efiboot: prevent overwriting variables with exactly the same value

include/efiboot.h

@@ -3593,6 +3593,16 @@
     auto [value, attributes] = variable;
     Raw_data bytes;
     size_t size = serialize(bytes, value);
+    // Skip overwriting with exactly the same value
+    if(const auto current = EFIBoot::get_variable<Raw_data>(guid, name); current)
+    {
+        const auto &[current_bytes, current_attributes] = *current;
+        if(current_attributes == attributes && current_bytes == bytes)
+            return true;
+    }
+
+    // Don't care about the error from get_variable
+    efi_error_clear();
     return efi_set_variable(guid, name.c_str(), bytes.data(), size, attributes, mode) == 0;
 }
 
@@ -3602,6 +3612,16 @@
     auto [value, attributes] = variable;
     Raw_data bytes;
     size_t size = serialize_list(bytes, value);
+    // Skip overwriting with exactly the same value
+    if(const auto current = EFIBoot::get_variable<Raw_data>(guid, name); current)
+    {
+        const auto &[current_bytes, current_attributes] = *current;
+        if(current_attributes == attributes && current_bytes == bytes)
+            return true;
+    }
+
+    // Don't care about the error from get_variable
+    efi_error_clear();
     return efi_set_variable(guid, name.c_str(), bytes.data(), size, attributes, mode) == 0;
 }
 

include/efivar-lite/efivar-lite.h

@@ -80,3 +80,4 @@ void efi_set_get_next_variable_name_progress_cb(void (*progress_cb)(size_t, size
 int efi_guid_cmp(const efi_guid_t *a, const efi_guid_t *b);
 
 int efi_error_get(unsigned int n, TCHAR **const filename, TCHAR **const function, int *line, TCHAR **const message, int *error) ATTR_NONNULL(2, 3, 4, 5, 6);
+void efi_error_clear(void);

src/efivar-lite.darwin.c

@@ -176,3 +176,8 @@ int efi_error_get(unsigned int n, char **const filename, char **const function,
     *message = mach_error_string(err);
     return 1;
 }
+
+void efi_error_clear(void)
+{
+    // Nothing to do
+}

src/efivar-lite.win32.c

@@ -215,3 +215,8 @@ int efi_error_get(unsigned int n, TCHAR **const filename, TCHAR **const function
     *message = error_buffer;
     return 1;
 }
+
+void efi_error_clear(void)
+{
+    // Nothing to do
+}