sql: Use SQLite API instead of PRAGMA to check for column existence.

sql::Database::DoesColumnExist currently compiles and runs a "PRAGMA
table_info" [1] SQL query and iterates over the result looking for
the desired column name. This is more brittle and expensive than using
the the sqlite3_table_column_metadata() [2] API, which pulls schema data
directly, bypassing query compilation and execution.

This CL introduces a behavior change -- the previous PRAGMA table_info
implementation works for views [3, 4], whereas
sqlite3_table_column_metadata() only works for tables. The behavior
change is not visible, because Chrome currently uses views in a single
feature [5], namely extensions [6].

DoesColumnExist should only be used for migrations, which should operate
on concrete tables. In steady-state, each feature should have the
database schema migrated to the current version, so column existence is
known statically. So, removing support for views in DoesColumnExist() is
acceptable.

[1] https://www.sqlite.org/pragma.html#pragma_table_info
[2] https://www.sqlite.org/c3ref/table_column_metadata.html
[3] https://www.sqlite.org/matrix/pragma.html
[4] https://www.sqlite.org/matrix/matrix_dpragma.html#R-35224-32827-55933-44478-17553-06933-55583-57822
[5] https://cs.chromium.org/search/?q=file:%5C.cc+sql::+VIEW+case:yes
[6] https://cs.chromium.org/chromium/src/chrome/browser/extensions/activity_log/counting_policy.cc?q=VIEW+case:yes

Bug: 910955
Change-Id: I44474585e6bb2523b5cd91c76c3444058822e601
Reviewed-on: https://chromium-review.googlesource.com/c/1357825
Reviewed-by: Chris Mumford <cmumford@google.com>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614663}

sql/database.cc

@@ -1490,23 +1490,15 @@ bool Database::DoesSchemaItemExist(const char* name, const char* type) const {
 
 bool Database::DoesColumnExist(const char* table_name,
                                const char* column_name) const {
-  std::string sql("PRAGMA TABLE_INFO(");
-  sql.append(table_name);
-  sql.append(")");
-
-  Statement statement(GetUntrackedStatement(sql.c_str()));
-
-  // This can happen if the database is corrupt and the error is a test
-  // expectation.
-  if (!statement.is_valid())
-    return false;
-
-  while (statement.Step()) {
-    if (base::EqualsCaseInsensitiveASCII(statement.ColumnString(1),
-                                         column_name))
-      return true;
-  }
-  return false;
+  // sqlite3_table_column_metadata uses out-params to return column definition
+  // details, such as the column type and whether it allows NULL values. These
+  // aren't needed to compute the current method's result, so we pass in nullptr
+  // for all the out-params.
+  int error = sqlite3_table_column_metadata(
+      db_, "main", table_name, column_name, /* pzDataType= */ nullptr,
+      /* pzCollSeq= */ nullptr, /* pNotNull= */ nullptr,
+      /* pPrimaryKey= */ nullptr, /* pAutoinc= */ nullptr);
+  return error == SQLITE_OK;
 }
 
 int64_t Database::GetLastInsertRowId() const {

sql/database.h

@@ -420,6 +420,12 @@ class SQL_EXPORT Database {
   bool DoesViewExist(const char* table_name) const;
 
   // Returns true if a column with the given name exists in the given table.
+  //
+  // Calling this method on a VIEW returns an unspecified result.
+  //
+  // This should only be used by migration code for legacy features that do not
+  // use MetaTable, and need an alternative way of figuring out the database's
+  // current version.
   bool DoesColumnExist(const char* table_name, const char* column_name) const;
 
   // Returns sqlite's internal ID for the last inserted row. Valid only