inotail: Support reopening rotated files

We changed the behaviour of the "follow" option so that it detects that
file has been moved or deleted it keeps monitoring for the file instead
of ignoring it.  This is useful when tailing syslog circular logs, where
syslog periodically moves the log to another filename and recreates the
log file.  Without the patch inotail would issue a "file has been moved"
or a "file has been deleted message" and stop tracking the file. Now it
will close the file and refresh the inotify +descriptor. When a
notification arrives with a CREATE or MODIFY event, it will reopen the
file and set the size to zero.

We are using the patched program in an embedded ARM system with
satisfactory results.

2 files changed, 42 insertions, 15 deletions

diff --git a/inotail.c b/inotail.c
index 735c4b7..de16a60 100644
--- a/inotail.c
+++ b/inotail.c
@@ -615,15 +615,28 @@ static int tail_file(struct file_struct *f, unsigned long n_units, char mode)
 	return 0;
 }
 
-static int handle_inotify_event(struct inotify_event *inev, struct file_struct *f)
+static int handle_inotify_event(int ifd, struct inotify_event *inev, struct file_struct *f)
 {
 	int ret = 0;
 
-	if (inev->mask & IN_MODIFY) {
+	if (inev->mask & (IN_MODIFY|IN_CREATE)) {
 		char *fbuf;
 		ssize_t bytes_read;
 		struct stat finfo;
 
+		if (f->fd < 0) {
+			fprintf(stderr, "File '%s' needs to get reopened.\n", f->name);
+			f->fd = open(f->name, O_RDONLY);
+			if (unlikely(f->fd < 0)) {
+				fprintf(stderr, "Error: Could not open file '%s' (%s)\n", f->name, strerror(errno));
+				ignore_file(f);
+				return -1;
+			}
+
+			/* File got rotated away, so start again */
+			f->size = 0;
+		}
+
 		if (verbose)
 			write_header(f->name);
 
@@ -653,10 +666,24 @@ static int handle_inotify_event(struct inotify_event *inev, struct file_struct *
 
 		free(fbuf);
 		return ret;
-	} else if (inev->mask & IN_DELETE_SELF) {
-		fprintf(stderr, "File '%s' deleted.\n", f->name);
-	} else if (inev->mask & IN_MOVE_SELF) {
-		fprintf(stderr, "File '%s' moved.\n", f->name);
+	} else if (inev->mask & (IN_DELETE_SELF|IN_MOVE_SELF)) {
+		inotify_rm_watch(ifd, f->i_watch);
+		close(f->fd);
+		f->fd = -1;
+
+		if (inev->mask & IN_DELETE_SELF)
+			fprintf(stderr, "File '%s' deleted.\n", f->name);
+		else
+			fprintf(stderr, "File '%s' moved.\n", f->name);
+
+		f->i_watch = inotify_add_watch(ifd, f->name, INOTAIL_WATCH_MASK);
+		if (f->i_watch < 0) {
+			fprintf(stderr, "Error: Could not create inotify watch on file '%s' (%s)\n",
+					f->name, strerror(errno));
+			ignore_file(f);
+			return -1;
+		}
+
 		return 0;
 	} else if (inev->mask & IN_UNMOUNT) {
 		fprintf(stderr, "Device containing file '%s' unmounted.\n", f->name);
@@ -685,8 +712,7 @@ static int watch_files(struct file_struct *files, int n_files)
 
 	for (i = 0; i < n_files; i++) {
 		if (!files[i].ignore) {
-			files[i].i_watch = inotify_add_watch(ifd, files[i].name,
-						IN_MODIFY|IN_DELETE_SELF|IN_MOVE_SELF|IN_UNMOUNT);
+			files[i].i_watch = inotify_add_watch(ifd, files[i].name, INOTAIL_WATCH_MASK);
 
 			if (files[i].i_watch < 0) {
 				fprintf(stderr, "Error: Could not create inotify watch on file '%s' (%s)\n",
@@ -720,9 +746,7 @@ static int watch_files(struct file_struct *files, int n_files)
 
 			/* Which file has produced the event? */
 			for (i = 0; i < n_files; i++) {
-				if (!files[i].ignore
-						&& files[i].fd >= 0
-						&& files[i].i_watch == inev->wd) {
+				if (!files[i].ignore && files[i].i_watch == inev->wd) {
 					f = &files[i];
 					break;
 				}
@@ -734,7 +758,7 @@ static int watch_files(struct file_struct *files, int n_files)
 				continue;
 			}
 
-			if (handle_inotify_event(inev, f) < 0 && n_ignored == n_files)
+			if (handle_inotify_event(ifd, inev, f) < 0 && n_ignored == n_files)
 				/* Got an error handling the event and no files
 				 * left unignored */
 				break;
diff --git a/inotail.h b/inotail.h
index 5b0ef0c..3320c3d 100644
--- a/inotail.h
+++ b/inotail.h
@@ -10,10 +10,13 @@
 #include <sys/types.h>
 #include <sys/inotify.h>
 
-#define DEFAULT_N_LINES 10	/* Number of items to tail. */
-
+/* Number of items to tail. */
+#define DEFAULT_N_LINES		10
 /* inotify event buffer length for one file */
-#define INOTIFY_BUFLEN (4 * sizeof(struct inotify_event))
+#define INOTIFY_BUFLEN		(4 * sizeof(struct inotify_event))
+/* inotify events to watch for on tailed files */
+#define INOTAIL_WATCH_MASK	\
+	(IN_MODIFY|IN_DELETE_SELF|IN_MOVE_SELF|IN_UNMOUNT|IN_CREATE)
 
 /* tail modes */
 enum tail_mode { M_LINES, M_BYTES };