esp32/mphalport: Print debug strings even before the GIL is ready.

If verbose debugging is enabled there is some stdout output happening
before the GIL is ready (for example, GC initialisation), and the code
assumed that no string printing occurred before the interpreter was fully
initialised.  Printing long strings would operate without holding the GIL,
which would crash if string output would happen too early in the startup
process.

This commit addresses that issue, making sure verbose debugging output will
work even before the interpreter is fully initialised (as if it is not yet
ready there's no GIL to take care of).

Also, the threshold that would indicate whether a string is "long" (and
thus requiring a GIL release/lock operation) or not was hardcoded to 20
bytes.  This commit makes that configurable, maintaining 20 bytes as a
default.

Fixes issue #15408.
Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

ports/esp32/mpconfigport.h

@@ -314,3 +314,8 @@ void boardctrl_startup(void);
 #endif
 #endif
 #endif
+
+// The minimum string length threshold for string printing to stdout operations to be GIL-aware.
+#ifndef MICROPY_PY_STRING_TX_GIL_THRESHOLD
+#define MICROPY_PY_STRING_TX_GIL_THRESHOLD  (20)
+#endif

ports/esp32/mphalport.c

@@ -47,6 +47,10 @@
 #include "usb_serial_jtag.h"
 #include "uart.h"
 
+#if MICROPY_PY_STRING_TX_GIL_THRESHOLD < 0
+#error "MICROPY_PY_STRING_TX_GIL_THRESHOLD must be positive"
+#endif
+
 TaskHandle_t mp_main_task_handle;
 
 static uint8_t stdin_ringbuf_array[260];
@@ -129,7 +133,13 @@ mp_uint_t mp_hal_stdout_tx_strn(const char *str, size_t len) {
     // Only release the GIL if many characters are being sent
     mp_uint_t ret = len;
     bool did_write = false;
-    bool release_gil = len > 20;
+    bool release_gil = len > MICROPY_PY_STRING_TX_GIL_THRESHOLD;
+    #if MICROPY_DEBUG_PRINTERS && MICROPY_DEBUG_VERBOSE && MICROPY_PY_THREAD_GIL
+    // If verbose debug output is enabled some strings are printed before the
+    // GIL mutex is set up.  When that happens, no Python code is running and
+    // therefore the interpreter doesn't care about the GIL not being ready.
+    release_gil = release_gil && (MP_STATE_VM(gil_mutex).handle != NULL);
+    #endif
     if (release_gil) {
         MP_THREAD_GIL_EXIT();
     }