tail: do not lose the tail of old file if new file (-F) is detected

function                                             old     new   delta
tail_main                                           1619    1645     +26
.rodata                                           103246  103250      +4
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 30/0)               Total: 30 bytes

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

1 files changed, 26 insertions, 12 deletions

diff --git a/coreutils/tail.c b/coreutils/tail.c
index 93f1514d0..6201eb023 100644
--- a/coreutils/tail.c
+++ b/coreutils/tail.c
@@ -346,9 +346,11 @@ int tail_main(int argc, char **argv)
                         int nread;
                         const char *filename = argv[i];
                         int fd = fds[i];
+                        int new_fd = -1;
+                        struct stat sbuf;
 
                         if (FOLLOW_RETRY) {
-                                struct stat sbuf, fsbuf;
+                                struct stat fsbuf;
 
                                 if (fd < 0
                                  || fstat(fd, &fsbuf) < 0
@@ -356,19 +358,21 @@ int tail_main(int argc, char **argv)
                                  || fsbuf.st_dev != sbuf.st_dev
                                  || fsbuf.st_ino != sbuf.st_ino
                                 ) {
-                                        int new_fd;
-
-                                        if (fd >= 0)
-                                                close(fd);
+                                        /* Looks like file has been created/renamed/deleted */
                                         new_fd = open(filename, O_RDONLY);
                                         if (new_fd >= 0) {
                                                 bb_error_msg("%s has %s; following end of new file",
                                                         filename, (fd < 0) ? "appeared" : "been replaced"
                                                 );
+                                                if (fd < 0) {
+                                                        /* No previously open fd for this file,
+                                                         * start using new_fd immediately. */
+                                                        fds[i] = fd = new_fd;
+                                                        new_fd = -1;
+                                                }
                                         } else if (fd >= 0) {
-                                                bb_perror_msg("%s has become inaccessible", filename);
+                                                bb_perror_msg("%s has been renamed or deleted", filename);
                                         }
-                                        fds[i] = fd = new_fd;
                                 }
                         }
                         if (ENABLE_FEATURE_FANCY_TAIL && fd < 0)
@@ -378,17 +382,27 @@ int tail_main(int argc, char **argv)
                         }
                         for (;;) {
                                 /* tail -f keeps following files even if they are truncated */
-                                struct stat sbuf;
                                 /* /proc files report zero st_size, don't lseek them */
-                                if (fstat(fd, &sbuf) == 0 && sbuf.st_size > 0) {
+                                if (fstat(fd, &sbuf) == 0
+                                 /* && S_ISREG(sbuf.st_mode) TODO? */
+                                 && sbuf.st_size > 0
+                                ) {
                                         off_t current = lseek(fd, 0, SEEK_CUR);
-                                        if (sbuf.st_size < current)
+                                        if (sbuf.st_size < current) {
+                                                //bb_perror_msg("%s: file truncated", filename); - says coreutils 8.32
                                                 xlseek(fd, 0, SEEK_SET);
+                                        }
                                 }
 
                                 nread = tail_read(fd, tailbuf, BUFSIZ);
-                                if (nread <= 0)
-                                        break;
+                                if (nread <= 0) {
+                                        if (new_fd < 0)
+                                                break;
+                                        /* Switch to "tail -F"ing the new file */
+                                        xmove_fd(new_fd, fd);
+                                        new_fd = -1;
+                                        continue;
+                                }
                                 if (fmt && (fd != prev_fd)) {
                                         tail_xprint_header(fmt, filename);
                                         fmt = NULL;