tail: track file size only in -f mode

This eliminates extra fstat and lseek calls on every read

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

diff --git a/coreutils/tail.c b/coreutils/tail.c
index 19fd8f6..fb07ca2 100644 (file)
--- a/coreutils/tail.c
+++ b/coreutils/tail.c
@@ -66,17 +66,21 @@ static void tail_xprint_header(const char *fmt, const char *filename)
                bb_perror_nomsg_and_die();
 }
 
-static ssize_t tail_read(int fd, char *buf, size_t count)
+static ssize_t tail_read(int fd, char *buf, size_t count, int follow)
 {
        ssize_t r;
-       off_t current;
-       struct stat sbuf;
-
-       /* /proc files report zero st_size, don't lseek them. */
-       if (fstat(fd, &sbuf) == 0 && sbuf.st_size > 0) {
-               current = lseek(fd, 0, SEEK_CUR);
-               if (sbuf.st_size < current)
-                       xlseek(fd, 0, SEEK_SET);
+
+       if (follow) {
+               /* tail -f keeps following files even if they are truncated */
+               off_t current;
+               struct stat sbuf;
+
+               /* /proc files report zero st_size, don't lseek them */
+               if (fstat(fd, &sbuf) == 0 && sbuf.st_size > 0) {
+                       current = lseek(fd, 0, SEEK_CUR);
+                       if (sbuf.st_size < current)
+                               xlseek(fd, 0, SEEK_SET);
+               }
        }
 
        r = full_read(fd, buf, count);
@@ -251,7 +255,7 @@ int tail_main(int argc, char **argv)
                 * Used only by +N code ("start from Nth", 1-based): */
                seen = 1;
                newlines_seen = 0;
-               while ((nread = tail_read(fd, buf, tailbufsize - taillen)) > 0) {
+               while ((nread = tail_read(fd, buf, tailbufsize - taillen, /*follow:*/ 0)) > 0) {
                        if (G.from_top) {
                                int nwrite = nread;
                                if (seen < count) {
@@ -363,7 +367,7 @@ int tail_main(int argc, char **argv)
                        if (nfiles > header_threshhold) {
                                fmt = header_fmt_str;
                        }
-                       while ((nread = tail_read(fd, tailbuf, BUFSIZ)) > 0) {
+                       while ((nread = tail_read(fd, tailbuf, BUFSIZ, /*follow:*/ 1)) > 0) {
                                if (fmt) {
                                        tail_xprint_header(fmt, filename);
                                        fmt = NULL;