[Bug-tar] `File shrank' errors during create

[Michael Teichgräber]

Hi,

when using "tar cf" on a fuse controlled file system, I stumbled
across messages like "File shrank by 500197 bytes; padding with
zeros", with these files being skipped from the archive.

Strace showed that tar was e.g. trying to read a block of 9728
(== 0x2600) bytes, but received only 4096. The FUSE controlled file
system reduced the requested size to that value, and made read()
return 4096. (It seems a normal, kernel controlled fs doesn't have
such a limitation, or perhaps there it is at 16384 bytes)

Apparently tar treates this as an error
 (tar-1.16/src/create.c:967: if (count != bufsize)).

This made me wonder whether in line 955 tar could use a function like
safe_readn instead of safe_read, so that actually n bytes would be
read unless there is an error. After temporarily adding such a
function to lib/safe-read.c tar didn't show these messages again and
seemed to create a valid archive.

Regards,
Michael