cp: work around linux kernel bug: short-read != EOF on /proc

Remove the optimization that avoided up to 50% of cp's read syscalls.
Do not assume that a short read on a regular file indicates EOF.
When reading from a file in /proc on linux [at least 2.6.9 - 2.6.29]
into a 4k-byte buffer or larger, a short read does not
always indicate EOF.  For example, "cp /proc/slabinfo /tmp"
copies only 4068 of the total 7493 bytes.  This optimization
(25719a33154f0c62ea9881f0c79ae312dd4cec7a, Improve performance a bit
by optimizing away; 2005-11-24) appears to have been worth less than
a 2% speed-up (and usually much less), so the impact of removing it
is negligible.

* src/copy.c (copy_reg): Don't exit the loop early.
* tests/cp/proc-short-read: New test, lightly based on a suggestion
from Mike Frysinger, to exercise this fix.
* tests/Makefile.am (TESTS): Add cp/proc-short-read.
* NEWS (Improve robustness): Mention this change.

1 files changed, 4 insertions, 3 deletions

diff --git a/src/copy.c b/src/copy.c
index 9b0e139c6..c45224c8a 100644
--- a/src/copy.c
+++ b/src/copy.c
@@ -700,9 +700,10 @@ copy_reg (char const *src_name, char const *dst_name,
 	      }
 	    last_write_made_hole = false;
 
-	    /* A short read on a regular file means EOF.  */
-	    if (n_read != buf_size && S_ISREG (src_open_sb.st_mode))
-	      break;
+            /* It is tempting to return early here upon a short read from a
+               regular file.  That would save the final read syscall for each
+               file.  Unfortunately that doesn't work for certain files in
+               /proc with linux kernels from at least 2.6.9 .. 2.6.29.  */
 	  }
       }