Add generic 'strbuf_readlink()' helper function

It was already what 'git apply' did in read_old_data(), just export it
as a real function, and make it be more generic.

In particular, this handles the case of the lstat() st_size data not
matching the readlink() return value properly (which apparently happens
at least on NTFS under Linux).  But as a result of this you could also
use the new function without even knowing how big the link is going to
be, and it will allocate an appropriately sized buffer.

So we pass in the st_size of the link as just a hint, rather than a
fixed requirement.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-apply.c

@@ -1559,10 +1559,8 @@ static int read_old_data(struct stat *st, const char *path, struct strbuf *buf)
 {
 	switch (st->st_mode & S_IFMT) {
 	case S_IFLNK:
-		strbuf_grow(buf, st->st_size);
-		if (readlink(path, buf->buf, st->st_size) != st->st_size)
-			return -1;
-		strbuf_setlen(buf, st->st_size);
+		if (strbuf_readlink(buf, path, st->st_size) < 0)
+			return error("unable to read symlink %s", path);
 		return 0;
 	case S_IFREG:
 		if (strbuf_read_file(buf, path, st->st_size) != st->st_size)

strbuf.c

@@ -288,6 +288,33 @@ ssize_t strbuf_read(struct strbuf *sb, int fd, size_t hint)
 	return sb->len - oldlen;
 }
 
+#define STRBUF_MAXLINK (2*PATH_MAX)
+
+int strbuf_readlink(struct strbuf *sb, const char *path, size_t hint)
+{
+	if (hint < 32)
+		hint = 32;
+
+	while (hint < STRBUF_MAXLINK) {
+		int len;
+
+		strbuf_grow(sb, hint);
+		len = readlink(path, sb->buf, hint);
+		if (len < 0) {
+			if (errno != ERANGE)
+				break;
+		} else if (len < hint) {
+			strbuf_setlen(sb, len);
+			return 0;
+		}
+
+		/* .. the buffer was too small - try again */
+		hint *= 2;
+	}
+	strbuf_release(sb);
+	return -1;
+}
+
 int strbuf_getline(struct strbuf *sb, FILE *fp, int term)
 {
 	int ch;

strbuf.h

@@ -124,6 +124,7 @@ extern size_t strbuf_fread(struct strbuf *, size_t, FILE *);
 /* XXX: if read fails, any partial read is undone */
 extern ssize_t strbuf_read(struct strbuf *, int fd, size_t hint);
 extern int strbuf_read_file(struct strbuf *sb, const char *path, size_t hint);
+extern int strbuf_readlink(struct strbuf *sb, const char *path, size_t hint);
 
 extern int strbuf_getline(struct strbuf *, FILE *, int);