protbuf: Add protobuf (PB) image entries read/write helpers v4

To not bloat util.h or image.h the protobuf.h introduced to handle all PB needs.

We represent our PB objects as pairs of records

  | 4 bytes for object packed size
  +---
  | X bytes -- object itself

v3:
 - Use plain read/write calls
 - Use local buffers for small objects

v4:
 - Add pb_pksize helper to calculate object
   size with header

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

protobuf.c

@@ -0,0 +1,132 @@
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <stdlib.h>
+
+#include "compiler.h"
+#include "types.h"
+#include "log.h"
+#include "util.h"
+
+#include "protobuf.h"
+
+/*
+ * To speed up reading of packed objects
+ * by providing space on stack, this should
+ * be more than enough for most objects.
+ */
+#define PB_PKOBJ_LOCAL_SIZE	1024
+
+/*
+ * Reads PB record (header + packed object) from file @fd and unpack
+ * it with @unpack procedure to the pointer @pobj
+ *
+ *  1 on success
+ * -1 on error (or EOF met and @eof set to false)
+ *  0 on EOF and @eof set to true
+ *
+ * Don't forget to free memory granted to unpacked object in calling code if needed
+ */
+int pb_read_object_with_header(int fd, void **pobj, pb_unpack_t *unpack, bool eof)
+{
+	u8 local[PB_PKOBJ_LOCAL_SIZE];
+	void *buf = (void *)&local;
+	u32 size;
+	int ret;
+
+	*pobj = NULL;
+
+	ret = read(fd, &size, sizeof(size));
+	if (ret == 0) {
+		if (eof) {
+			return 0;
+		} else {
+			pr_err("Unexpected EOF\n");
+			return -1;
+		}
+	} else if (ret < sizeof(size)) {
+		pr_perror("Read %d bytes while %d expected",
+			  ret, (int)sizeof(size));
+		return -1;
+	}
+
+	if (size > sizeof(local)) {
+		ret = -1;
+		buf = xmalloc(size);
+		if (!buf)
+			goto err;
+	}
+
+	ret = read(fd, buf, size);
+	if (ret < 0) {
+		pr_perror("Can't read %d bytes from file", size);
+		goto err;
+	} else if (ret != size) {
+		pr_perror("Read %d bytes while %d expected", ret, size);
+		goto err;
+	}
+
+	*pobj = unpack(NULL, size, buf);
+	if (!*pobj) {
+		ret = -1;
+		pr_err("Failed unpacking object %p\n", pobj);
+		goto err;
+	}
+
+	ret = 1;
+err:
+	if (buf != (void *)&local)
+		xfree(buf);
+
+	return ret;
+}
+
+/*
+ * Writes PB record (header + packed object pointed by @obj)
+ * to file @fd, using @getpksize to get packed size and @pack
+ * to implement packing
+ *
+ *  0 on success
+ * -1 on error
+ */
+int pb_write_object_with_header(int fd, void *obj, pb_getpksize_t *getpksize, pb_pack_t *pack)
+{
+	u8 local[PB_PKOBJ_LOCAL_SIZE];
+	void *buf = (void *)&local;
+	u32 size, packed;
+	int ret = -1;
+
+	size = getpksize(obj);
+	if (size > (u32)sizeof(local)) {
+		buf = xmalloc(size);
+		if (!buf)
+			goto err;
+	}
+
+	packed = pack(obj, buf);
+	if (packed != size) {
+		pr_err("Failed packing PB object %p\n", obj);
+		goto err;
+	}
+
+	ret = write(fd, &size, sizeof(size));
+	if (ret != sizeof(size)) {
+		ret = -1;
+		pr_perror("Can't write %d bytes", (int)sizeof(size));
+		goto err;
+	}
+
+	ret = write(fd, buf, size);
+	if (ret != size) {
+		ret = -1;
+		pr_perror("Can't write %d bytes", size);
+		goto err;
+	}
+
+	ret = 0;
+err:
+	if (buf != (void *)&local)
+		xfree(buf);
+	return ret;
+}