fpi-log: Add atomic and unique check for FP_DEBUG_TRANSFER

Rather than check it all the times the variable, just rely on an atomic
check that is just done once for all

doc/libfprint-2-sections.txt

@@ -353,6 +353,7 @@ fp_dbg_hex_dump_data
 fp_info
 fp_warn
 fp_err
+fpi_log_is_debug_transfer_enabled
 BUG_ON
 BUG
 </SECTION>

libfprint/fpi-log.c

@@ -21,6 +21,27 @@
 
 #include "fpi-log.h"
 
+/**
+ * fpi_log_is_debug_transfer_enabled:
+ *
+ * Checks if the %FP_DEBUG_TRANSFER environment variable is set.
+ *
+ * Returns: %TRUE if %FP_DEBUG_TRANSFER is set, %FALSE otherwise
+ */
+gboolean
+fpi_log_is_debug_transfer_enabled (void)
+{
+  static gsize debug_transfer_enabled = 0;
+
+  if (g_once_init_enter (&debug_transfer_enabled))
+    {
+      gsize enabled = g_getenv ("FP_DEBUG_TRANSFER") != NULL ? TRUE : G_MAXSIZE;
+      g_once_init_leave (&debug_transfer_enabled, enabled);
+    }
+
+  return debug_transfer_enabled == TRUE;
+}
+
 /**
  * fpi_dbg_hex_dump_data:
  * @buf: Bytes buffer to dump

libfprint/fpi-log.h

@@ -124,3 +124,5 @@ void (fpi_dbg_hex_dump_bytes) (const gchar *log_domain,
 #define fp_dbg_hex_dump_bytes(bytes) \
   (fpi_dbg_hex_dump_bytes) \
   (G_LOG_DOMAIN, (bytes))
+
+gboolean fpi_log_is_debug_transfer_enabled (void);

libfprint/fpi-spi-transfer.c

@@ -52,7 +52,7 @@ G_DEFINE_BOXED_TYPE (FpiSpiTransfer, fpi_spi_transfer, fpi_spi_transfer_ref, fpi
 static void
 log_transfer (FpiSpiTransfer *transfer, gboolean submit, GError *error)
 {
-  if (g_getenv ("FP_DEBUG_TRANSFER"))
+  if (fpi_log_is_debug_transfer_enabled ())
     {
       if (submit)
         {

libfprint/fpi-usb-transfer.c

@@ -18,6 +18,7 @@
  */
 
 #include "fpi-usb-transfer.h"
+#include "fpi-log.h"
 
 /**
  * SECTION:fpi-usb-transfer
@@ -38,7 +39,7 @@ G_DEFINE_BOXED_TYPE (FpiUsbTransfer, fpi_usb_transfer, fpi_usb_transfer_ref, fpi
 static void
 log_transfer (FpiUsbTransfer *transfer, gboolean submit, GError *error)
 {
-  if (g_getenv ("FP_DEBUG_TRANSFER"))
+  if (fpi_log_is_debug_transfer_enabled ())
     {
       if (!submit)
         {