pkt-line: make packet_reader.line non-const

The "line" member of a packet_reader struct is marked as const. This
kind of makes sense, because it's not its own allocated buffer that
should be freed, and we often use const to indicate that. But it is
always writable, because it points into the non-const "buffer" member.

And we rely on this writability in places like send-pack and
receive-pack, where we parse incoming packet contents by writing NULs
over delimiters. This has traditionally worked because we implicitly
cast away the constness with strchr() like:

  const char *head;
  char *p;

  head = reader->line;
  p = strchr(head, ' ');

Since C23 libc provides a generic strchr() to detect this implicit
const removal, this now generate a compiler warning on some platforms
(like recent glibc).

We can fix it by marking "line" as non-const, as well as a few
intermediate variables (like "head" in the above example). Note that by
itself, switching to a non-const variable would cause problems with this
line in send-pack.c:

  if (!skip_prefix(reader->line, "unpack ", &reader->line))

But due to our skip_prefix() magic introduced in the previous commit,
this compiles fine (both the in and out-parameters are non-const, so we
know it is safe).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

builtin/receive-pack.c

@@ -1025,8 +1025,8 @@ static int read_proc_receive_report(struct packet_reader *reader,
 
 	for (;;) {
 		struct object_id old_oid, new_oid;
-		const char *head;
-		const char *refname;
+		char *head;
+		char *refname;
 		char *p;
 		enum packet_read_status status;
 
@@ -1050,7 +1050,8 @@ static int read_proc_receive_report(struct packet_reader *reader,
 		}
 		*p++ = '\0';
 		if (!strcmp(head, "option")) {
-			const char *key, *val;
+			char *key;
+			const char *val;
 
 			if (!hint || !(report || new_report)) {
 				if (!once++)

pkt-line.h

@@ -184,7 +184,7 @@ struct packet_reader {
 	int pktlen;
 
 	/* the last line read */
-	const char *line;
+	char *line;
 
 	/* indicates if a line has been peeked */
 	int line_peeked;

send-pack.c

@@ -175,8 +175,8 @@ static int receive_status(struct repository *r,
 	ret = receive_unpack_status(reader);
 	while (1) {
 		struct object_id old_oid, new_oid;
-		const char *head;
-		const char *refname;
+		char *head;
+		char *refname;
 		char *p;
 		if (packet_reader_read(reader) != PACKET_READ_NORMAL)
 			break;
@@ -190,7 +190,8 @@ static int receive_status(struct repository *r,
 		*p++ = '\0';
 
 		if (!strcmp(head, "option")) {
-			const char *key, *val;
+			char *key;
+			const char *val;
 
 			if (!hint || !(report || new_report)) {
 				if (!once++)