New -n option for read builtin

Usage: read -n nchars
Reads maximum of nchars characters. If nchars <= 0, there's no limit.

builtin.cpp

@@ -2313,6 +2313,8 @@ static int builtin_read(parser_t &parser, wchar_t **argv)
     const wchar_t *commandline = L"";
     int exit_res=STATUS_BUILTIN_OK;
     const wchar_t *mode_name = READ_MODE_NAME;
+    int nchars=0;
+    wchar_t *end;
     int shell = 0;
     int array = 0;
 
@@ -2355,6 +2357,10 @@ static int builtin_read(parser_t &parser, wchar_t **argv)
                 L"mode-name", required_argument, 0, 'm'
             }
             ,
+            {
+                L"nchars", required_argument, 0, 'n'
+            }
+            ,
             {
                 L"shell", no_argument, 0, 's'
             }
@@ -2377,7 +2383,7 @@ static int builtin_read(parser_t &parser, wchar_t **argv)
 
         int opt = wgetopt_long(argc,
                                argv,
-                               L"xglUup:c:hm:sa",
+                               L"xglUup:c:hm:n:sa",
                                long_options,
                                &opt_index);
         if (opt == -1)
@@ -2428,6 +2434,32 @@ static int builtin_read(parser_t &parser, wchar_t **argv)
                 mode_name = woptarg;
                 break;
 
+            case L'n':
+                errno = 0;
+                nchars = fish_wcstoi(woptarg, &end, 10);
+                if (errno || *end != 0)
+                {
+                    switch (errno)
+                    {
+                        case ERANGE:
+                            append_format(stderr_buffer,
+                                    _(L"%ls: Argument '%ls' is out of range\n"),
+                                    argv[0],
+                                    woptarg);
+                            builtin_print_help(parser, argv[0], stderr_buffer);
+                            return STATUS_BUILTIN_ERROR;
+
+                        default:
+                            append_format(stderr_buffer,
+                                    _(L"%ls: Argument '%ls' must be an integer\n"),
+                                    argv[0],
+                                    woptarg);
+                            builtin_print_help(parser, argv[0], stderr_buffer);
+                            return STATUS_BUILTIN_ERROR;
+                    }
+                }
+                break;
+
             case 's':
                 shell = 1;
                 break;
@@ -2530,11 +2562,24 @@ static int builtin_read(parser_t &parser, wchar_t **argv)
         proc_push_interactive(1);
 
         event_fire_generic(L"fish_prompt");
-        line = reader_readline();
+        line = reader_readline(nchars);
         proc_pop_interactive();
         if (line)
         {
-            buff = wcsdup(line);
+            if (0 < nchars && nchars < wcslen(line))
+            {
+                // line may be longer than nchars if a keybinding used `commandline -i`
+                // note: we're deliberately throwing away the tail of the commandline.
+                // It shouldn't be unread because it was produced with `commandline -i`,
+                // not typed.
+                buff = (wchar_t *)malloc(((size_t)nchars + 1) * sizeof(wchar_t));
+                wmemcpy(buff, line, (size_t)nchars);
+                buff[nchars] = 0;
+            }
+            else
+            {
+                buff = wcsdup(line);
+            }
         }
         else
         {
@@ -2594,6 +2639,11 @@ static int builtin_read(parser_t &parser, wchar_t **argv)
                 break;
 
             sb.push_back(res);
+
+            if (0 < nchars && (size_t)nchars <= sb.size())
+            {
+                break;
+            }
         }
 
         if (sb.size() < 2 && eof)

