main: redesign boot_out.txt writing

New design:
 * capture output to a vstr
 * compare the complete vstr to boot_out.txt
 * rewrite if not a complete match

This is resilient against future changes to the automatic
text written to boot_out.txt.

This also fixes rewriting boot_out.txt in the case where
boot.py prints something.

Perhaps it also saves a bit of code space. Some tricks:
 * no need to close a file in read mode
 * no need to switch on/off USB write access, going down to the
   oofatfs layer doesn't check it anyway

main.c

@@ -635,7 +635,7 @@ STATIC bool run_code_py(safe_mode_t safe_mode) {
     return skip_repl;
 }
 
-FIL* boot_output_file;
+vstr_t *boot_output;
 
 STATIC void __attribute__ ((noinline)) run_boot_py(safe_mode_t safe_mode) {
     // If not in safe mode, run boot before initing USB and capture output in a file.
@@ -645,66 +645,11 @@ STATIC void __attribute__ ((noinline)) run_boot_py(safe_mode_t safe_mode) {
         && safe_mode == NO_SAFE_MODE
         && MP_STATE_VM(vfs_mount_table) != NULL;
 
+    if (!ok_to_run) {
+        return;
+    }
+
     static const char * const boot_py_filenames[] = STRING_LIST("boot.py", "boot.txt");
-    bool skip_boot_output = false;
-    #ifdef CIRCUITPY_BOOT_OUTPUT_FILE
-    FIL file_pointer;
-    #endif
-
-    if (ok_to_run) {
-
-        #ifdef CIRCUITPY_BOOT_OUTPUT_FILE
-        boot_output_file = &file_pointer;
-
-        // Get the base filesystem.
-        FATFS *fs = &((fs_user_mount_t *) MP_STATE_VM(vfs_mount_table)->obj)->fatfs;
-
-        bool have_boot_py = first_existing_file_in_list(boot_py_filenames) != NULL;
-
-        // If there's no boot.py file that might write some changing output,
-        // read the existing copy of CIRCUITPY_BOOT_OUTPUT_FILE and see if its contents
-        // match the version info we would print anyway. If so, skip writing CIRCUITPY_BOOT_OUTPUT_FILE.
-        // This saves wear and tear on the flash and also prevents filesystem damage if power is lost
-        // during the write, which may happen due to bobbling the power connector or weak power.
-
-        static const size_t NUM_CHARS_TO_COMPARE = 160;
-        if (!have_boot_py && f_open(fs, boot_output_file, CIRCUITPY_BOOT_OUTPUT_FILE, FA_READ) == FR_OK) {
-
-            char file_contents[NUM_CHARS_TO_COMPARE];
-            UINT chars_read = 0;
-            f_read(boot_output_file, file_contents, NUM_CHARS_TO_COMPARE, &chars_read);
-            f_close(boot_output_file);
-            skip_boot_output =
-                // + 2 accounts for  \r\n.
-                chars_read == strlen(MICROPY_FULL_VERSION_INFO) + 2 &&
-                strncmp(file_contents, MICROPY_FULL_VERSION_INFO, strlen(MICROPY_FULL_VERSION_INFO)) == 0;
-        }
-
-        if (!skip_boot_output) {
-            // Wait 1.5 seconds before opening CIRCUITPY_BOOT_OUTPUT_FILE for write,
-            // in case power is momentary or will fail shortly due to, say a low, battery.
-            if (common_hal_mcu_processor_get_reset_reason() == RESET_REASON_POWER_ON) {
-                mp_hal_delay_ms(1500);
-            }
-            // USB isn't up, so we can write the file.
-            filesystem_set_internal_writable_by_usb(false);
-            f_open(fs, boot_output_file, CIRCUITPY_BOOT_OUTPUT_FILE, FA_WRITE | FA_CREATE_ALWAYS);
-
-            // Switch the filesystem back to non-writable by Python now instead of later,
-            // since boot.py might change it back to writable.
-            filesystem_set_internal_writable_by_usb(true);
-
-            // Write version info to boot_out.txt.
-            mp_hal_stdout_tx_str(MICROPY_FULL_VERSION_INFO);
-            // Write the board ID (board directory and ID on circuitpython.org)
-            mp_hal_stdout_tx_str("\r\n" "Board ID:");
-            mp_hal_stdout_tx_str(CIRCUITPY_BOARD_ID);
-            mp_hal_stdout_tx_str("\r\n");
-        }
-        #endif
-
-        filesystem_flush();
-    }
 
     // Do USB setup even if boot.py is not run.
 
