tail: fix "tail +N file_shorter_than_N"

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

2 files changed, 27 insertions, 6 deletions

diff --git a/coreutils/tail.c b/coreutils/tail.c
index ef1326c83..5f98bff82 100644
--- a/coreutils/tail.c
+++ b/coreutils/tail.c
@@ -50,12 +50,12 @@ static ssize_t tail_read(int fd, char *buf, size_t count)
         off_t current;
         struct stat sbuf;
 
-        /* (A good comment is missing here) */
-        current = lseek(fd, 0, SEEK_CUR);
         /* /proc files report zero st_size, don't lseek them. */
-        if (fstat(fd, &sbuf) == 0 && sbuf.st_size)
+        if (fstat(fd, &sbuf) == 0 && sbuf.st_size) {
+                current = lseek(fd, 0, SEEK_CUR);
                 if (sbuf.st_size < current)
                         lseek(fd, 0, SEEK_SET);
+        }
 
         r = full_read(fd, buf, count);
         if (r < 0) {
@@ -119,10 +119,11 @@ int tail_main(int argc, char **argv)
 #if ENABLE_FEATURE_FANCY_TAIL
         /* q: make it impossible for nfiles to be > header_threshhold */
         if (opt & 0x8) header_threshhold = UINT_MAX; // -q
+        //if (opt & 0x10) // -s
         if (opt & 0x20) header_threshhold = 0; // -v
-#define FOLLOW_RETRY (opt & 0x40)
+# define FOLLOW_RETRY (opt & 0x40)
 #else
-#define FOLLOW_RETRY 0
+# define FOLLOW_RETRY 0
 #endif
         argc -= optind;
         argv += optind;
@@ -189,7 +190,7 @@ int tail_main(int argc, char **argv)
                         off_t current = lseek(fds[i], 0, SEEK_END);
                         if (current > 0) {
                                 if (count == 0)
-                                        continue; /* showing zero lines is easy :) */
+                                        continue; /* showing zero bytes is easy :) */
                                 current -= count;
                                 if (current < 0)
                                         current = 0;
@@ -201,12 +202,15 @@ int tail_main(int argc, char **argv)
 
                 buf = tailbuf;
                 taillen = 0;
+                /* "We saw 1st line/byte".
+                 * Used only by +N code ("start from Nth", 1-based) */
                 seen = 1;
                 newlines_seen = 0;
                 while ((nread = tail_read(fds[i], buf, tailbufsize-taillen)) > 0) {
                         if (from_top) {
                                 int nwrite = nread;
                                 if (seen < count) {
+                                        /* We need to skip a few more bytes/lines */
                                         if (COUNT_BYTES) {
                                                 nwrite -= (count - seen);
                                                 seen = count;
diff --git a/testsuite/tail.tests b/testsuite/tail.tests
new file mode 100755
index 000000000..d97bf7bbe
--- /dev/null
+++ b/testsuite/tail.tests
@@ -0,0 +1,17 @@
+#!/bin/sh
+
+# Copyright 2009 by Denys Vlasenko <vda.linux@googlemail.com>
+# Licensed under GPL v2, see file LICENSE for details.
+
+. testing.sh
+
+# testing "test name" "command" "expected result" "file input" "stdin"
+#   file input will be file called "input"
+#   test can create a file "actual" instead of writing to stdout
+
+testing "tail: +N with N > file length" \
+        "tail -c +55 2>&1; echo \$?" \
+        "0\n" \
+        "" "qw"
+
+exit $FAILCOUNT