shell_builtin_read: set cc[VMIN] to 1; lineedit: don't clear c_cc[VINTR]

First change fixes "read -n NUM". Apparently poll() won't report
data availability if cc[VMIN] > 1 until there are at least cc[VMIN] bytes.

function                                             old     new   delta
read_line_input                                     3885    3877      -8
shell_builtin_read                                  1097    1087     -10
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-18)             Total: -18 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

2 files changed, 16 insertions, 7 deletions

diff --git a/libbb/lineedit.c b/libbb/lineedit.c
index 1390ed62d..460e27fed 100644
--- a/libbb/lineedit.c
+++ b/libbb/lineedit.c
@@ -2206,14 +2206,17 @@ int FAST_FUNC read_line_input(line_input_t *st, const char *prompt, char *comman
 #define command command_must_not_be_used
 
         new_settings = initial_settings;
-        new_settings.c_lflag &= ~ICANON;        /* unbuffered input */
-        /* Turn off echoing and CTRL-C, so we can trap it */
-        new_settings.c_lflag &= ~(ECHO | ECHONL | ISIG);
-        /* Hmm, in linux c_cc[] is not parsed if ICANON is off */
+        /* ~ICANON: unbuffered input (most c_cc[] are disabled, VMIN/VTIME are enabled) */
+        /* ~ECHO, ~ECHONL: turn off echoing, including newline echoing */
+        /* ~ISIG: turn off INTR (ctrl-C), QUIT, SUSP */
+        new_settings.c_lflag &= ~(ICANON | ECHO | ECHONL | ISIG);
+        /* reads would block only if < 1 char is available */
         new_settings.c_cc[VMIN] = 1;
+        /* no timeout (reads block forever) */
         new_settings.c_cc[VTIME] = 0;
-        /* Turn off CTRL-C, so we can trap it */
-        new_settings.c_cc[VINTR] = _POSIX_VDISABLE;
+        /* Should be not needed if ISIG is off: */
+        /* Turn off CTRL-C */
+        /* new_settings.c_cc[VINTR] = _POSIX_VDISABLE; */
         tcsetattr_stdin_TCSANOW(&new_settings);
 
 #if ENABLE_USERNAME_OR_HOMEDIR
diff --git a/shell/shell_common.c b/shell/shell_common.c
index bbc22ed34..51c92d60e 100644
--- a/shell/shell_common.c
+++ b/shell/shell_common.c
@@ -138,7 +138,13 @@ shell_builtin_read(void FAST_FUNC (*setvar)(const char *name, const char *val),
                 old_tty = tty;
                 if (nchars) {
                         tty.c_lflag &= ~ICANON;
-                        tty.c_cc[VMIN] = nchars < 256 ? nchars : 255;
+                        // Setting it to more than 1 breaks poll():
+                        // it blocks even if there's data. !??
+                        //tty.c_cc[VMIN] = nchars < 256 ? nchars : 255;
+                        /* reads would block only if < 1 char is available */
+                        tty.c_cc[VMIN] = 1;
+                        /* no timeout (reads block forever) */
+                        tty.c_cc[VTIME] = 0;
                 }
                 if (read_flags & BUILTIN_READ_SILENT) {
                         tty.c_lflag &= ~(ECHO | ECHOK | ECHONL);