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tail: refactor to skip stat call on failure

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* src/tail.c (tail_bytes): New function.
(tail_bytes, tail_lines, tail): Accept struct stat pointer from
caller instead of calling fstat ourselves.  All callers changed.
(tail_file): Skip a call to fstat if fstat already failed.
* tests/tail/follow-stdin.sh: Adjust to match new behavior
on failure, which omits a redundant diagnostic.
This commit is contained in:
Paul Eggert
2025-07-29 08:24:58 -07:00
parent d58aa27d06
commit 09fc945b37
2 changed files with 47 additions and 48 deletions
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94
src/tail.c
View File

@@ -1840,21 +1840,30 @@ tail_forever_inotify (int wd, struct File_spec *f, size_t n_files,
}
#endif
/* Output the last N_BYTES bytes of file FILENAME open for reading in FD.
Return true if successful. */
/* Get the status for file F, which has descriptor FD, into *ST.
Return true on success, false (diagnosing the failure) otherwise. */
static bool
tail_bytes (char const *pretty_filename, int fd, uintmax_t n_bytes,
uintmax_t *read_pos)
get_file_status (struct File_spec *f, int fd, struct stat *st)
{
struct stat stats;
if (fstat (fd, &stats))
if (fstat (fd, st) < 0)
{
error (0, errno, _("cannot fstat %s"), quoteaf (pretty_filename));
f->errnum = errno;
error (0, f->errnum, _("cannot fstat %s"), quoteaf (pretty_name (f)));
return false;
}
return true;
}
/* Output the last bytes of the file PRETTY_FILENAME open for reading
in FD and with status ST. Output the last N_BYTES bytes, and set *READ_POS
to the resulting read position if the file is a regular file, and
to an unspecified value otherwise. Return true if and only if successful. */
static bool
tail_bytes (char const *pretty_filename, int fd, struct stat const *st,
uintmax_t n_bytes, uintmax_t *read_pos)
{
if (from_start)
{
off_t pos = (presume_input_pipe
@@ -1878,14 +1887,14 @@ tail_bytes (char const *pretty_filename, int fd, uintmax_t n_bytes,
if (0 <= current_pos)
{
if (usable_st_size (&stats))
if (usable_st_size (st))
{
/* Use st_size only if it's so large that this is
probably not a /proc or similar file, where st_size
is notional. */
off_t smallish_size = STP_BLKSIZE (&stats);
if (smallish_size < stats.st_size)
end_pos = stats.st_size;
off_t smallish_size = STP_BLKSIZE (st);
if (smallish_size < st->st_size)
end_pos = st->st_size;
}
else
{
@@ -1933,21 +1942,15 @@ tail_bytes (char const *pretty_filename, int fd, uintmax_t n_bytes,
return true;
}
/* Output the last N_LINES lines of file FILENAME open for reading in FD.
Return true if successful. */
/* Output the last lines of the file PRETTY_FILENAME open for reading
in FD and with status ST. Output the last N_LINES lines, and set *READ_POS
to the resulting read position if the file is a regular file, and
to an unspecified value otherwise. Return true if and only if successful. */
static bool
tail_lines (char const *pretty_filename, int fd, uintmax_t n_lines,
uintmax_t *read_pos)
tail_lines (char const *pretty_filename, int fd, struct stat const *st,
uintmax_t n_lines, uintmax_t *read_pos)
{
struct stat stats;
if (fstat (fd, &stats))
{
error (0, errno, _("cannot fstat %s"), quoteaf (pretty_filename));
return false;
}
if (from_start)
{
/* If skipping all input use lseek if possible, for speed. */
@@ -1970,13 +1973,13 @@ tail_lines (char const *pretty_filename, int fd, uintmax_t n_lines,
/* Use file_lines only if FD refers to a regular file for
which lseek (... SEEK_END) works. */
if ( ! presume_input_pipe
&& S_ISREG (stats.st_mode)
&& S_ISREG (st->st_mode)
&& (start_pos = lseek (fd, 0, SEEK_CUR)) != -1
&& start_pos < (end_pos = lseek (fd, 0, SEEK_END)))
{
*read_pos = end_pos;
if (end_pos != 0
&& ! file_lines (pretty_filename, fd, &stats, n_lines,
&& ! file_lines (pretty_filename, fd, st, n_lines,
start_pos, end_pos, read_pos))
return false;
}
@@ -1995,8 +1998,9 @@ tail_lines (char const *pretty_filename, int fd, uintmax_t n_lines,
return true;
}
/* Display the last N_UNITS units of file FILENAME, open for reading
via FD. Set *READ_POS to the position of the input stream pointer.
/* Display the last N_UNITS units of file FILENAME,
open for reading via FD and with status *ST.
Set *READ_POS to the position of the input stream pointer.
*READ_POS is usually the number of bytes read and corresponds to an
offset from the beginning of a file. However, it may be larger than
OFF_T_MAX (as for an input pipe), and may also be larger than the
@@ -2006,14 +2010,12 @@ tail_lines (char const *pretty_filename, int fd, uintmax_t n_lines,
Return true if successful. */
static bool
tail (char const *filename, int fd, uintmax_t n_units,
uintmax_t *read_pos)
tail (char const *filename, int fd, struct stat const *st,
uintmax_t n_units, uintmax_t *read_pos)
{
*read_pos = 0;
if (count_lines)
return tail_lines (filename, fd, n_units, read_pos);
else
return tail_bytes (filename, fd, n_units, read_pos);
return ((count_lines ? tail_lines : tail_bytes)
(filename, fd, st, n_units, read_pos));
}
/* Display the last N_UNITS units of the file described by F.
@@ -2061,29 +2063,22 @@ tail_file (struct File_spec *f, uintmax_t n_files, uintmax_t n_units)
if (print_headers)
write_header (pretty_name (f));
ok = tail (pretty_name (f), fd, n_units, &read_pos);
struct stat stats;
bool stat_ok = get_file_status (f, fd, &stats);
ok = stat_ok && tail (pretty_name (f), fd, &stats, n_units, &read_pos);
if (forever)
{
struct stat stats;
#if TAIL_TEST_SLEEP
/* Before the tail function provided 'read_pos', there was
a race condition described in the URL below. This sleep
call made the window big enough to exercise the problem. */
xnanosleep (1);
#endif
f->errnum = ok - 1;
if (fstat (fd, &stats) < 0)
if (stat_ok && !IS_TAILABLE_FILE_TYPE (stats.st_mode))
{
ok = false;
f->errnum = errno;
error (0, errno, _("error reading %s"),
quoteaf (pretty_name (f)));
}
else if (!IS_TAILABLE_FILE_TYPE (stats.st_mode))
{
ok = false;
f->errnum = -1;
f->tailable = false;
f->ignore = ! reopen_inaccessible_files;
error (0, 0, _("%s: cannot follow end of this type of file%s"),
@@ -2091,6 +2086,11 @@ tail_file (struct File_spec *f, uintmax_t n_files, uintmax_t n_units)
f->ignore ? _("; giving up on this name") : "");
}
if (ok && !get_file_status (f, fd, &stats))
ok = false;
else if (stat_ok)
f->errnum = ok - 1;
if (!ok)
{
f->ignore = ! reopen_inaccessible_files;

1
tests/tail/follow-stdin.sh
View File

@@ -59,7 +59,6 @@ done
returns_ 1 timeout 10 tail -f - <&- 2>errt || fail=1
cat <<\EOF >exp || framework_failure_
tail: cannot fstat 'standard input'
tail: error reading 'standard input'
tail: no files remaining
EOF
sed 's/\(tail:.*\):.*/\1/' errt > err || framework_failure_