[PATCH v3] lineedit: fix some tab completions written to stdout

[Natanael Copa]

From: Sertonix <sertonix at posteo.net>

In fd47f056765 (lineedit: print prompt and editing operations to stderr)
some output was left printing to stdout. This causes a race condition
between stderr and stdout which in some cases leads to output written in
the wrong places.

Downstream issue: https://gitlab.alpinelinux.org/alpine/aports/-/issues/16566
Fixes: fd47f056765

function                                             old     new   delta
fputs_stderr                                           -      12     +12
put_cur_glyph_and_inc_cursor                         182     178      -4
put_prompt_custom                                     52      45      -7
input_delete                                         161     154      -7
draw_custom                                           84      77      -7
BB_PUTCHAR                                           164     157      -7
read_line_input                                     3612    3579     -33
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/6 up/down: 12/-65)            Total: -53 bytes
   text    data     bss     dec     hex filename
 824213   14108    2008  840329   cd289 busybox_old
 824160   14108    2008  840276   cd254 busybox_unstripped

Signed-off-by: Sertonix <sertonix at posteo.net>
Signed-off-by: Natanael Copa <ncopa at alpinelinux.org>
---

v2 -> v3:
Introduce a fputs_stderr() func to reduce size.

Denys, It would be nice if this could be applied and backported to
1_37_stable.

Thanks!


 libbb/lineedit.c | 28 +++++++++++++++++-----------
 1 file changed, 17 insertions(+), 11 deletions(-)

diff --git a/libbb/lineedit.c b/libbb/lineedit.c
index 151208c1c..21fd6037c 100644
--- a/libbb/lineedit.c
+++ b/libbb/lineedit.c
@@ -345,6 +345,12 @@ static unsigned save_string(char *dst, unsigned maxsize)
 		return i;
 	}
 }
+
+static void fputs_stderr(const char *buf)
+{
+	fputs(buf, stderr);
+}
+
 /* I thought just fputwc(c, stderr) would work. But no... */
 static void BB_PUTCHAR(wchar_t c)
 {
@@ -354,7 +360,7 @@ static void BB_PUTCHAR(wchar_t c)
 		ssize_t len = wcrtomb(buf, c, &mbst);
 		if (len > 0) {
 			buf[len] = '\0';
-			fputs(buf, stderr);
+			fputs_stderr(buf);
 		}
 	} else {
 		/* In this case, c is always one byte */
@@ -451,7 +457,7 @@ static void put_cur_glyph_and_inc_cursor(void)
 		 * have automargin (IOW: it is moving cursor to next line
 		 * by itself (which is wrong for VT-10x terminals)),
 		 * this will break things: there will be one extra empty line */
-		puts("\r"); /* + implicit '\n' */
+		bb_putchar_stderr('\r'); /* + implicit '\n' */
 #else
 		/* VT-10x terminals don't wrap cursor to next line when last char
 		 * on the line is printed - cursor stays "over" this char.
@@ -505,7 +511,7 @@ static void put_prompt_custom(bool is_full)
 	/* https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html
 	 * says that shells must write $PSn to stderr, not stdout.
 	 */
-	fputs((is_full ? cmdedit_prompt : prompt_last_line), stderr);
+	fputs_stderr((is_full ? cmdedit_prompt : prompt_last_line));
 	cursor = 0;
 	cmdedit_y = cmdedit_prmt_len / cmdedit_termw; /* new quasireal y */
 	cmdedit_x = cmdedit_prmt_len % cmdedit_termw;
@@ -607,7 +613,7 @@ static void draw_custom(int y, int back_cursor, bool is_full)
 	bb_putchar_stderr('\r');
 	put_prompt_custom(is_full);
 	put_till_end_and_adv_cursor();
-	fputs(SEQ_CLEAR_TILL_END_OF_SCREEN, stderr);
+	fputs_stderr(SEQ_CLEAR_TILL_END_OF_SCREEN);
 	input_backward(back_cursor);
 }
 
@@ -652,7 +658,7 @@ static void input_delete(int save)
 	command_len--;
 	put_till_end_and_adv_cursor();
 	/* Last char is still visible, erase it (and more) */
-	fputs(SEQ_CLEAR_TILL_END_OF_SCREEN, stderr);
+	fputs_stderr(SEQ_CLEAR_TILL_END_OF_SCREEN);
 	input_backward(cursor - j);     /* back to old pos cursor */
 }
 
@@ -1170,9 +1176,9 @@ static void showfiles(void)
 			);
 		}
 		if (ENABLE_UNICODE_SUPPORT)
-			puts(printable_string(matches[n]));
+			fputs_stderr(printable_string(matches[n]));
 		else
-			puts(matches[n]);
+			fputs_stderr(matches[n]);
 	}
 }
 
@@ -1903,7 +1909,7 @@ static void ask_terminal(void)
 	pfd.events = POLLIN;
 	if (safe_poll(&pfd, 1, 0) == 0) {
 		S.sent_ESC_br6n = 1;
-		fputs(ESC"[6n", stderr);
+		fputs_stderr(ESC"[6n");
 		fflush_all(); /* make terminal see it ASAP! */
 	}
 }
@@ -2642,13 +2648,13 @@ int FAST_FUNC read_line_input(line_input_t *st, const char *prompt, char *comman
 			/* Control-k -- clear to end of line */
 			command_ps[cursor] = BB_NUL;
 			command_len = cursor;
-			fputs(SEQ_CLEAR_TILL_END_OF_SCREEN, stderr);
+			fputs_stderr(SEQ_CLEAR_TILL_END_OF_SCREEN);
 			break;
 		case CTRL('L'):
 		vi_case(CTRL('L')|VI_CMDMODE_BIT:)
 			/* Control-l -- clear screen */
 			/* cursor to top,left; clear to the end of screen */
-			fputs(ESC"[H" ESC"[J", stderr);
+			fputs_stderr(ESC"[H" ESC"[J");
 			draw_full(command_len - cursor);
 			break;
 #if MAX_HISTORY > 0
@@ -3033,7 +3039,7 @@ int FAST_FUNC read_line_input(const char* prompt, char* command, int maxsize)
 	/* https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html
 	 * says that shells must write $PSn to stderr, not stdout.
 	 */
-	fputs(prompt, stderr);
+	fputs_stderr(prompt);
 	fflush_all();
 	if (!fgets(command, maxsize, stdin))
 		return -1;
-- 
2.47.1