wc: ensure we update file offset

* src/wc.c (wc): Update the offset when not reading,
and do read if we can't update the offset.
* tests/misc/wc-proc.sh: Add a test case.
* NEWS: Mention the bug fix.
Fixes https://bugs.gnu.org/61300

3 files changed, 20 insertions, 1 deletions

diff --git a/NEWS b/NEWS
index 09dba0b80..9d7836d7b 100644
--- a/NEWS
+++ b/NEWS
@@ -27,6 +27,10 @@ GNU coreutils NEWS                                    -*- outline -*-
   wc will now diagnose if any total counts have overflowed.
   [This bug was present in "the beginning".]
 
+  `wc -c` will again correctly update the read offset of inputs.
+  Previously it deduced the size of inputs while leaving the offset unchanged.
+  [bug introduced in coreutils-8.27]
+
 ** Changes in behavior
 
   'cp -n' and 'mv -n' now issue an error diagnostic if skipping a file,
diff --git a/src/wc.c b/src/wc.c
index 801396a15..becceda98 100644
--- a/src/wc.c
+++ b/src/wc.c
@@ -450,7 +450,10 @@ wc (int fd, char const *file_x, struct fstatus *fstatus, off_t current_pos)
                  beyond the end of the file.  As in the example above.  */
 
               bytes = end_pos < current_pos ? 0 : end_pos - current_pos;
-              skip_read = true;
+              if (bytes && 0 <= lseek (fd, bytes, SEEK_CUR))
+                skip_read = true;
+              else
+                bytes = 0;
             }
           else
             {
diff --git a/tests/misc/wc-proc.sh b/tests/misc/wc-proc.sh
index 5eb43b982..2307f2c38 100755
--- a/tests/misc/wc-proc.sh
+++ b/tests/misc/wc-proc.sh
@@ -42,6 +42,18 @@ cat <<\EOF > exp
 EOF
 compare exp out || fail=1
 
+# Ensure we update the offset even when not reading,
+# which wasn't the case from coreutils-8.27 to coreutils-9.2
+{ wc -c; wc -c; } < no_read >  out || fail=1
+{ wc -c; wc -c; } < do_read >> out || fail=1
+cat <<\EOF > exp
+2
+0
+1048576
+0
+EOF
+compare exp out || fail=1
+
 # Ensure we don't read too much when reading,
 # as was the case on 32 bit systems
 # from coreutils-8.24 to coreutils-9.1