sql: Add support for measuring per-database memory usage.

Change-Id: I836b9412f915cdebeff46883c535b83026ed6472
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463873
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817189}

sql/database.cc

@@ -1401,6 +1401,33 @@ int Database::GetLastChangeCount() const {
   return sqlite3_changes(db_);
 }
 
+int Database::GetMemoryUsage() {
+  if (!db_) {
+    DCHECK(poisoned_) << "Illegal use of Database without a db";
+    return 0;
+  }
+
+  int highwater_should_always_be_zero;
+  int cache_memory = 0, schema_memory = 0, statement_memory = 0;
+
+  int error =
+      sqlite3_db_status(db_, SQLITE_DBSTATUS_CACHE_USED, &cache_memory,
+                        &highwater_should_always_be_zero, /*resetFlg=*/false);
+  DCHECK_EQ(error, SQLITE_OK);
+
+  error =
+      sqlite3_db_status(db_, SQLITE_DBSTATUS_SCHEMA_USED, &schema_memory,
+                        &highwater_should_always_be_zero, /*resetFlg=*/false);
+  DCHECK_EQ(error, SQLITE_OK);
+
+  error =
+      sqlite3_db_status(db_, SQLITE_DBSTATUS_STMT_USED, &statement_memory,
+                        &highwater_should_always_be_zero, /*resetFlg=*/false);
+  DCHECK_EQ(error, SQLITE_OK);
+
+  return cache_memory + schema_memory + statement_memory;
+}
+
 int Database::GetErrorCode() const {
   if (!db_)
     return SQLITE_ERROR;

sql/database.h

@@ -463,6 +463,15 @@ class COMPONENT_EXPORT(SQL) Database {
   // is closed.
   int GetLastChangeCount() const;
 
+  // Approximates the amount of memory used by SQLite for this database.
+  //
+  // This measures the memory used for the page cache (most likely the biggest
+  // consumer), database schema, and prepared statements.
+  //
+  // The memory used by the page cache can be recovered by calling TrimMemory(),
+  // which will cause SQLite to drop the page cache.
+  int GetMemoryUsage();
+
   // Errors --------------------------------------------------------------------
 
   // Returns the error code associated with the last sqlite operation.

sql/database_unittest.cc

@@ -1251,6 +1251,29 @@ TEST_F(SQLDatabaseTest, GetAppropriateMmapSizeAltStatus) {
             ExecuteWithResult(&db(), "SELECT * FROM MmapStatus"));
 }
 
+TEST_F(SQLDatabaseTest, GetMemoryUsage) {
+  // Databases with mmap enabled may not follow the assumptions below.
+  db().Close();
+  db().set_mmap_disabled();
+  ASSERT_TRUE(db().Open(db_path()));
+
+  int initial_memory = db().GetMemoryUsage();
+  EXPECT_GT(initial_memory, 0)
+      << "SQLite should always use some memory for a database";
+
+  ASSERT_TRUE(db().Execute("CREATE TABLE foo (a, b)"));
+  ASSERT_TRUE(db().Execute("INSERT INTO foo(a, b) VALUES (12, 13)"));
+
+  int post_query_memory = db().GetMemoryUsage();
+  EXPECT_GT(post_query_memory, initial_memory)
+      << "Page cache usage should go up after executing queries";
+
+  db().TrimMemory();
+  int post_trim_memory = db().GetMemoryUsage();
+  EXPECT_GT(post_query_memory, post_trim_memory)
+      << "Page cache usage should go down after calling TrimMemory()";
+}
+
 TEST_F(SQLDatabaseTest, LockingModeExclusive) {
   db().Close();
   db().set_exclusive_locking();