grep: store grep_source buffer as const

Our grep_buffer() function takes a non-const buffer, which is confusing:
we don't take ownership of nor write to the buffer.

This mostly comes from the fact that the underlying grep_source struct
in which we store the buffer uses non-const pointer. The memory pointed
to by the struct is sometimes owned by us (for FILE or OID sources), and
sometimes not (for BUF sources).

Let's store it as const, which lets us err on the side of caution (i.e.,
the compiler will warn us if any of our code writes to or tries to free
it).

As a result, we must annotate the one place where we do free it by
casting away the constness. But that's a small price to pay for the
extra safety and clarity elsewhere (and indeed, it already had a comment
explaining why GREP_SOURCE_BUF _didn't_ free it).

And then we can mark grep_buffer() as taking a const buffer.

Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

grep.c

@@ -1821,7 +1821,8 @@ int grep_source(struct grep_opt *opt, struct grep_source *gs)
 	return grep_source_1(opt, gs, 0);
 }
 
-static void grep_source_init_buf(struct grep_source *gs, char *buf,
+static void grep_source_init_buf(struct grep_source *gs,
+				 const char *buf,
 				 unsigned long size)
 {
 	gs->type = GREP_SOURCE_BUF;
@@ -1833,7 +1834,7 @@ static void grep_source_init_buf(struct grep_source *gs, char *buf,
 	gs->identifier = NULL;
 }
 
-int grep_buffer(struct grep_opt *opt, char *buf, unsigned long size)
+int grep_buffer(struct grep_opt *opt, const char *buf, unsigned long size)
 {
 	struct grep_source gs;
 	int r;
@@ -1885,7 +1886,9 @@ void grep_source_clear_data(struct grep_source *gs)
 	switch (gs->type) {
 	case GREP_SOURCE_FILE:
 	case GREP_SOURCE_OID:
-		FREE_AND_NULL(gs->buf);
+		/* these types own the buffer */
+		free((char *)gs->buf);
+		gs->buf = NULL;
 		gs->size = 0;
 		break;
 	case GREP_SOURCE_BUF:

grep.h

@@ -189,7 +189,7 @@ void append_grep_pattern(struct grep_opt *opt, const char *pat, const char *orig
 void append_header_grep_pattern(struct grep_opt *, enum grep_header_field, const char *);
 void compile_grep_patterns(struct grep_opt *opt);
 void free_grep_patterns(struct grep_opt *opt);
-int grep_buffer(struct grep_opt *opt, char *buf, unsigned long size);
+int grep_buffer(struct grep_opt *opt, const char *buf, unsigned long size);
 
 struct grep_source {
 	char *name;
@@ -202,7 +202,7 @@ struct grep_source {
 	void *identifier;
 	struct repository *repo; /* if GREP_SOURCE_OID */
 
-	char *buf;
+	const char *buf;
 	unsigned long size;
 
 	char *path; /* for attribute lookups */