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, 110 insertions, 32 deletions

diff --git a/libbb/lineedit.c b/libbb/lineedit.c
index 3953cc904..af1b62764 100644
--- a/libbb/lineedit.c
+++ b/libbb/lineedit.c
@@ -990,64 +990,141 @@ static int get_next_history(void)
 }
 
 #if ENABLE_FEATURE_EDITING_SAVEHISTORY
+/* We try to ensure that concurrent additions to the history
+ * do not overwrite each other, and that additions to the history
+ * by one user are noticed by others.
+ * Otherwise shell users get unhappy.
+ *
+ * History file is trimmed lazily, when it grows several times longer
+ * than configured MAX_HISTORY lines.
+ */
+
 /* state->flags is already checked to be nonzero */
-static void load_history(const char *fromfile)
+static void load_history(void)
 {
+        char *temp_h[MAX_HISTORY];
+        char *line;
         FILE *fp;
-        int hi;
+        unsigned idx, i, line_len;
 
         /* NB: do not trash old history if file can't be opened */
 
-        fp = fopen_for_read(fromfile);
+        fp = fopen_for_read(state->hist_file);
         if (fp) {
                 /* clean up old history */
-                for (hi = state->cnt_history; hi > 0;) {
-                        hi--;
-                        free(state->history[hi]);
-                        state->history[hi] = NULL;
+                for (idx = state->cnt_history; idx > 0;) {
+                        idx--;
+                        free(state->history[idx]);
+                        state->history[idx] = NULL;
                 }
 
-                for (hi = 0; hi < MAX_HISTORY;) {
-                        char *hl = xmalloc_fgetline(fp);
-                        int l;
-
-                        if (!hl)
-                                break;
-                        l = strlen(hl);
-                        if (l >= MAX_LINELEN)
-                                hl[MAX_LINELEN-1] = '\0';
-                        if (l == 0) {
-                                free(hl);
+                /* fill temp_h[], retaining only last MAX_HISTORY lines */
+                memset(temp_h, 0, sizeof(temp_h));
+                state->cnt_history_in_file = idx = 0;
+                while ((line = xmalloc_fgetline(fp)) != NULL) {
+                        if (line[0] == '\0') {
+                                free(line);
                                 continue;
                         }
-                        state->history[hi++] = hl;
+                        free(temp_h[idx]);
+                        temp_h[idx] = line;
+                        state->cnt_history_in_file++;
+                        idx++;
+                        if (idx == MAX_HISTORY)
+                                idx = 0;
                 }
+                state->last_history_end = lseek(fileno(fp), 0, SEEK_CUR);
                 fclose(fp);
-                state->cnt_history = hi;
+
+                /* find first non-NULL temp_h[], if any */
+                if (state->cnt_history_in_file) {
+                        while (temp_h[idx] == NULL) {
+                                idx++;
+                                if (idx == MAX_HISTORY)
+                                        idx = 0;
+                        }
+                }
+
+                /* copy temp_h[] to state->history[] */
+                for (i = 0; i < MAX_HISTORY;) {
+                        line = temp_h[idx];
+                        if (!line)
+                                break;
+                        idx++;
+                        if (idx == MAX_HISTORY)
+                                idx = 0;
+                        line_len = strlen(line);
+                        if (line_len >= MAX_LINELEN)
+                                line[MAX_LINELEN-1] = '\0';
+                        state->history[i++] = line;
+                }
+                state->cnt_history = i;
         }
 }
 
 /* state->flags is already checked to be nonzero */
-static void save_history(const char *tofile)
+static void save_history(char *str)
 {
-        FILE *fp;
+        off_t end;
+        int fd;
+        int len;
 
-        fp = fopen_for_write(tofile);
-        if (fp) {
-                int i;
+        len = strlen(str);
+ again:
+        fd = open(state->hist_file, O_WRONLY | O_CREAT | O_APPEND, 0666);
+        if (fd < 0)
+                return;
 
-                for (i = 0; i < state->cnt_history; i++) {
-                        fprintf(fp, "%s\n", state->history[i]);
+        end = lseek(fd, 0, SEEK_END);
+
+        if (str) {
+                str[len] = '\n';
+                full_write(fd, str, len + 1);
+                str[len] = '\0';
+                str = NULL;
+                state->cnt_history_in_file++;
+        }
+        close(fd);
+
+        /* if it was not a 1st write */
+        if (state->last_history_end >= 0) {
+                /* did someone else write anything there? */
+                if (state->last_history_end != end) {
+                        load_history(); /* note: updates cnt_history_in_file */
+                        state->last_history_end = -1;
+                        goto again;
                 }
-                fclose(fp);
+        }
+        state->last_history_end = end + len + 1;
+
+        /* did we write so much that history file needs trimming? */
+        if (state->cnt_history_in_file > MAX_HISTORY * 4) {
+                FILE *fp;
+                char *new_name;
+
+                new_name = xasprintf("%s.new", state->hist_file);
+                fp = fopen_for_write(new_name);
+                if (fp) {
+                        int i;
+
+                        for (i = 0; i < state->cnt_history; i++) {
+                                fprintf(fp, "%s\n", state->history[i]);
+                        }
+                        state->cnt_history_in_file = i; /* == cnt_history */
+                        state->last_history_end = lseek(fileno(fp), 0, SEEK_CUR);
+                        fclose(fp);
+                        /* replace hist_file atomically */
+                        rename(new_name, state->hist_file);
+                }
+                free(new_name);
         }
 }
 #else
-#define load_history(a) ((void)0)
+#define load_history()  ((void)0)
 #define save_history(a) ((void)0)
 #endif /* FEATURE_COMMAND_SAVEHISTORY */
 
-static void remember_in_history(const char *str)
+static void remember_in_history(char *str)
 {
         int i;
 
@@ -1078,7 +1155,7 @@ static void remember_in_history(const char *str)
         state->cnt_history = i;
 #if ENABLE_FEATURE_EDITING_SAVEHISTORY
         if ((state->flags & SAVE_HISTORY) && state->hist_file)
-                save_history(state->hist_file);
+                save_history(str);
 #endif
         USE_FEATURE_EDITING_FANCY_PROMPT(num_ok_lines++;)
 }
@@ -1413,7 +1490,7 @@ int FAST_FUNC read_line_input(const char *prompt, char *command, int maxsize, li
         state = st ? st : (line_input_t*) &const_int_0;
 #if ENABLE_FEATURE_EDITING_SAVEHISTORY
         if ((state->flags & SAVE_HISTORY) && state->hist_file)
-                load_history(state->hist_file);
+                load_history();
 #endif
         if (state->flags & DO_HISTORY)
                 state->cur_history = state->cnt_history;
@@ -1875,6 +1952,7 @@ line_input_t* FAST_FUNC new_line_input_t(int flags)
 {
         line_input_t *n = xzalloc(sizeof(*n));
         n->flags = flags;
+        n->last_history_end = -1;
         return n;
 }