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f51819afa

Denys Vlasenko

2026-02-27

SUCCESS Duration: 145s

Subject

less: do not retry reads of stdin if got EAGAIN

Binary

No binary (build failed)

Architecture & Compiler Flags

Architecture: x86_64-linux-gnu Compiler: gcc (Ubuntu 11.4.0-1ubuntu1~22.04.3) 11.4.0

gcc -Wp,-MD,applets/.usage.d -Wall -Wstrict-prototypes -O2 -fomit-frame-pointer      -Iinclude -Iinclude -o applets/usage applets/usage.c

Build Log

(log yok)

Commit Diff GitHub →

miscutils/less.c +35 -35

miscutils/less.c+35 -35

@@ -201,9 +201,10 @@ struct globals {
	#if ENABLE_FEATURE_LESS_WINCH
	unsigned winch_counter;
	#endif
-	ssize_t eof_error; /* eof if 0, error if < 0 */
-	ssize_t readpos;
-	ssize_t readeof; /* must be signed */
+	int stdin_GETFL_flags;
+	ssize_t eof_error; /* eof if 0, error if < 0, > 0 if last read got some chars */
+	size_t readpos;
+	size_t read_size;
	const char **buffer;
	const char **flines;
	const char *empty_line_marker;
@@ -253,7 +254,7 @@ struct globals {
	#define WINCH_COUNTER ((volatile unsigned )&winch_counter)
	#define eof_error (G.eof_error )
	#define readpos (G.readpos )
-	#define readeof (G.readeof )
+	#define read_size (G.read_size )
	#define buffer (G.buffer )
	#define flines (G.flines )
	#define empty_line_marker (G.empty_line_marker )
@@ -458,7 +459,7 @@ static int at_end(void)
	* that it was seen])
	* max_lineno - last line's number, this one doesn't increment
	* on line wrap, only on "real" new lines.
-	* readbuf[0..readeof-1] - small preliminary buffer.
+	* readbuf[0..read_size-1] - small preliminary buffer.
	* readbuf[readpos] - next character to add to current line.
	* last_line_pos - screen line position of next char to be read
	* (takes into account tabs and backspaces)
@@ -472,8 +473,6 @@ static void read_lines(void)
	char current_line, p;
	int w = width;
	char last_terminated = terminated;
-	time_t last_time = 0;
-	int retry_EAGAIN = 2;
	#if ENABLE_FEATURE_LESS_REGEXP
	unsigned old_max_fline = max_fline;
	#endif
@@ -505,30 +504,23 @@ static void read_lines(void)
	while (1) { /* read chars until we have a line */
	char c;
	/* if no unprocessed chars left, eat more */
-	if (readpos >= readeof) {
-	int flags = ndelay_on(0);
+	if (readpos >= read_size) {
+	// Read stdin, temporarily making it nonblocking (if it's not already).
+	// NB: we do NOT check eof_error status before reading.
+	// This has the effect that e.g. PageDown on a regular file
+	// where we already reached EOF will try reading anyway,
+	// if the file is a growing log file, less will show the new data.
+	int flags = G.stdin_GETFL_flags;
+	if (!(flags & O_NONBLOCK))
+	fcntl(STDIN_FILENO, F_SETFL, flags\|O_NONBLOCK);
+	eof_error = safe_read(STDIN_FILENO, readbuf, COMMON_BUFSIZE);
+	if (!(flags & O_NONBLOCK))
+	fcntl(STDIN_FILENO, F_SETFL, flags);

-	while (1) {
-	time_t t;
-
-	errno = 0;
-	eof_error = safe_read(STDIN_FILENO, readbuf, COMMON_BUFSIZE);
-	if (errno != EAGAIN)
-	break;
-	t = time(NULL);
-	if (t != last_time) {
-	last_time = t;
-	if (--retry_EAGAIN < 0)
-	break;
-	}
-	sched_yield();
-	}
-	fcntl(0, F_SETFL, flags); /* ndelay_off(0) */
	readpos = 0;
-	readeof = eof_error;
+	read_size = (eof_error < 0 ? 0 : eof_error);
	if (eof_error <= 0)
	goto reached_eof;
-	retry_EAGAIN = 1;
	}
	c = readbuf[readpos];
	/* backspace? [needed for manpages] */
@@ -632,7 +624,7 @@ static void read_lines(void)

	if (eof_error < 0) {
	if (errno == EAGAIN) {
-	eof_error = 1;
+	eof_error = 1; /* "neither EOF nor error" */
	} else {
	print_statusline(bb_msg_read_error);
	}
@@ -914,7 +906,7 @@ static void buffer_print(void)
	if ((option_mask32 & (FLAG_E\|FLAG_F))
	&& eof_error <= 0
	) {
-	i = option_mask32 & FLAG_F ? 0 : cur_fline;
+	i = (option_mask32 & FLAG_F) ? 0 : cur_fline;
	if (max_fline - i <= max_displayed_line)
	less_exit();
	}
@@ -1063,8 +1055,9 @@ static void open_file_and_read_lines(void)
	num_lines = REOPEN_STDIN;
	#endif
	}
+	G.stdin_GETFL_flags = fcntl(STDIN_FILENO, F_GETFL);
	readpos = 0;
-	readeof = 0;
+	read_size = 0;
	last_line_pos = 0;
	terminated = 1;
	read_lines();
@@ -1093,9 +1086,13 @@ static void reinitialize(void)
	buffer_fill_and_print();
	}

+	/* Poll stdin and keyboard.
+	* If stdin has more data, redraw and repeat.
+	* Return keycode when a key is pressed.
+	*/
	static int64_t getch_nowait(void)
	{
-	int rd;
+	int dont_poll_stdin;
	int64_t key64;
	struct pollfd pfd[2];

@@ -1111,11 +1108,11 @@ static int64_t getch_nowait(void)
	* Even if select/poll says that input is available, read CAN block
	* (switch fd into O_NONBLOCK'ed mode to avoid it)
	*/
-	rd = 1;
+	dont_poll_stdin = 1;
	/* Are we interested in stdin? */
	if (at_end()) {
	if (eof_error > 0) /* did NOT reach eof yet */
-	rd = 0; /* yes, we are interested in stdin */
+	dont_poll_stdin = 0; /* yes, we are interested in stdin */
	}
	/* Position cursor if line input is done */
	if (less_gets_pos >= 0)
@@ -1127,21 +1124,24 @@ static int64_t getch_nowait(void)
	while (1) {
	int r;
	/* NB: SIGWINCH interrupts poll() */
-	r = poll(pfd + rd, 2 - rd, -1);
+	r = poll(pfd + dont_poll_stdin, 2 - dont_poll_stdin, -1);
	if (/r < 0 && errno == EINTR &&/ winch_counter)
	return '\\'; /* anything which has no defined function */
	if (r) break;
	}
	#else
-	safe_poll(pfd + rd, 2 - rd, -1);
+	safe_poll(pfd + dont_poll_stdin, 2 - dont_poll_stdin, -1);
	#endif
	}

+	if (pfd[1].revents == 0)
+	goto no_kbd_input;
	/* We have kbd_fd in O_NONBLOCK mode, read inside safe_read_key()
	* would not block even if there is no input available */
	key64 = safe_read_key(kbd_fd, kbd_input, /do not poll:/ -2);
	if ((int)key64 == -1) {
	if (errno == EAGAIN) {
+	no_kbd_input:
	/* No keyboard input available. Since poll() did return,
	* we should have input on stdin */
	read_lines();