[PATCH v5] kconfig: avoid an infinite loop in oldconfig/syncconfig

[Yoann Congal <yoann.congal@smile.fr>]

Exit on error when asking for value and reading stdin returns an error
(mainly if it has reached EOF or is closed).

This infinite loop happens in particular for hex/int configs without an
explicit default value.

Previously, this case would loop:
* oldconfig prompts for the value but stdin has reached EOF
* It gets the global default value : an empty string
* This is not a valid hex/int value so it prompts again, hence the
  infinite loop.

This case happens with a configuration like this (a hex config without a
valid default value):
  config TEST_KCONFIG
       hex "Test KConfig"
       # default 0x0

And using:
  make oldconfig < /dev/null

This was discovered when working on Yocto bug[0] on a downstream
kconfig user (U-boot)

[0]: https://bugzilla.yoctoproject.org/show_bug.cgi?id=14136

CC: Brandon Maier <brandon.maier@collins.com>
Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
---
v4->v5:
 * Switched to Masahiro Yamada's suggested code.
v3->v4:
 * Added Brandon Maier's "Tested-by". Thanks!
v2->v3:
 * Simplify the patch by fusing comments of :
   * Masahiro Yamada : Exit as soon as reading stdin hits an error
   * Randy Dunlap : Display the name of the currently read symbol
v1->v2:
 * Improve coding style
 * Put more info in the commit message
---
 scripts/kconfig/conf.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/scripts/kconfig/conf.c b/scripts/kconfig/conf.c
index 33d19e419908..62de1fbaff97 100644
--- a/scripts/kconfig/conf.c
+++ b/scripts/kconfig/conf.c
@@ -76,8 +76,10 @@ static void strip(char *str)
 /* Helper function to facilitate fgets() by Jean Sacren. */
 static void xfgets(char *str, int size, FILE *in)
 {
-	if (!fgets(str, size, in))
+	if (!fgets(str, size, in)) {
 		fprintf(stderr, "\nError in reading or end of file.\n");
+		exit(1);
+	}
 
 	if (!tty_stdio)
 		printf("%s", str);
-- 
2.30.2