awk: fix FS assignment behavior. Closes 5108

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

editors/awk.c

@@ -1812,6 +1812,18 @@ static void handle_special(var *v)
 		is_f0_split = FALSE;
 
 	} else if (v == intvar[FS]) {
+		/*
+		 * The POSIX-2008 standard says that changing FS should have no effect on the
+		 * current input line, but only on the next one. The language is:
+		 *
+		 * > Before the first reference to a field in the record is evaluated, the record
+		 * > shall be split into fields, according to the rules in Regular Expressions,
+		 * > using the value of FS that was current at the time the record was read.
+		 *
+		 * So, split up current line before assignment to FS:
+		 */
+		split_f0();
+
 		mk_splitter(getvar_s(v), &fsplitter);
 
 	} else if (v == intvar[RS]) {

testsuite/awk.tests

@@ -217,4 +217,11 @@ end d
 testing "awk handles empty ()" \
 	"awk 'BEGIN {print()}' 2>&1" "awk: cmd. line:1: Empty sequence\n" "" ""
 
+testing "awk FS assignment" "awk '{FS=\":\"; print \$1}'" \
+	"a:b\ne\n" \
+	"" \
+	"a:b c:d\ne:f g:h"
+
+# testing "description" "command" "result" "infile" "stdin"
+
 exit $FAILCOUNT