.,cgosqlite: add log handler callback

SQLite logs can contain useful diagnostic information such as access
misuse and other out of band failure modes.

Updates tailscale/corp#33477
Updates tailscale/corp#33305
Updates tailscale/corp#33577

Author: raggi

cgosqlite/logcallback.go

@@ -0,0 +1,55 @@
+package cgosqlite
+
+// #include <sqlite3.h>
+//
+// void logCallbackGo(void* userData, int errCode, char* msgC);
+//
+// static void log_callback_into_go(void *userData, int errCode, const char *msg) {
+// 	logCallbackGo(userData, errCode, (char*)msg);
+// }
+//
+// static int ts_sqlite3_config_log(void) {
+// 	// TODO(raggi): if the library gains new uses of sqlite3_config they need to
+// 	// share a mutex.
+// 	return sqlite3_config(SQLITE_CONFIG_LOG, log_callback_into_go, NULL);
+// }
+import "C"
+import (
+	"sync"
+	"unsafe"
+
+	"github.com/tailscale/sqlite/sqliteh"
+)
+
+// LogCallback receives SQLite log messages.
+type LogCallback func(code sqliteh.Code, msg string)
+
+var (
+	logCallbackMu sync.Mutex
+	logCallback   LogCallback
+)
+
+//export logCallbackGo
+func logCallbackGo(userData unsafe.Pointer, errCode C.int, msgC *C.char) {
+	logCallbackMu.Lock()
+	cb := logCallback
+	logCallbackMu.Unlock()
+
+	if cb == nil {
+		return
+	}
+
+	msg := C.GoString(msgC)
+	cb(sqliteh.Code(errCode), msg)
+}
+
+// SetLogCallback sets the global SQLite log callback.
+// If callback is nil, logs are discarded.
+func SetLogCallback(callback LogCallback) error {
+	logCallbackMu.Lock()
+	logCallback = callback
+	logCallbackMu.Unlock()
+
+	res := C.ts_sqlite3_config_log()
+	return errCode(res)
+}

sqlite_cgo.go

@@ -3,8 +3,20 @@
 
 package sqlite
 
-import "github.com/tailscale/sqlite/cgosqlite"
+import (
+	"github.com/tailscale/sqlite/cgosqlite"
+	"github.com/tailscale/sqlite/sqliteh"
+)
 
 func init() {
 	Open = cgosqlite.Open
 }
+
+// LogCallback receives SQLite log messages.
+type LogCallback func(code sqliteh.Code, msg string)
+
+// SetLogCallback sets the global SQLite log callback.
+// If callback is nil, logs are discarded.
+func SetLogCallback(callback LogCallback) error {
+	return cgosqlite.SetLogCallback(cgosqlite.LogCallback(callback))
+}

sqlite_cgo_test.go

@@ -0,0 +1,45 @@
+//go:build cgo
+// +build cgo
+
+package sqlite
+
+import (
+	"sync"
+	"testing"
+
+	"github.com/tailscale/sqlite/cgosqlite"
+	"github.com/tailscale/sqlite/sqliteh"
+)
+
+// ensure LogCallback is convertible to cgosqlite.LogCallback
+var _ cgosqlite.LogCallback = cgosqlite.LogCallback(LogCallback(func(code sqliteh.Code, msg string) {}))
+
+func TestSetLogCallback(t *testing.T) {
+	var mu sync.Mutex
+	var logs []string
+
+	err := SetLogCallback(func(code sqliteh.Code, msg string) {
+		mu.Lock()
+		defer mu.Unlock()
+		logs = append(logs, msg)
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer SetLogCallback(nil)
+
+	db := openTestDB(t)
+
+	_, err = db.Exec("SELECT * FROM nonexistent_table")
+	if err == nil {
+		t.Fatal("expected error from invalid SQL")
+	}
+
+	mu.Lock()
+	gotLogs := len(logs) > 0
+	mu.Unlock()
+
+	if !gotLogs {
+		t.Fatal("expected to receive log messages")
+	}
+}