Brightness (Linux): fixes random built-in flag and stdout leak in DDC/CI detection (#2616)

DDC/CI results never set `builtin`, and FF_LIST_ADD does not zero the new
element, so external monitors were reported as built-in or external at
random. Set it to false, as the BSD DDC/CI backend does.

FF_SUPPRESS_IO() was declared inside the `if (ffddca_init)` block, so its
cleanup restored stdout before display detection ran and libddcutil trace
lines could reach the output, ahead of `--format json` too. Declare it at
function scope so it covers the whole detection, including the
FF_DISABLE_DLOPEN build.

Closes #2615

CHANGELOG.md

@@ -58,6 +58,7 @@ Features:
 
 Bugfixes:
 * Fixed Snapdragon X2 series model detection on Linux. (#2611, CPU / Linux)
+* Fixed external monitors detected via DDC/CI being randomly reported as built-in, and libddcutil trace messages occasionally leaking into the output and breaking `--format json`. (#2615, Brightness / Linux)
 * Fixed image logo caching, padding, and positioning issues, including an iTerm display bug. (Logo)
 * Fixed several Windows issues, including Windows Terminal font detection, redirected `--gen-config` output, and gamepad battery reporting for DualShock 4 and Switch controllers. (#2573, TerminalFont; Gamepad; General)
 * Fixed formatting and display issues, including ANSI-aware truncation, date/time formatting, and bright key/title colors. (Format / DateTime / Display)

src/detection/brightness/brightness_linux.c

@@ -98,10 +98,12 @@ static const char* detectWithDdcci([[maybe_unused]] FFBrightnessOptions* options
     FF_LIBRARY_LOAD_SYMBOL_MESSAGE(libddcutil, ddca_free_any_vcp_value)
     FF_LIBRARY_LOAD_SYMBOL_MESSAGE(libddcutil, ddca_close_display)
 
+    // libddcutil may print trace messages to stdout during display detection, not only in ddca_init
+    FF_SUPPRESS_IO();
+
     #ifndef FF_DISABLE_DLOPEN
     FF_LIBRARY_LOAD_SYMBOL_LAZY(libddcutil, ddca_init)
     if (ffddca_init) {
-        FF_SUPPRESS_IO();
         // Ref: https://github.com/rockowitz/ddcutil/issues/344
         if (ffddca_init(nullptr, -1 /*DDCA_SYSLOG_NOT_SET*/, 1 /*DDCA_INIT_OPTIONS_DISABLE_CONFIG_FILE*/) < 0) {
             return "ddca_init() failed";
@@ -148,6 +150,7 @@ static const char* detectWithDdcci([[maybe_unused]] FFBrightnessOptions* options
                 brightness->max = max;
                 brightness->min = 0;
                 brightness->current = current;
+                brightness->builtin = false;
                 ffStrbufInitS(&brightness->name, display->model_name);
             }
             ffddca_close_display(handle);