libbb: make history saving/loading concurrent-safe

* all history writers always append (not overwrite) history files
* they reload history if they detect that file length has changed since last
write
* they trim history file only when it grows 4 times longer than MAXLINES
* they do this atomically by creating new file and renaming it to old

Unfortunately, this comes at a price:

function                                             old     new   delta
load_history                                           -     346    +346
read_line_input                                     3155    3358    +203
new_line_input_t                                      17      31     +14
...irrelevant small jitter...
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 5/5 up/down: 573/-13)           Total: 560 bytes

1 files changed, 7 insertions, 3 deletions

diff --git a/include/libbb.h b/include/libbb.h
index a2042fe5c..7bf9469cb 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -1187,9 +1187,9 @@ unsigned long long bb_makedev(unsigned int major, unsigned int minor) FAST_FUNC;
 #if ENABLE_FEATURE_EDITING
 /* It's NOT just ENABLEd or disabled. It's a number: */
 #ifdef CONFIG_FEATURE_EDITING_HISTORY
-#define MAX_HISTORY (CONFIG_FEATURE_EDITING_HISTORY + 0)
+# define MAX_HISTORY (CONFIG_FEATURE_EDITING_HISTORY + 0)
 #else
-#define MAX_HISTORY 0
+# define MAX_HISTORY 0
 #endif
 typedef struct line_input_t {
         int flags;
@@ -1197,7 +1197,11 @@ typedef struct line_input_t {
 #if MAX_HISTORY
         int cnt_history;
         int cur_history;
-        USE_FEATURE_EDITING_SAVEHISTORY(const char *hist_file;)
+#if ENABLE_FEATURE_EDITING_SAVEHISTORY
+        unsigned cnt_history_in_file;
+        off_t last_history_end;
+        const char *hist_file;
+#endif
         char *history[MAX_HISTORY + 1];
 #endif
 } line_input_t;