* connection)
   if (db_->page_size_)
     recover_db_.set_page_size(db_->page_size_);
 
+  // Files with I/O errors cannot be safely memory-mapped.
+  recover_db_.set_mmap_disabled();
+
   // TODO(shess): This may not handle cases where the default page
   // size is used, but the default has changed.  I do not think this
   // has ever happened.  This could be handled by using "PRAGMA

sql/recovery_unittest.cc

@@ -692,4 +692,16 @@ TEST_F(SQLRecoveryTest, Bug387868) {
 }
 #endif  // !defined(USE_SYSTEM_SQLITE)
 
+// Memory-mapped I/O interacts poorly with I/O errors.  Make sure the recovery
+// database doesn't accidentally enable it.
+TEST_F(SQLRecoveryTest, NoMmap) {
+  scoped_ptr<sql::Recovery> recovery = sql::Recovery::Begin(&db(), db_path());
+  ASSERT_TRUE(recovery.get());
+
+  // In the current implementation, the PRAGMA successfully runs with no result
+  // rows.  Running with a single result of |0| is also acceptable.
+  sql::Statement s(recovery->db()->GetUniqueStatement("PRAGMA mmap_size"));
+  EXPECT_TRUE(!s.Step() || !s.ColumnInt64(0));
+}
+
 }  // namespace