Remove support for the "command" encoder

The "command" encoder was mostly intended for use in `%rename` - most
uses can be achieved using the "regex" encoder, so we recommend using
that instead.

The "command" encoder suffers from a number of issues - as the
documentation for it admitted, "[it] is extremely slow compared to all
the other [encoders] as it involves spawning a separate process and
using it for many declarations is not recommended" and that it "should
generally be avoided because of performance considerations".

But it's also not portable.  The design assumes that `/bin/sh` supports
`<<<` but that's a bash-specific feature so it doesn't work on platforms
where `/bin/sh` is not bash - it fails on Debian, Ubuntu and probably
some other Linux distros, plus most non-Linux platforms.  Microsoft
Windows doesn't even have a /bin/sh as standard.

Finally, no escaping of the passed string is done, so it has potential
security issues (though at least with %rename the input is limited to
valid C/C++ symbol names).

Fixes #1806

CHANGES.current

@@ -7,6 +7,29 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.1.0 (in progress)
 ===========================
 
+2022-02-07: olly
+	    #1806 Remove support for the "command" encoder, which was mostly
+	    intended for use in `%rename` - most uses can be achieved using
+	    the "regex" encoder, so we recommend using that instead.
+
+	    The "command" encoder suffers from a number of issues - as the
+	    documentation for it admitted, "[it] is extremely slow compared to
+	    all the other [encoders] as it involves spawning a separate process
+	    and using it for many declarations is not recommended" and that it
+	    "should generally be avoided because of performance
+	    considerations".
+
+	    But it's also not portable.  The design assumes that `/bin/sh`
+	    supports `<<<` but that's a bash-specific feature so it doesn't
+	    work on platforms where `/bin/sh` is not bash - it fails on
+	    Debian, Ubuntu and probably some other Linux distros, plus most
+	    non-Linux platforms.  Microsoft Windows doesn't even have a
+	    /bin/sh as standard.
+
+	    Finally, no escaping of the passed string is done, so it has
+	    potential security issues (though at least with %rename the input
+	    is limited to valid C/C++ symbol names).
+
 2022-02-06: olly
 	    #2193 -DFOO on the SWIG command line now sets FOO to 1 for
 	    consistency with C/C++ compiler preprocessors.  Previously

Doc/Manual/SWIG.html

@@ -2064,23 +2064,10 @@ and a more descriptive one, but the two functions are otherwise equivalent:
     <tt>%rename("regex:/(\\w+)_(.*)/\\u\\2/")</tt></td>
     <td><tt>prefix_print</tt></td><td><tt>Print</tt></td>
 </tr>
-<tr>
-    <td><tt>command:cmd</tt></td>
-    <td>Output of an external command <tt>cmd</tt> with the string passed to
-    it as input. Notice that this function is extremely slow compared to all
-    the other ones as it involves spawning a separate process and using it for
-    many declarations is not recommended. The <i>cmd</i> is not enclosed in
-    square brackets but must be terminated with a triple <tt>'&lt;'</tt> sign,
-    e.g. <tt>%rename("command:tr&nbsp;-d&nbsp;aeiou &lt;&lt;&lt;")</tt>
-    (nonsensical example removing all vowels)</td>
-    <td><tt>Print</tt></td><td><tt>Prnt</tt></td>
-</tr>
 </table>
 
 <p>
-The most general function of all of the above ones (not counting
-<tt>command</tt> which is even more powerful in principle but which should
-generally be avoided because of performance considerations) is the
+The most general function of all of the above ones is the
 <tt>regex</tt> one. Here are some more examples of its use:
 </p>
 

Examples/test-suite/errors/swig_command_encoder.i

@@ -0,0 +1,6 @@
+%module xxx
+
+// This feature was removed in SWIG 4.1.0 so check it gives an error.
+%define SedCmd "%(command:sed -e 's/\([a-z]\)/\U\\1/' -e 's/\(_\)\([a-z]\)/\U\\2/g' <<<)s" %enddef
+%rename(SedCmd) "";
+int x;

Examples/test-suite/errors/swig_command_encoder.stderr

@@ -0,0 +1 @@
+SWIG:1: Error: Command encoder no longer supported - use regex encoder instead.

Examples/test-suite/rename_camel.i

@@ -20,18 +20,14 @@
 }
 
 
-%define SedCmd "%(command:sed -e 's/\([a-z]\)/\U\\1/' -e 's/\(_\)\([a-z]\)/\U\\2/g' <<<)s" %enddef
-
 %rename(CamelCase1) camel_case_1;
-%rename(SedCmd)     camel_case_2;
+%rename("%(camelcase)s") camel_case_2;
 %rename("%(ctitle)s") camel_case_3;
 
 
 %rename("%(utitle)s") CamelCase_5;
 
-%define awk_cmd "%(command:awk '/^i/{print toupper($1)}' <<<)s" %enddef
-
-%rename(awk_cmd) "";
+%rename("%(regex:/\\(.*i.*\\)/\\U\\1/)s") "";
 
 %rename("%(title)s",regexmatch$parentNode$type="enum .*") "";
 

Source/Swig/misc.c

@@ -1157,47 +1157,14 @@ int Swig_scopename_check(const String *s) {
 /* -----------------------------------------------------------------------------
  * Swig_string_command()
  *
- * Executes a external command via popen with the string as a command
- * line parameter. For example:
- *
- *  Printf(stderr,"%(command:sed 's/[a-z]/\U\\1/' <<<)s","hello") -> Hello
+ * Feature removed in SWIG 4.1.0.
  * ----------------------------------------------------------------------------- */
-#if defined(_MSC_VER)
-#  define popen _popen
-#  define pclose _pclose
-#  if !defined(HAVE_POPEN)
-#    define HAVE_POPEN 1
-#  endif
-#else
-#  if !defined(_WIN32)
-/* These Posix functions are not ISO C and so are not always defined in stdio.h */
-extern FILE *popen(const char *command, const char *type);
-extern int pclose(FILE *stream);
-#  endif
-#endif
 
 String *Swig_string_command(String *s) {
-  String *res = NewStringEmpty();
-#if defined(HAVE_POPEN)
-  if (Len(s)) {
-    char *command = Char(s);
-    FILE *fp = popen(command, "r");
-    if (fp) {
-      char buffer[1025];
-      while (fscanf(fp, "%1024s", buffer) != EOF) {
-	Append(res, buffer);
-      }
-      pclose(fp);
-    } else {
-      Swig_error("SWIG", Getline(s), "Command encoder fails attempting '%s'.\n", s);
-      SWIG_exit(EXIT_FAILURE);
-    }
-  }
-#endif
-  return res;
+  Swig_error("SWIG", Getline(s), "Command encoder no longer supported - use regex encoder instead.\n");
+  SWIG_exit(EXIT_FAILURE);
 }
 
-
 /* -----------------------------------------------------------------------------
  * Swig_string_strip()
  *