sql: Add sequence checks to sql::Statement.

sql::Statement is not thread-safe. Add checks documenting this.

The checks are currently excluded on Android, because the Android
implementation of History uses sql::Statement in a thread-unsafe way --
see https://crbug.com/866218. Landing the checks on all other platforms
now is a bit ugly, but has the benefit of preventing new errors from
creeping into the codebase while the Android issues are figured out.

Bug: 863724
Change-Id: Ia0d087f40bd892c2ed084e3acfc3cf1ecf12e6af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1137851
Commit-Queue: Victor Costan <pwnall@chromium.org>
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Darwin Huang <huangdarwin@chromium.org>
Auto-Submit: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841165}

sql/statement.h

@@ -6,6 +6,7 @@
 #define SQL_STATEMENT_H_
 
 #include <stdint.h>
+
 #include <string>
 #include <vector>
 
@@ -15,6 +16,7 @@
 #include "base/sequence_checker.h"
 #include "base/strings/string16.h"
 #include "base/strings/string_piece_forward.h"
+#include "build/build_config.h"  // TODO(crbug.com/866218): Remove this include.
 #include "sql/database.h"
 
 namespace sql {
@@ -29,6 +31,11 @@ enum class ColumnType {
   kNull = 5,
 };
 
+// Compiles and executes SQL statements.
+//
+// This class is not thread-safe. An instance must be accessed from a single
+// sequence. This is enforced in DCHECK-enabled builds.
+//
 // Normal usage:
 //   sql::Statement s(connection_.GetUniqueStatement(...));
 //   s.BindInt(0, a);
@@ -66,7 +73,13 @@ class COMPONENT_EXPORT(SQL) Statement {
   // default value. This is because the statement can become invalid in the
   // middle of executing a command if there is a serious error and the database
   // has to be reset.
-  bool is_valid() const { return ref_->is_valid(); }
+  bool is_valid() const {
+#if !defined(OS_ANDROID)  // TODO(crbug.com/866218): Remove this conditional
+    DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
+#endif  // !defined(OS_ANDROID)
+
+    return ref_->is_valid();
+  }
 
   // Running -------------------------------------------------------------------
 
@@ -195,6 +208,8 @@ class COMPONENT_EXPORT(SQL) Statement {
   // See Succeeded() for what this holds.
   bool succeeded_ = false;
 
+  SEQUENCE_CHECKER(sequence_checker_);
+
   DISALLOW_COPY_AND_ASSIGN(Statement);
 };