Split up file reading from 'parse-xml.c' into 'file.c'

We're going to eventually import non-xml files too, so let's begin
splitting the logic up.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

file.c

@@ -0,0 +1,73 @@
+#include <unistd.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <stdlib.h>
+#include <errno.h>
+
+#include "dive.h"
+
+struct memblock {
+	void *buffer;
+	size_t size;
+};
+
+static int readfile(const char *filename, struct memblock *mem)
+{
+	int ret, fd = open(filename, O_RDONLY);
+	struct stat st;
+
+	mem->buffer = NULL;
+	mem->size = 0;
+
+	fd = open(filename, O_RDONLY);
+	if (fd < 0)
+		return fd;
+	ret = fstat(fd, &st);
+	if (ret < 0)
+		goto out;
+	ret = -EINVAL;
+	if (!S_ISREG(st.st_mode))
+		goto out;
+	ret = 0;
+	if (!st.st_size)
+		goto out;
+	mem->buffer = malloc(st.st_size);
+	ret = -1;
+	errno = ENOMEM;
+	if (!mem->buffer)
+		goto out;
+	mem->size = st.st_size;
+	ret = read(fd, mem->buffer, mem->size);
+	if (ret < 0)
+		goto free;
+	if (ret == mem->size)
+		goto out;
+	errno = EIO;
+	ret = -1;
+free:
+	free(mem->buffer);
+	mem->buffer = NULL;
+	mem->size = 0;
+out:
+	close(fd);
+	return ret;
+}
+
+void parse_file(const char *filename, GError **error)
+{
+	struct memblock mem;
+
+	if (readfile(filename, &mem) < 0) {
+		fprintf(stderr, "Failed to read '%s'.\n", filename);
+		if (error) {
+			*error = g_error_new(g_quark_from_string("subsurface"),
+					     DIVE_ERROR_PARSE,
+					     "Failed to read '%s'",
+					     filename);
+		}
+		return;
+	}
+
+	parse_xml_buffer(filename, mem.buffer, mem.size, error);
+	free(mem.buffer);
+}