odb: guard against data loss checking out a huge file

This introduces an additional guard for platforms where `unsigned long`
and `size_t` are not of the same size. If the size of an object in the
database would overflow `unsigned long`, instead we now exit with an
error.

A complete fix will have to update _many_ other functions throughout the
codebase to use `size_t` instead of `unsigned long`. It will have to be
implemented at some stage.

This commit puts in a stop-gap for the time being.

Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Matt Cooper <vtbassmatt@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

delta.h

@@ -90,15 +90,15 @@ static inline unsigned long get_delta_hdr_size(const unsigned char **datap,
 					       const unsigned char *top)
 {
 	const unsigned char *data = *datap;
-	unsigned long cmd, size = 0;
+	size_t cmd, size = 0;
 	int i = 0;
 	do {
 		cmd = *data++;
-		size |= (cmd & 0x7f) << i;
+		size |= st_left_shift(cmd & 0x7f, i);
 		i += 7;
 	} while (cmd & 0x80 && data < top);
 	*datap = data;
-	return size;
+	return cast_size_t_to_ulong(size);
 }
 
 #endif

object-file.c

@@ -1306,7 +1306,7 @@ static void *unpack_loose_rest(git_zstream *stream,
 int parse_loose_header(const char *hdr, struct object_info *oi)
 {
 	const char *type_buf = hdr;
-	unsigned long size;
+	size_t size;
 	int type, type_len = 0;
 
 	/*
@@ -1341,12 +1341,12 @@ int parse_loose_header(const char *hdr, struct object_info *oi)
 			if (c > 9)
 				break;
 			hdr++;
-			size = size * 10 + c;
+			size = st_add(st_mult(size, 10), c);
 		}
 	}
 
 	if (oi->sizep)
-		*oi->sizep = size;
+		*oi->sizep = cast_size_t_to_ulong(size);
 
 	/*
 	 * The length must be followed by a zero byte

packfile.c

@@ -1060,7 +1060,7 @@ unsigned long unpack_object_header_buffer(const unsigned char *buf,
 		unsigned long len, enum object_type *type, unsigned long *sizep)
 {
 	unsigned shift;
-	unsigned long size, c;
+	size_t size, c;
 	unsigned long used = 0;
 
 	c = buf[used++];
@@ -1074,10 +1074,10 @@ unsigned long unpack_object_header_buffer(const unsigned char *buf,
 			break;
 		}
 		c = buf[used++];
-		size += (c & 0x7f) << shift;
+		size = st_add(size, st_left_shift(c & 0x7f, shift));
 		shift += 7;
 	}
-	*sizep = size;
+	*sizep = cast_size_t_to_ulong(size);
 	return used;
 }