pygrub: cope better with big files in the guest.

Only read the first megabyte of a configuration file (grub etc.) and read the
kernel and ramdisk files from the guest in one megabyte pieces so pygrub
doesn't use a lot of memory if the files are large.  With --not-really option
check that the chosen kernel and ramdisk files exist.  If there are problems
writing the copy of the kernel or ramdisk, delete the copied files and exit in
case they have filled the filesystem.

Signed-off-by: Michael Young <m.a.young@durham.ac.uk>
Acked-by: Matt Wilson <msw@amazon.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>

tools/pygrub/src/pygrub

@@ -28,6 +28,7 @@ import grub.LiloConf
 import grub.ExtLinuxConf
 
 PYGRUB_VER = 0.6
+FS_READ_MAX = 1024 * 1024
 
 def enable_cursor(ison):
     if ison:
@@ -448,7 +449,8 @@ class Grub:
         if self.__dict__.get('cf', None) is None:
             raise RuntimeError, "couldn't find bootloader config file in the image provided."
         f = fs.open_file(self.cf.filename)
-        buf = f.read()
+        # limit read size to avoid pathological cases
+        buf = f.read(FS_READ_MAX)
         del f
         self.cf.parse(buf)
 
@@ -697,6 +699,37 @@ if __name__ == "__main__":
     def usage():
         print >> sys.stderr, "Usage: %s [-q|--quiet] [-i|--interactive] [-n|--not-really] [--output=] [--kernel=] [--ramdisk=] [--args=] [--entry=] [--output-directory=] [--output-format=sxp|simple|simple0] <image>" %(sys.argv[0],)
 
+    def copy_from_image(fs, file_to_read, file_type, output_directory,
+                        not_really):
+        if not_really:
+            if fs.file_exists(file_to_read):
+                return "<%s:%s>" % (file_type, file_to_read)
+            else:
+                sys.exit("The requested %s file does not exist" % file_type)
+        try:
+            datafile = fs.open_file(file_to_read)
+        except Exception, e:
+            print >>sys.stderr, e
+            sys.exit("Error opening %s in guest" % file_to_read)
+        (tfd, ret) = tempfile.mkstemp(prefix="boot_"+file_type+".",
+                                      dir=output_directory)
+        dataoff = 0
+        while True:
+            data = datafile.read(FS_READ_MAX, dataoff)
+            if len(data) == 0:
+                os.close(tfd)
+                del datafile
+                return ret
+            try:
+                os.write(tfd, data)
+            except Exception, e:
+                print >>sys.stderr, e
+                os.close(tfd)
+                os.unlink(ret)
+                del datafile
+                sys.exit("Error writing temporary copy of "+file_type)
+            dataoff += len(data)
+
     try:
         opts, args = getopt.gnu_getopt(sys.argv[1:], 'qinh::',
                                    ["quiet", "interactive", "not-really", "help", 
@@ -821,24 +854,18 @@ if __name__ == "__main__":
     if not fs:
         raise RuntimeError, "Unable to find partition containing kernel"
 
-    if not_really:
-        bootcfg["kernel"] = "<kernel:%s>" % chosencfg["kernel"]
-    else:
-        data = fs.open_file(chosencfg["kernel"]).read()
-        (tfd, bootcfg["kernel"]) = tempfile.mkstemp(prefix="boot_kernel.",
-                                                    dir=output_directory)
-        os.write(tfd, data)
-        os.close(tfd)
+    bootcfg["kernel"] = copy_from_image(fs, chosencfg["kernel"], "kernel",
+                                        output_directory, not_really)
 
     if chosencfg["ramdisk"]:
-        if not_really:
-            bootcfg["ramdisk"] = "<ramdisk:%s>" % chosencfg["ramdisk"]
-        else:
-            data = fs.open_file(chosencfg["ramdisk"],).read()
-            (tfd, bootcfg["ramdisk"]) = tempfile.mkstemp(
-                prefix="boot_ramdisk.", dir=output_directory)
-            os.write(tfd, data)
-            os.close(tfd)
+        try:
+            bootcfg["ramdisk"] = copy_from_image(fs, chosencfg["ramdisk"],
+                                                 "ramdisk", output_directory,
+                                                 not_really)
+        except:
+            if not not_really:
+                os.unlink(bootcfg["kernel"])
+            raise
     else:
         initrd = None