libmisc/yesno.c: Use getline(3) and rpmatch(3)

getline(3) is much more readable than manually looping.  It has some
overhead due to the allocation of a buffer, but that shouldn't be a
problem here.  If that was a problem, we could reuse the buffer (thus
making the function non-reentrant), but I don't think that's worth the
extra complexity.

Using rpmatch(3) instead of a simple y/n test provides i18n to the
response checking.  We have a fall-back minimalistic implementation for
systems that lack this function (e.g., musl libc).

While we're at it, apply some other minor improvements to this file:

-  Remove comment saying which files use this function.  That's likely
   to get outdated.  And anyway, it's just a grep(1) away, so it doesn't
   really add any value.

-  Remove unnecessary casts to (void) that were used to verbosely ignore
   errors from stdio calls.  They add clutter without really adding much
   value to the code (or I don't see it).

-  Remove comments from the function body.  They make the function less
   readable.  Instead, centralize the description of the function into a
   man-page-like comment before the function definition.  This keeps the
   function body short and sweet.

-  Add '#include <stdbool.h>', which was missing.

-  Minor whitespace style changes (it doesn't hurt the diff at this
   point, since most of the affected lines were already touched by other
   changes, so I applied my preferred style :).

Acked-by: Samanta Navarro <ferivoz@riseup.net>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

configure.ac

@@ -50,6 +50,7 @@ AC_CHECK_FUNCS(arc4random_buf futimes \
 	initgroups lckpwdf lutimes mempcpy \
 	setgroups updwtmp updwtmpx innetgr \
 	getspnam_r \
+	rpmatch \
 	memset_explicit explicit_bzero stpecpy stpeprintf)
 AC_SYS_LARGEFILE
 

libmisc/yesno.c

@@ -1,59 +1,84 @@
 /*
  * SPDX-FileCopyrightText: 1992 - 1994, Julianne Frances Haugh
  * SPDX-FileCopyrightText: 2007 - 2008, Nicolas François
+ * SPDX-FileCopyrightText: 2023, Alejandro Colomar <alx@kernel.org>
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
-/*
- * Common code for yes/no prompting
- *
- * Used by pwck.c and grpck.c
- */
 
 #include <config.h>
 
 #ident "$Id$"
 
+#include <stdbool.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include "prototypes.h"
 
-/*
- * yes_or_no - get answer to question from the user
- *
- *	It returns false if no.
- *
- *	If the read_only flag is set, it will print No, and will return
- *	false.
- */
-bool yes_or_no (bool read_only)
-{
-	int c;
-	bool result;
 
-	/*
-	 * In read-only mode all questions are answered "no".
-	 */
+/*
+ * Synopsis
+ *	bool yes_or_no(bool read_only);
+ *
+ * Arguments
+ *	read_only
+ *		In read-only mode, all questions are answered "no".  It
+ *		will print "No" to stdout.
+ *
+ * Description
+ *	After a yes/no question, this function gets the answer from the
+ *	user.
+ *
+ *	Calls to this function will normally be preceeded by a prompt on
+ *	stdout, so we should fflush(3).
+ *
+ * Return value
+ *	false	"no"
+ *	true	"yes"
+ *
+ * See also
+ *	rpmatch(3)
+ */
+
+
+#if !defined(HAVE_RPMATCH)
+static int rpmatch(const char *response);
+#endif
+
+
+bool
+yes_or_no(bool read_only)
+{
+	bool  ret;
+	char  *buf;
+
 	if (read_only) {
-		(void) puts (_("No"));
+		puts(_("No"));
 		return false;
 	}
 
-	/*
-	 * Typically, there's a prompt on stdout, sometimes unflushed.
-	 */
-	(void) fflush (stdout);
+	fflush(stdout);
 
-	/*
-	 * Get a line and see what the first character is.
-	 */
-	c = fgetc(stdin);
-	/* TODO: use gettext */
-	result = (c == 'y' || c == 'Y');
+	ret = false;
+	if (getline(&buf, NULL, stdin) != NULL)
+		ret = rpmatch(buf) == 1;
 
-	while (c != '\n' && c != EOF)
-		c = fgetc(stdin);
-
-	return result;
+	free(buf);
+	return ret;
 }
 
+
+#if !defined(HAVE_RPMATCH)
+static int
+rpmatch(const char *response)
+{
+	if (response[0] == 'y' || response[0] == 'Y')
+		return 1;
+
+	if (response[0] == 'n' || response[0] == 'n')
+		return 0;
+
+	return -1;
+}
+#endif