AP_Logger: prevent potential infinite recursion in log-open codepath

If anything in start_new_log did logging (for example, by sending a
statustext), we end up infinitely recursing.

With the patch:

diff --git a/libraries/AP_Logger/AP_Logger_File.cpp b/libraries/AP_Logger/AP_Logger_File.cpp
index 69b8ef0431..eb422d10f8 100644
--- a/libraries/AP_Logger/AP_Logger_File.cpp
+++ b/libraries/AP_Logger/AP_Logger_File.cpp
@@ -778,6 +778,7 @@ void AP_Logger_File::PrepForArming()
  */
 void AP_Logger_File::start_new_log(void)
 {
+    gcs().send_text(MAV_SEVERITY_WARNING, "Starting new log");
     stop_logging();

     start_new_log_reset_variables();
pbarker@bluebottle:~/rc/ardupilot(master)$

We see:

    at ../../libraries/AP_Logger/AP_Logger_File.cpp:781
    this=0x555555ad9d30, pBuffer=0x7fffff8209d0, size=75, is_critical=true)
    at ../../libraries/AP_Logger/AP_Logger_Backend.cpp:372
    this=0x555555ad9d30, pBuffer=0x7fffff8209d0, size=75)
    at ../../libraries/AP_Logger/AP_Logger_Backend.h:32
    this=0x555555ad9d30, message=0x7fffff820b10 "Starting new log")
    at ../../libraries/AP_Logger/LogFile.cpp:466
    this=0x555555a6d758 <copter+11384>,
    message=0x7fffff820b10 "Starting new log")
    at ../../libraries/AP_Logger/AP_Logger.cpp:752
    this=0x555555a6e708 <copter+15400>, severity=MAV_SEVERITY_WARNING,
    fmt=0x5555557d64d0 "Starting new log", arg_list=0x7fffff820be0,
    dest_bitmask=1 '\001') at ../../libraries/GCS_MAVLink/GCS_Common.cpp:1847
    this=0x555555a6e708 <copter+15400>, severity=MAV_SEVERITY_WARNING,
    fmt=0x5555557d64d0 "Starting new log", arg_list=0x7fffff820be0)
    at ../../libraries/GCS_MAVLink/GCS.cpp:53
    this=0x555555a6e708 <copter+15400>, severity=MAV_SEVERITY_WARNING,
    fmt=0x5555557d64d0 "Starting new log")
    at ../../libraries/GCS_MAVLink/GCS.cpp:60
    at ../../libraries/AP_Logger/AP_Logger_File.cpp:781
    this=0x555555ad9d30, pBuffer=0x7fffff820dc0, size=75, is_critical=true)
    at ../../libraries/AP_Logger/AP_Logger_Backend.cpp:372

I'm not aware of any instances in the code where this will actually
happen - but it could easily sneak in.

libraries/AP_Logger/AP_Logger_File.cpp

@@ -778,16 +778,24 @@ void AP_Logger_File::PrepForArming()
  */
 void AP_Logger_File::start_new_log(void)
 {
-    stop_logging();
-
-    start_new_log_reset_variables();
-
     if (_open_error) {
         // we have previously failed to open a file - don't try again
         // to prevent us trying to open files while in flight
         return;
     }
 
+    // set _open_error here to avoid infinite recursion.  Simply
+    // writing a prioritised block may try to open a log - which means
+    // if anything in the start_new_log path does a gcs().send_text()
+    // (for example), you will end up recursing if we don't take
+    // precautions.  We will reset _open_error if we actually manage
+    // to open the log...
+    _open_error = true;
+
+    stop_logging();
+
+    start_new_log_reset_variables();
+
     if (_read_fd != -1) {
         AP::FS().close(_read_fd);
         _read_fd = -1;
@@ -795,7 +803,6 @@ void AP_Logger_File::start_new_log(void)
 
     if (disk_space_avail() < _free_space_min_avail && disk_space() > 0) {
         hal.console->printf("Out of space for logging\n");
-        _open_error = true;
         return;
     }
 
@@ -808,7 +815,6 @@ void AP_Logger_File::start_new_log(void)
         log_num = 1;
     }
     if (!write_fd_semaphore.take(1)) {
-        _open_error = true;
         return;
     }
     if (_write_filename) {
@@ -817,7 +823,6 @@ void AP_Logger_File::start_new_log(void)
     }
     _write_filename = _log_file_name(log_num);
     if (_write_filename == nullptr) {
-        _open_error = true;
         write_fd_semaphore.give();
         return;
     }
@@ -834,7 +839,6 @@ void AP_Logger_File::start_new_log(void)
 
     if (_write_fd == -1) {
         _initialised = false;
-        _open_error = true;
         write_fd_semaphore.give();
         int saved_errno = errno;
         ::printf("Log open fail for %s - %s\n",
@@ -851,13 +855,10 @@ void AP_Logger_File::start_new_log(void)
     // now update lastlog.txt with the new log number
     char *fname = _lastlog_file_name();
 
-    // we avoid fopen()/fprintf() here as it is not available on as many
-    // systems as open/write
     EXPECT_DELAY_MS(3000);
     int fd = AP::FS().open(fname, O_WRONLY|O_CREAT);
     free(fname);
     if (fd == -1) {
-        _open_error = true;
         return;
     }
 
@@ -868,9 +869,9 @@ void AP_Logger_File::start_new_log(void)
     AP::FS().close(fd);
 
     if (written < to_write) {
-        _open_error = true;
         return;
     }
+    _open_error = false;
 
     return;
 }