Ensure MDBX_USE_FALLOCATE is enabled (#203)

* Enable MDBX_USE_FALLOCATE

* Fix lints

Author: anacrolix

check_fallocate.go

@@ -0,0 +1,34 @@
+//go:build ignore
+
+package main
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/erigontech/mdbx-go/mdbx"
+)
+
+func main() {
+	buildOpts := mdbx.BuildOptions()
+
+	fmt.Println("=== MDBX Build Options ===")
+	fmt.Println(buildOpts)
+	fmt.Println()
+
+	// Check for MDBX_USE_FALLOCATE specifically
+	if strings.Contains(buildOpts, "MDBX_USE_FALLOCATE=1") || strings.Contains(buildOpts, "MDBX_USE_FALLOCATE=AUTO=1") {
+		fmt.Println("✓ MDBX_USE_FALLOCATE is ENABLED (1)")
+	} else if strings.Contains(buildOpts, "MDBX_USE_FALLOCATE=0") || strings.Contains(buildOpts, "MDBX_USE_FALLOCATE=AUTO=0") {
+		fmt.Println("✗ MDBX_USE_FALLOCATE is DISABLED (0)")
+	} else {
+		fmt.Println("? MDBX_USE_FALLOCATE status unknown")
+	}
+
+	// Also check _GNU_SOURCE status
+	if strings.Contains(buildOpts, "_GNU_SOURCE=YES") {
+		fmt.Println("✓ _GNU_SOURCE is defined")
+	} else if strings.Contains(buildOpts, "_GNU_SOURCE=NO") {
+		fmt.Println("✗ _GNU_SOURCE is NOT defined")
+	}
+}

go.mod

@@ -6,4 +6,4 @@ toolchain go1.24.1
 
 require github.com/ianlancetaylor/cgosymbolizer v0.0.0-20241129212102-9c50ad6b591e
 
-require golang.org/x/sys v0.31.0 // indirect
+require golang.org/x/sys v0.31.0

mdbx/mdbx.go

@@ -127,7 +127,7 @@ details about dealing with such situations.
 package mdbx
 
 /*
-#cgo !windows CFLAGS: -O2 -g -DNDEBUG=1 -std=gnu11 -fvisibility=hidden -ffast-math  -fPIC -pthread -Wno-error=attributes -W -Wall -Wextra -Wpedantic -Wno-deprecated-declarations -Wno-format -Wno-format-security -Wno-implicit-fallthrough -Wno-unused-parameter -Wno-unused-function -Wno-format-extra-args -Wno-missing-field-initializers -Wno-unknown-warning-option -Wno-enum-int-mismatch -Wno-strict-prototypes
+#cgo !windows CFLAGS: -D_GNU_SOURCE -O2 -g -DNDEBUG=1 -std=gnu11 -fvisibility=hidden -ffast-math  -fPIC -pthread -Wno-error=attributes -W -Wall -Wextra -Wpedantic -Wno-deprecated-declarations -Wno-format -Wno-format-security -Wno-implicit-fallthrough -Wno-unused-parameter -Wno-unused-function -Wno-format-extra-args -Wno-missing-field-initializers -Wno-unknown-warning-option -Wno-enum-int-mismatch -Wno-strict-prototypes
 #cgo windows CFLAGS:  -O2 -g -DNDEBUG=1 -std=gnu11 -fvisibility=hidden -ffast-math -fexceptions -fno-common -W -Wno-deprecated-declarations -Wno-bad-function-cast -Wno-cast-function-type -Wall -Wno-format -Wno-format-security -Wno-implicit-fallthrough -Wno-unused-parameter -Wno-unused-function -Wno-format-extra-args -Wno-missing-field-initializers -Wno-unknown-warning-option -Wno-enum-int-mismatch -Wno-strict-prototypes
 
 #cgo windows LDFLAGS: -lntdll
@@ -175,6 +175,16 @@ func Version() string {
 	return C.GoString(C.mdbx_version.git.describe)
 }
 
+// BuildOptions returns the build-time configuration options used when
+// compiling libmdbx. This includes settings like MDBX_USE_FALLOCATE,
+// MDBX_DEBUG, and other compile-time flags.
+//
+// Example output: " MDBX_DEBUG=0 ... MDBX_USE_FALLOCATE=1 ..."
+func BuildOptions() string {
+	//nolint:gocritic // C variable access pattern
+	return C.GoString(C.mdbx_build.options)
+}
+
 func GetSysRamInfo() (pageSize, totalPages, availablePages int, err error) {
 	var cPageSize, cTotalPages, cAvailablePages C.intptr_t
 

mdbx/txn.go

@@ -442,6 +442,7 @@ func (txn *Txn) renew() error {
 // null bytes in the name argument.
 //
 // See mdbx_dbi_open.
+//
 // Deprecated: use OpenDBISimple instead
 func (txn *Txn) OpenDBI(name string, flags uint, cmp, dcmp CmpFunc) (DBI, error) {
 	cname := C.CString(name)
@@ -495,7 +496,8 @@ type Cmp func(k1, k2 []byte) int
 // returned in those cases.  This is not a big deal for now because
 // applications are expected to handle any error encountered opening a
 // database.
-// Deprecated: openDBISimple instead because using comparators now is a deprecated
+//
+// Deprecated: use OpenDBISimple instead because using comparators is now deprecated
 func (txn *Txn) openDBI(cname *C.char, flags uint, cmp, dcmp *C.MDBX_cmp_func) (DBI, error) {
 	var dbi C.MDBX_dbi
 	ret := C.mdbx_dbi_open_ex(txn._txn, cname, C.MDBX_db_flags_t(flags), &dbi, cmp, dcmp)