tar-tree: Use the prefix field of a tar header

... to store parts of the path, if possible.  This allows us to avoid
writing extended headers in certain cases (long pathes can only be
split at '/' chars).

Also adds a file to the test repo with a 100 chars long directory name.
Even old versions of tar that don't understand POSIX extended headers
should be able to handle this testcase.

Btw.: The longest path in the kernel tree currently has 70 chars.
Together with a 30 chars long prefix this would already cross the
field limit of 100 chars.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>

t/t5000-tar-tree.sh

@@ -34,6 +34,9 @@ test_expect_success \
      mkdir a/bin &&
      cp /bin/sh a/bin &&
      ln -s a a/l1 &&
+     (p=long_path_to_a_file && cd a &&
+      for depth in 1 2 3 4 5; do mkdir $p && cd $p; done &&
+      echo text >file_with_long_path) &&
      (cd a && find .) | sort >a.lst'
 
 test_expect_success \

tar-tree.c

@@ -161,6 +161,16 @@ static unsigned int ustar_header_chksum(const struct ustar_header *header)
 	return chksum;
 }
 
+static int get_path_prefix(const struct strbuf *path, int maxlen)
+{
+	int i = path->len;
+	if (i > maxlen)
+		i = maxlen;
+	while (i > 0 && path->buf[i] != '/')
+		i--;
+	return i;
+}
+
 static void write_entry(const unsigned char *sha1, struct strbuf *path,
                         unsigned int mode, void *buffer, unsigned long size)
 {
@@ -195,9 +205,17 @@ static void write_entry(const unsigned char *sha1, struct strbuf *path,
 			return;
 		}
 		if (path->len > sizeof(header.name)) {
-			sprintf(header.name, "%s.data", sha1_to_hex(sha1));
-			strbuf_append_ext_header(&ext_header, "path",
-				                 path->buf, path->len);
+			int plen = get_path_prefix(path, sizeof(header.prefix));
+			int rest = path->len - plen - 1;
+			if (plen > 0 && rest <= sizeof(header.name)) {
+				memcpy(header.prefix, path->buf, plen);
+				memcpy(header.name, path->buf + plen + 1, rest);
+			} else {
+				sprintf(header.name, "%s.data",
+				        sha1_to_hex(sha1));
+				strbuf_append_ext_header(&ext_header, "path",
+				                         path->buf, path->len);
+			}
 		} else
 			memcpy(header.name, path->buf, path->len);
 	}