AP_Logger: added LOG_DISARMED=3

when LOG_DISARMED is set to 3 then we log while disarmed but if we
reboot without ever arming the log is discarded. This allows for using
LOG_DISARMED without filling the microSD.

libraries/AP_Logger/AP_Logger.cpp

@@ -90,8 +90,8 @@ const AP_Param::GroupInfo AP_Logger::var_info[] = {
 
     // @Param: _DISARMED
     // @DisplayName: Enable logging while disarmed
-    // @Description: If LOG_DISARMED is set to 1 then logging will be enabled at all times including when disarmed. Logging before arming can make for very large logfiles but can help a lot when tracking down startup issues and is necessary if logging of EKF replay data is selected via the LOG_REPLAY parameter. If LOG_DISARMED is set to 2, then logging will be enabled when disarmed, but not if a USB connection is detected. This can be used to prevent unwanted data logs being generated when the vehicle is connected via USB for log downloading or parameter changes.
+    // @Description: If LOG_DISARMED is set to 1 then logging will be enabled at all times including when disarmed. Logging before arming can make for very large logfiles but can help a lot when tracking down startup issues and is necessary if logging of EKF replay data is selected via the LOG_REPLAY parameter. If LOG_DISARMED is set to 2, then logging will be enabled when disarmed, but not if a USB connection is detected. This can be used to prevent unwanted data logs being generated when the vehicle is connected via USB for log downloading or parameter changes. If LOG_DISARMED is set to 3 then logging will happen while disarmed, but if the vehicle never arms then the logs using the filesystem backend will be discarded on the next boot.
-    // @Values: 0:Disabled,1:Enabled,2:Disabled on USB connection
+    // @Values: 0:Disabled,1:Enabled,2:Disabled on USB connection,3:Discard log on reboot if never armed
     // @User: Standard
     AP_GROUPINFO("_DISARMED",  2, AP_Logger, _params.log_disarmed,       0),
 
@@ -1469,6 +1469,7 @@ bool AP_Logger::log_while_disarmed(void) const
         return true;
     }
     if (_params.log_disarmed == LogDisarmed::LOG_WHILE_DISARMED ||
+        _params.log_disarmed == LogDisarmed::LOG_WHILE_DISARMED_DISCARD ||
         (_params.log_disarmed == LogDisarmed::LOG_WHILE_DISARMED_NOT_USB && !hal.gpio->usb_connected())) {
         return true;
     }

libraries/AP_Logger/AP_Logger.h

@@ -319,6 +319,7 @@ public:
         NONE = 0,
         LOG_WHILE_DISARMED = 1,
         LOG_WHILE_DISARMED_NOT_USB = 2,
+        LOG_WHILE_DISARMED_DISCARD = 3,
     };
 
     // parameter support

libraries/AP_Logger/AP_Logger_File.cpp

@@ -94,6 +94,16 @@ void AP_Logger_File::Init()
         _log_directory = custom_dir;
     }
 
+    uint16_t last_log_num = find_last_log();
+    if (last_log_is_marked_discard) {
+        // delete the last log leftover from LOG_DISARMED=3
+        char *filename = _log_file_name(last_log_num);
+        if (filename != nullptr) {
+            AP::FS().unlink(filename);
+            free(filename);
+        }
+    }
+
     Prep_MinSpace();
 }
 
@@ -518,8 +528,13 @@ uint16_t AP_Logger_File::find_last_log()
     EXPECT_DELAY_MS(3000);
     FileData *fd = AP::FS().load_file(fname);
     free(fname);
+    last_log_is_marked_discard = false;
     if (fd != nullptr) {
-        ret = strtol((const char *)fd->data, nullptr, 10);
+        char *endptr = nullptr;
+        ret = strtol((const char *)fd->data, &endptr, 10);
+        if (endptr != nullptr) {
+            last_log_is_marked_discard = *endptr == 'D';
+        }
         delete fd;
     }
     return ret;
@@ -846,6 +861,18 @@ void AP_Logger_File::start_new_log(void)
     _writebuf.clear();
     write_fd_semaphore.give();
 
+    // now update lastlog.txt with the new log number
+    last_log_is_marked_discard = _front._params.log_disarmed == AP_Logger::LogDisarmed::LOG_WHILE_DISARMED_DISCARD;
+    if (!write_lastlog_file(log_num)) {
+        _open_error_ms = AP_HAL::millis();
+    }
+}
+
+/*
+  write LASTLOG.TXT, possibly with a discard marker
+ */
+bool AP_Logger_File::write_lastlog_file(uint16_t log_num)
+{
     // now update lastlog.txt with the new log number
     char *fname = _lastlog_file_name();
 
@@ -853,25 +880,17 @@ void AP_Logger_File::start_new_log(void)
     int fd = AP::FS().open(fname, O_WRONLY|O_CREAT);
     free(fname);
     if (fd == -1) {
-        _open_error_ms = AP_HAL::millis();
+        return false;
-        return;
     }
 
     char buf[30];
-    snprintf(buf, sizeof(buf), "%u\r\n", (unsigned)log_num);
+    snprintf(buf, sizeof(buf), "%u%s\r\n", (unsigned)log_num, last_log_is_marked_discard?"D":"");
     const ssize_t to_write = strlen(buf);
     const ssize_t written = AP::FS().write(fd, buf, to_write);
     AP::FS().close(fd);
-
+    return written == to_write;
-    if (written < to_write) {
-        _open_error_ms = AP_HAL::millis();
-        return;
-    }
-
-    return;
 }
 
-
 #if CONFIG_HAL_BOARD == HAL_BOARD_SITL || CONFIG_HAL_BOARD == HAL_BOARD_LINUX
 void AP_Logger_File::flush(void)
 #if APM_BUILD_TYPE(APM_BUILD_Replay)
@@ -921,6 +940,13 @@ void AP_Logger_File::io_timer(void)
         return;
     }
 
+    if (last_log_is_marked_discard && hal.util->get_soft_armed()) {
+        // time to make the log permanent
+        const auto log_num = find_last_log();
+        last_log_is_marked_discard = false;
+        write_lastlog_file(log_num);
+    }
+
     uint32_t nbytes = _writebuf.available();
     if (nbytes == 0) {
         return;

libraries/AP_Logger/AP_Logger_File.h

@@ -70,6 +70,7 @@ protected:
 private:
     int _write_fd = -1;
     char *_write_filename;
+    bool last_log_is_marked_discard;
     uint32_t _last_write_ms;
 #if CONFIG_HAL_BOARD == HAL_BOARD_CHIBIOS
     bool _need_rtc_update;
@@ -101,6 +102,7 @@ private:
     bool log_exists(const uint16_t lognum) const;
 
     bool dirent_to_log_num(const dirent *de, uint16_t &log_num) const;
+    bool write_lastlog_file(uint16_t log_num);
 
     // write buffer
     ByteBuffer _writebuf{0};