[PATCH v4] kconfig: avoid an infinite loop in oldconfig/syncconfig

[Yoann Congal <yoann.congal@xxxxxxxx>]

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

Signed-off-by: Yoann Congal <yoann.congal@xxxxxxxx>
Tested-by: Brandon Maier <brandon.maier@xxxxxxxxxxx>
---
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 | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/scripts/kconfig/conf.c b/scripts/kconfig/conf.c
index 33d19e419908..68f0c649a805 100644
--- a/scripts/kconfig/conf.c
+++ b/scripts/kconfig/conf.c
@@ -74,13 +74,17 @@ static void strip(char *str)
 }
 
 /* Helper function to facilitate fgets() by Jean Sacren. */
-static void xfgets(char *str, int size, FILE *in)
+static int xfgets(char *str, int size, FILE *in)
 {
+	int ret = 0;
+
 	if (!fgets(str, size, in))
-		fprintf(stderr, "\nError in reading or end of file.\n");
+		ret = -1;
 
 	if (!tty_stdio)
 		printf("%s", str);
+
+	return ret;
 }
 
 static void set_randconfig_seed(void)
@@ -339,7 +343,10 @@ static int conf_askvalue(struct symbol *sym, const char *def)
 		/* fall through */
 	default:
 		fflush(stdout);
-		xfgets(line, sizeof(line), stdin);
+		if (xfgets(line, sizeof(line), stdin) != 0) {
+			fprintf(stderr, "Error while reading value of symbol \"%s\"\n", sym->name);
+			exit(1);
+		}
 		break;
 	}
 
@@ -521,7 +528,11 @@ static int conf_choice(struct menu *menu)
 			/* fall through */
 		case oldaskconfig:
 			fflush(stdout);
-			xfgets(line, sizeof(line), stdin);
+			if (xfgets(line, sizeof(line), stdin) != 0) {
+				fprintf(stderr, "Error while reading value of symbol \"%s\"\n",
+						sym->name);
+				exit(1);
+			}
 			strip(line);
 			if (line[0] == '?') {
 				print_help(menu);
-- 
2.30.2