Add support for SQLITE_ENABLE_MULTITHREADED_CHECKS

The build tag `sqlite_enable_multithreaded_checks` enables
`SQLITE_ENABLE_MULTITHREADED_CHECKS` in sqlite.

sqlite3.c is modified to respect `SQLITE_ENABLE_MULTITHREADED_CHECKS` even
on connections that use `SQLITE_OPEN_FULLMUTEX`.

Updates tailscale/corp#34473

Signed-off-by: Percy Wegmann <percy@tailscale.com>

Author: oxtoacart

.github/workflows/test-sqlite.yml

@@ -28,7 +28,9 @@ jobs:
 
     - name: With sqlite_enable_tmstmpvfs
       run: go test -v -tags sqlite_enable_tmstmpvfs
-      
+
+    - name: With sqlite_enable_multithreaded_checks
+      run: go test -v -tags sqlite_enable_multithreaded_checks
     - name: Race
       run: go test -v -race ./...
 

cgosqlite/cgosqlite.go

@@ -63,6 +63,14 @@ int tmstmpvfs_enabled=1;
 int tmstmpvfs_enabled=0;
 #endif
 
+// Enable mutex contention warnings.
+#cgo sqlite_enable_multithreaded_checks CFLAGS: -DSQLITE_ENABLE_MULTITHREADED_CHECKS
+#ifdef SQLITE_ENABLE_MULTITHREADED_CHECKS
+int multithreaded_checks_enabled=1;
+#else
+int multithreaded_checks_enabled=0;
+#endif
+
 #include "cgosqlite.h"
 */
 import "C"
@@ -529,3 +537,8 @@ func APIArmorEnabled() bool {
 func TimestampVFSEnabled() bool {
 	return C.tmstmpvfs_enabled == 1
 }
+
+// MultithreadedChecksEnabled reports whether or not sqlite was compiled with SQLITE_ENABLE_MULTITHREADED_CHECKS.
+func MultithreadedChecksEnabled() bool {
+	return C.multithreaded_checks_enabled == 1
+}

cgosqlite/multithreaded_checks_disabled_test.go

@@ -0,0 +1,11 @@
+//go:build !sqlite_enable_multithreaded_checks
+
+package cgosqlite
+
+import (
+	"testing"
+)
+
+func TestMultithreadedChecksDisabled(t *testing.T) {
+	testMultithreadedChecks(t, false)
+}

cgosqlite/multithreaded_checks_enabled_test.go

@@ -0,0 +1,11 @@
+//go:build sqlite_enable_multithreaded_checks
+
+package cgosqlite
+
+import (
+	"testing"
+)
+
+func TestMultithreadedChecksEnabled(t *testing.T) {
+	testMultithreadedChecks(t, true)
+}

cgosqlite/multithreaded_checks_test.go

@@ -0,0 +1,80 @@
+//
+
+package cgosqlite
+
+import (
+	"path/filepath"
+	"runtime"
+	"sync"
+	"sync/atomic"
+	"testing"
+
+	"github.com/tailscale/sqlite/sqliteh"
+)
+
+// testMultithreadedChecks provides a common function for testing SQLITE_ENABLE_MULTITHREADED_CHECKS.
+func testMultithreadedChecks(t *testing.T, wantThreadingWarning bool) {
+	if wantThreadingWarning && !MultithreadedChecksEnabled() {
+		t.Fatal("Multithreaded checks are not enabled")
+	} else if !wantThreadingWarning && MultithreadedChecksEnabled() {
+		t.Fatal("Multithreaded checks are enabled")
+	}
+
+	var gotMisuseLog atomic.Bool
+	err := SetLogCallback(func(code sqliteh.Code, msg string) {
+		if code == sqliteh.SQLITE_MISUSE && msg == "illegal multi-threaded access to database connection" {
+			gotMisuseLog.Store(true)
+		}
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Lock this goroutine to a thread (preventing other goroutines from using that thread)
+	runtime.LockOSThread()
+
+	flags := sqliteh.SQLITE_OPEN_READWRITE |
+		sqliteh.SQLITE_OPEN_CREATE |
+		sqliteh.SQLITE_OPEN_WAL |
+		sqliteh.SQLITE_OPEN_URI |
+		sqliteh.SQLITE_OPEN_FULLMUTEX
+		// sqliteh.SQLITE_OPEN_FULLMUTEX currently contention checks do not work when opening connection with SQLITE_OPEN_FULLMUTEX
+	db, err := Open(filepath.Join(t.TempDir(), "test.db"), flags, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+
+	hitAPI := func() {
+		for i := 0; i < 1000 && !gotMisuseLog.Load(); i++ {
+			// Prepare a statement on this thread, mostly ignoring errors.
+			stmt, _, err := db.Prepare("CREATE TABLE t(c INTEGER PRIMARY KEY)", 0)
+			if err != nil {
+				continue
+			}
+			if _, err := stmt.Step(nil); err != nil {
+				continue
+			}
+			_ = stmt.Finalize()
+		}
+	}
+
+	// Hit API on a separate goroutine as well as in this goroutine.
+	// Because the original goroutine locked the OS thread, this new goroutine
+	// will execute on a separate thread.
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		hitAPI()
+	}()
+	hitAPI()
+	wg.Wait()
+
+	if wantThreadingWarning && !gotMisuseLog.Load() {
+		t.Fatal("did not get SQLITE_MISUSE in LogCallback")
+	}
+	if !wantThreadingWarning && gotMisuseLog.Load() {
+		t.Fatal("got SQLITE_MISUSE in LogCallback")
+	}
+}

cgosqlite/sqlite3.c

@@ -188262,7 +188262,7 @@ static int openDatabase(
       db = 0;
       goto opendb_out;
     }
-    if( isThreadsafe==0 ){
+    if( isThreadsafe==0 || sqlite3GlobalConfig.bCoreMutex ){
       sqlite3MutexWarnOnContention(db->mutex);
     }
   }