If the kevent() registration fails (probably due to the underlying

filesystem not being kq-aware), then fall back to using sleep.  This
allows tail to work with NFS filesystems again without chewing up CPU time.

When given the -F flag, resort to sleep/stat after the file was moved
or deleted.  This allows a window where the file being tailed does not
exist at all, which is typically the case during log rotation.  Switch
back to using kq (if possible) after the file is reopened.

1 files changed, 53 insertions, 19 deletions

diff --git a/usr.bin/tail/forward.c b/usr.bin/tail/forward.c
index 5fff64f..526ff2f 100644
--- a/usr.bin/tail/forward.c
+++ b/usr.bin/tail/forward.c
@@ -58,6 +58,11 @@ static char sccsid[] = "@(#)forward.c	8.1 (Berkeley) 6/6/93";
 
 static void rlines __P((FILE *, long, struct stat *));
 
+/* defines for inner loop actions */
+#define USE_SLEEP	0
+#define USE_KQUEUE	1
+#define ADD_EVENTS	2
+
 /*
  * forward -- display the file, from an offset, forward.
  *
@@ -87,8 +92,10 @@ forward(fp, style, off, sbp)
 	long off;
 	struct stat *sbp;
 {
-	int ch, add_events = 0, kq = -1;
+	int ch, kq = -1;
+	int action = USE_SLEEP;
 	struct kevent ev[2];
+	struct stat sb2;
 
 	switch(style) {
 	case FBYTES:
@@ -167,7 +174,7 @@ forward(fp, style, off, sbp)
 		kq = kqueue();
 		if (kq < 0)
 			err(1, "kqueue");
-		add_events = 1;
+		action = ADD_EVENTS;
 	}
 
 	for (;;) {
@@ -183,7 +190,8 @@ forward(fp, style, off, sbp)
 			break;
 		clearerr(fp);
 
-		if (add_events) {
+		switch (action) {
+		case ADD_EVENTS: {
 			int n = 0;
 			struct kevent *evp[2];
 			struct timespec ts = { 0, 0 };
@@ -202,27 +210,53 @@ forward(fp, style, off, sbp)
 			evp[n] = &ev[n];
 			n++;
 
-			if (kevent(kq, n, evp, 0, NULL, &ts) < 0)
-				err(1, "kevent");
-			add_events = 0;
+			if (kevent(kq, n, evp, 0, NULL, &ts) < 0) {
+				close(kq);
+				kq = -1;
+				action = USE_SLEEP;
+			} else {
+				action = USE_KQUEUE;
+			}
+			break;
 		}
 
-		if (kevent(kq, 0, NULL, 1, ev, NULL) < 0)
-			err(1, "kevent");
+		case USE_KQUEUE:
+			if (kevent(kq, 0, NULL, 1, ev, NULL) < 0)
+				err(1, "kevent");
 
-		if (ev->filter == EVFILT_VNODE) {
-			fp = freopen(fname, "r", fp);
-			if (fp == NULL) {
-				ierr();
-				break;
+			if (ev->filter == EVFILT_VNODE) {
+				/* file was rotated, wait until it reappears */
+				action = USE_SLEEP;
+			} else if (ev->data < 0) {
+				/* file shrank, reposition to end */
+				if (fseek(fp, 0L, SEEK_END) == -1) {
+					ierr();
+					return;
+				}
 			}
-			add_events = 1;
-		} else if (ev->data < 0) {
-			/* file shrank, reposition to end */
-			if (fseek(fp, 0L, SEEK_END) == -1) {
-				ierr();
-				return;
+			break;
+
+		case USE_SLEEP:
+                	(void) usleep(250000);
+	                clearerr(fp);
+
+			if (Fflag && fileno(fp) != STDIN_FILENO &&
+			    stat(fname, &sb2) != -1) {
+				if (sb2.st_ino != sbp->st_ino ||
+				    sb2.st_dev != sbp->st_dev ||
+				    sb2.st_rdev != sbp->st_rdev ||
+				    sb2.st_nlink == 0) {
+					fp = freopen(fname, "r", fp);
+					if (fp == NULL) {
+						ierr();
+						break;
+					}
+					*sbp = sb2;
+					if (kq != -1)
+						action = ADD_EVENTS;
+				}
 			}
+			break;
 		}
 	}
 }