@@ -716,19 +661,54 @@ STATIC void __attribute__ ((noinline)) run_boot_py(safe_mode_t safe_mode) {
     usb_set_defaults();
 #endif
 
+    #ifdef CIRCUITPY_BOOT_OUTPUT_FILE
+    vstr_t boot_text;
+    vstr_init(&boot_text, 512);
+    boot_output = &boot_text;
+    #endif
+
+    // Write version info
+    mp_printf(&mp_plat_print, "%s\nBoard ID:%s\n", MICROPY_FULL_VERSION_INFO, CIRCUITPY_BOARD_ID);
+
     pyexec_result_t result = {0, MP_OBJ_NULL, 0};
-    if (ok_to_run) {
+
     bool found_boot = maybe_run_list(boot_py_filenames, &result);
     (void) found_boot;
 
+
     #ifdef CIRCUITPY_BOOT_OUTPUT_FILE
-        if (!skip_boot_output) {
-            f_close(boot_output_file);
+    // Get the base filesystem.
+    fs_user_mount_t *vfs = (fs_user_mount_t *) MP_STATE_VM(vfs_mount_table)->obj;
+    FATFS *fs = &vfs->fatfs;
+
+    boot_output = NULL;
+    bool write_boot_output = (common_hal_mcu_processor_get_reset_reason() == RESET_REASON_POWER_ON);
+    FIL boot_output_file;
+    if (f_open(fs, &boot_output_file, CIRCUITPY_BOOT_OUTPUT_FILE, FA_READ) == FR_OK) {
+        char *file_contents = m_new(char, boot_text.alloc);
+        UINT chars_read;
+        if (f_read(&boot_output_file, file_contents, 1+boot_text.len, &chars_read) == FR_OK) {
+            write_boot_output =
+                (chars_read != boot_text.len) || (memcmp(boot_text.buf, file_contents, chars_read) != 0);
+        }
+        // no need to f_close the file
+    }
+
+    if (write_boot_output) {
+        // Wait 1 second before opening CIRCUITPY_BOOT_OUTPUT_FILE for write,
+        // in case power is momentary or will fail shortly due to, say a low, battery.
+        mp_hal_delay_ms(1000);
+
+        // USB isn't up, so we can write the file.
+        // operating at the oofatfs (f_open) layer means the usb concurrent write permission
+        // is not even checked!
+        f_open(fs, &boot_output_file, CIRCUITPY_BOOT_OUTPUT_FILE, FA_WRITE | FA_CREATE_ALWAYS);
+        UINT chars_written;
+        f_write(&boot_output_file, boot_text.buf, boot_text.len, &chars_written);
+        f_close(&boot_output_file);
         filesystem_flush();
     }
-        boot_output_file = NULL;
     #endif
-    }
 
 #if CIRCUITPY_USB
     // Some data needs to be carried over from the USB settings in boot.py

py/makeversionhdr.py

@@ -107,7 +107,7 @@ def make_version_header(filename):
 #define MICROPY_VERSION_MINOR (%s)
 #define MICROPY_VERSION_MICRO (%s)
 #define MICROPY_VERSION_STRING "%s"
-#define MICROPY_FULL_VERSION_INFO ("Adafruit CircuitPython " MICROPY_GIT_TAG " on " MICROPY_BUILD_DATE "; " MICROPY_HW_BOARD_NAME " with " MICROPY_HW_MCU_NAME)
+#define MICROPY_FULL_VERSION_INFO "Adafruit CircuitPython " MICROPY_GIT_TAG " on " MICROPY_BUILD_DATE "; " MICROPY_HW_BOARD_NAME " with " MICROPY_HW_MCU_NAME
 """ % (
         git_tag,
         git_hash,

supervisor/serial.h

@@ -34,9 +34,9 @@
 #include "py/mpconfig.h"
 
 #ifdef CIRCUITPY_BOOT_OUTPUT_FILE
-#include "lib/oofatfs/ff.h"
+#include "py/misc.h"
 
-extern FIL *boot_output_file;
+extern vstr_t *boot_output;
 #endif
 
 void serial_early_init(void);

supervisor/shared/micropython.c

@@ -59,9 +59,8 @@ void mp_hal_stdout_tx_strn(const char *str, size_t len) {
     toggle_tx_led();
 
     #ifdef CIRCUITPY_BOOT_OUTPUT_FILE
-    if (boot_output_file != NULL) {
-        UINT bytes_written = 0;
-        f_write(boot_output_file, str, len, &bytes_written);
+    if (boot_output != NULL) {
+        vstr_add_strn(boot_output, str, len);
     }
     #endif