libbb: revent previous version of "concurrent history updating"

and replace it with one which does not "snoop" history written
by others. (1) it is what bug 185 needs, and (2) it is less bloaty:

function                                             old     new   delta
load_history                                           -     252    +252
read_line_input                                     3155    3287    +132
next_token                                           914     918      +4
qrealloc                                              36      33      -3
getoptscmd                                           713     708      -5
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 2/2 up/down: 388/-8)            Total: 380 bytes

2 files changed, 56 insertions, 60 deletions

diff --git a/include/libbb.h b/include/libbb.h
index 7bf9469cb..015374b4a 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -1199,7 +1199,6 @@ typedef struct line_input_t {
 	int cur_history;
 #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];
@@ -1215,6 +1214,7 @@ enum {
 	FOR_SHELL = DO_HISTORY | SAVE_HISTORY | TAB_COMPLETION | USERNAME_COMPLETION,
 };
 line_input_t *new_line_input_t(int flags) FAST_FUNC;
+/* so far static: void free_line_input_t(line_input_t *n) FAST_FUNC; */
 /* Returns:
  * -1 on read errors or EOF, or on bare Ctrl-D,
  * 0  on ctrl-C (the line entered is still returned in 'command'),
diff --git a/libbb/lineedit.c b/libbb/lineedit.c
index af1b62764..7bcdb954c 100644
--- a/libbb/lineedit.c
+++ b/libbb/lineedit.c
@@ -954,6 +954,14 @@ static void input_tab(smallint *lastWasTab)
 #endif  /* FEATURE_COMMAND_TAB_COMPLETION */
 
 
+line_input_t* FAST_FUNC new_line_input_t(int flags)
+{
+	line_input_t *n = xzalloc(sizeof(*n));
+	n->flags = flags;
+	return n;
+}
+
+
 #if MAX_HISTORY > 0
 
 static void save_command_ps_at_cur_history(void)
@@ -991,16 +999,23 @@ 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.
+ * do not overwrite each other.
  * Otherwise shell users get unhappy.
  *
  * History file is trimmed lazily, when it grows several times longer
  * than configured MAX_HISTORY lines.
  */
 
+static void free_line_input_t(line_input_t *n)
+{
+	int i = n->cnt_history;
+	while (i > 0)
+		free(n->history[--i]);
+	free(n);
+}
+
 /* state->flags is already checked to be nonzero */
-static void load_history(void)
+static void load_history(line_input_t *st_parm)
 {
 	char *temp_h[MAX_HISTORY];
 	char *line;
@@ -1009,18 +1024,18 @@ static void load_history(void)
 
 	/* NB: do not trash old history if file can't be opened */
 
-	fp = fopen_for_read(state->hist_file);
+	fp = fopen_for_read(st_parm->hist_file);
 	if (fp) {
 		/* clean up old history */
-		for (idx = state->cnt_history; idx > 0;) {
+		for (idx = st_parm->cnt_history; idx > 0;) {
 			idx--;
-			free(state->history[idx]);
-			state->history[idx] = NULL;
+			free(st_parm->history[idx]);
+			st_parm->history[idx] = NULL;
 		}
 
 		/* fill temp_h[], retaining only last MAX_HISTORY lines */
 		memset(temp_h, 0, sizeof(temp_h));
-		state->cnt_history_in_file = idx = 0;
+		st_parm->cnt_history_in_file = idx = 0;
 		while ((line = xmalloc_fgetline(fp)) != NULL) {
 			if (line[0] == '\0') {
 				free(line);
@@ -1028,16 +1043,15 @@ static void load_history(void)
 			}
 			free(temp_h[idx]);
 			temp_h[idx] = line;
-			state->cnt_history_in_file++;
+			st_parm->cnt_history_in_file++;
 			idx++;
 			if (idx == MAX_HISTORY)
 				idx = 0;
 		}
-		state->last_history_end = lseek(fileno(fp), 0, SEEK_CUR);
 		fclose(fp);
 
 		/* find first non-NULL temp_h[], if any */
-		if (state->cnt_history_in_file) {
+		if (st_parm->cnt_history_in_file) {
 			while (temp_h[idx] == NULL) {
 				idx++;
 				if (idx == MAX_HISTORY)
@@ -1045,7 +1059,7 @@ static void load_history(void)
 			}
 		}
 
-		/* copy temp_h[] to state->history[] */
+		/* copy temp_h[] to st_parm->history[] */
 		for (i = 0; i < MAX_HISTORY;) {
 			line = temp_h[idx];
 			if (!line)
@@ -1056,71 +1070,60 @@ static void load_history(void)
 			line_len = strlen(line);
 			if (line_len >= MAX_LINELEN)
 				line[MAX_LINELEN-1] = '\0';
-			state->history[i++] = line;
+			st_parm->history[i++] = line;
 		}
-		state->cnt_history = i;
+		st_parm->cnt_history = i;
 	}
 }
 
 /* state->flags is already checked to be nonzero */
 static void save_history(char *str)
 {
-	off_t end;
 	int fd;
-	int len;
+	int len, len2;
 
-	len = strlen(str);
- again:
 	fd = open(state->hist_file, O_WRONLY | O_CREAT | O_APPEND, 0666);
 	if (fd < 0)
 		return;
-
-	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++;
-	}
+	xlseek(fd, 0, SEEK_END); /* paranoia */
+	len = strlen(str);
+	str[len] = '\n'; /* we (try to) do atomic write */
+	len2 = full_write(fd, str, len + 1);
+	str[len] = '\0';
 	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;
-		}
-	}
-	state->last_history_end = end + len + 1;
+	if (len2 != len + 1)
+		return; /* "wtf?" */
 
 	/* did we write so much that history file needs trimming? */
+	state->cnt_history_in_file++;
 	if (state->cnt_history_in_file > MAX_HISTORY * 4) {
 		FILE *fp;
 		char *new_name;
+		line_input_t *st_temp;
+		int i;
+
+		/* we may have concurrently written entries from others.
+		 * load them */
+		st_temp = new_line_input_t(state->flags);
+		st_temp->hist_file = state->hist_file;
+		load_history(st_temp);
 
-		new_name = xasprintf("%s.new", state->hist_file);
+		/* write out temp file and replace hist_file atomically */
+		new_name = xasprintf("%s.%u.new", state->hist_file, (int) getpid());
 		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);
+			for (i = 0; i < st_temp->cnt_history; i++)
+				fprintf(fp, "%s\n", st_temp->history[i]);
 			fclose(fp);
-			/* replace hist_file atomically */
-			rename(new_name, state->hist_file);
+			if (rename(new_name, state->hist_file) == 0)
+				state->cnt_history_in_file = st_temp->cnt_history;
 		}
 		free(new_name);
+		free_line_input_t(st_temp);
 	}
 }
 #else
-#define load_history()  ((void)0)
+#define load_history(a) ((void)0)
 #define save_history(a) ((void)0)
 #endif /* FEATURE_COMMAND_SAVEHISTORY */
 
@@ -1490,7 +1493,8 @@ 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();
+		if (state->cnt_history == 0)
+			load_history(state);
 #endif
 	if (state->flags & DO_HISTORY)
 		state->cur_history = state->cnt_history;
@@ -1948,14 +1952,6 @@ int FAST_FUNC read_line_input(const char *prompt, char *command, int maxsize, li
 	return len; /* can't return command_len, DEINIT_S() destroys it */
 }
 
-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;
-}
-
 #else
 
 #undef read_line_input