sed: make N command behave as in GNU sed

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

editors/sed.c

@@ -992,6 +992,8 @@ static void process_files(void)
 		}
 
 		/* actual sedding */
+		//bb_error_msg("pattern_space:'%s' next_line:'%s' cmd:%c",
+		//pattern_space, next_line, sed_cmd->cmd);
 		switch (sed_cmd->cmd) {
 
 		/* Print line number */
@@ -1118,10 +1120,16 @@ static void process_files(void)
 		{
 			int len;
 			/* If no next line, jump to end of script and exit. */
+			/* http://www.gnu.org/software/sed/manual/sed.html:
+			 * "Most versions of sed exit without printing anything
+			 * when the N command is issued on the last line of
+			 * a file. GNU sed prints pattern space before exiting
+			 * unless of course the -n command switch has been
+			 * specified. This choice is by design."
+			 */
 			if (next_line == NULL) {
-				free(next_line);
-				next_line = NULL;
-				goto discard_line;
+				//goto discard_line;
+				goto discard_commands; /* GNU behavior */
 			}
 			/* Append next_line, read new next_line. */
 			len = strlen(pattern_space);

testsuite/sed.tests

@@ -80,10 +80,18 @@ test x"$SKIP_KNOWN_BUGS" = x"" && {
 # Query: how does this interact with no newline at EOF?
 testing "sed n (flushes pattern space, terminates early)" "sed -e 'n;p'" \
 	"a\nb\nb\nc\n" "" "a\nb\nc\n"
-# N does _not_ flush pattern space, therefore c is still in there @ script end.
-testing "sed N (doesn't flush pattern space when terminating)" "sed -e 'N;p'" \
-	"a\nb\na\nb\nc\n" "" "a\nb\nc\n"
 }
+# non-GNU sed: N does _not_ flush pattern space, therefore c is eaten @ script end
+# GNU sed: N flushes pattern space, therefore c is printed too @ script end
+testing "sed N (flushes pattern space (GNU behavior))" "sed -e 'N;p'" \
+	"a\nb\na\nb\nc\n" "" "a\nb\nc\n"
+
+testing "sed N test2" "sed ':a;N;s/\n/ /;ta'" \
+	"a b c\n" "" "a\nb\nc\n"
+
+testing "sed N test3" "sed 'N;s/\n/ /'" \
+	"a b\nc\n" "" "a\nb\nc\n"
+
 testing "sed address match newline" 'sed "/b/N;/b\\nc/i woo"' \
 	"a\nwoo\nb\nc\nd\n" "" "a\nb\nc\nd\n"