doc_src/read.txt

@@ -14,6 +14,7 @@ The following options are available:
 - <tt>-g</tt> or <tt>--global</tt> makes the variables global.
 - <tt>-l</tt> or <tt>--local</tt> makes the variables local.
 - <tt>-m NAME</tt> or <tt>--mode-name=NAME</tt> specifies that the name NAME should be used to save/load the history file. If NAME is fish, the regular fish history will be available.
+- <tt>-n NCHARS</tt> or <tt>--nchars=NCHARS</tt> causes \c read to return after reading NCHARS characters rather than waiting for a complete line of input.
 - <tt>-p PROMPT_CMD</tt> or <tt>--prompt=PROMPT_CMD</tt> uses the output of the shell command \c PROMPT_CMD as the prompt for the interactive mode. The default prompt command is <tt>set_color green; echo read; set_color normal; echo "> "</tt>.
 - <code>-s</code> or <code>--shell</code> enables syntax highlighting, tab completions and command termination suitable for entering shellscript code in the interactive mode.
 - <code>-u</code> or <code>--unexport</code> prevents the variables from being exported to child processes (default behaviour).

reader.cpp

@@ -2955,7 +2955,7 @@ static int read_i(void)
           during evaluation.
         */
 
-        const wchar_t *tmp = reader_readline();
+        const wchar_t *tmp = reader_readline(0);
 
         if (data->end_loop)
         {
@@ -3044,7 +3044,7 @@ static wchar_t unescaped_quote(const wcstring &str, size_t pos)
 }
 
 
-const wchar_t *reader_readline(void)
+const wchar_t *reader_readline(int nchars)
 {
     wint_t c;
     int last_char=0;
@@ -3084,6 +3084,13 @@ const wchar_t *reader_readline(void)
 
     while (!finished && !data->end_loop)
     {
+        if (0 < nchars && (size_t)nchars <= data->command_line.size())
+        {
+            // we've already hit the specified character limit
+            finished = 1;
+            break;
+        }
+
         /*
          Sometimes strange input sequences seem to generate a zero
          byte. I believe these simply mean a character was pressed
@@ -3104,12 +3111,14 @@ const wchar_t *reader_readline(void)
                 {
 
                     wchar_t arr[READAHEAD_MAX+1];
-                    int i;
+                    size_t i;
+                    size_t limit = 0 < nchars ? std::min((size_t)nchars - data->command_line.size(), (size_t)READAHEAD_MAX)
+                                              : READAHEAD_MAX;
 
                     memset(arr, 0, sizeof(arr));
                     arr[0] = c;
 
-                    for (i=1; i<READAHEAD_MAX; i++)
+                    for (i = 1; i < limit; ++i)
                     {
 
                         if (!can_read(0))
@@ -3137,6 +3146,12 @@ const wchar_t *reader_readline(void)
 
             if (c != 0)
                 break;
+
+            if (0 < nchars && (size_t)nchars <= data->command_line.size())
+            {
+                c = R_NULL;
+                break;
+            }
         }
 
         /* If we get something other than a repaint, then stop coalescing them */

reader.h

@@ -200,11 +200,13 @@ int reader_reading_interrupted();
 bool reader_thread_job_is_stale();
 
 /**
-   Read one line of input. Before calling this function, reader_push()
-   must have been called in order to set up a valid reader
-   environment.
+   Read one line of input. Before calling this function, reader_push() must have
+   been called in order to set up a valid reader environment. If nchars > 0,
+   return after reading that many characters even if a full line has not yet
+   been read. Note: the returned value may be longer than nchars if a single
+   keypress resulted in multiple characters being inserted into the commandline.
 */
-const wchar_t *reader_readline();
+const wchar_t *reader_readline(int nchars);
 
 /**
    Push a new reader environment.

tests/read.in

@@ -75,3 +75,19 @@ print_vars ary
 echo '' | read -la ary
 print_vars ary
 set -le IFS
+
+# read -n tests
+
+echo
+echo '# read -n tests'
+echo 'testing' | read -n 3 foo
+echo $foo
+echo 'test' | read -n 10 foo
+echo $foo
+echo 'test' | read -n 0 foo
+echo $foo
+echo 'testing' | begin; read -n 3 foo; read -n 3 bar; end
+echo $foo
+echo $bar
+echo 'test' | read -n 1 foo
+echo $foo

tests/read.out

@@ -34,3 +34,11 @@ two
 5 'h' 'e' 'l' 'l' 'o'
 1 'h'
 0
+
+# read -n tests
+tes
+test
+test
+tes
+tin
+t