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>

1 files changed, 15 insertions, 11 deletions

diff --git a/coreutils/tail.c b/coreutils/tail.c
index 19fd8f695..fb07ca27b 100644
--- 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;