tail: Add rudimentary support to detect file truncation

We cannot in general detect that truncation happened.  At the moment
we use a heuristic to compare the file size before and after a write
happened.  If the new file size is smaller than the old, we correctly
handle truncation and dump the entire file to stdout.

If it so happened that the new size is larger or equal to the old size
after the file had been truncated without any reads in between, we will
assume the data was appended to the file.

There is no known way around this other than using inotify or kevent
which is outside the scope of sbase.

tail.c

@@ -1,4 +1,6 @@
 /* See LICENSE file for copyright and license details. */
+#include <sys/stat.h>
+
 #include <limits.h>
 #include <stdint.h>
 #include <stdio.h>
@@ -59,6 +61,7 @@ usage(void)
 int
 main(int argc, char *argv[])
 {
+	struct stat st1, st2;
 	FILE *fp;
 	size_t n = 10, tmpsize;
 	int ret = 0, newline, many;
@@ -96,6 +99,8 @@ main(int argc, char *argv[])
 			if (many)
 				printf("%s==> %s <==\n",
 				       newline ? "\n" : "", argv[0]);
+			if (stat(argv[0], &st1) < 0)
+				eprintf("stat %s:", argv[0]);
 			newline = 1;
 			tail(fp, argv[0], n);
 
@@ -108,8 +113,18 @@ main(int argc, char *argv[])
 						fflush(stdout);
 					}
 					if (ferror(fp))
-						eprintf("readline '%s':", argv[0]);
+						eprintf("readline %s:", argv[0]);
 					clearerr(fp);
+					/* ignore error in case file was removed, we continue
+					 * tracking the existing open file descriptor */
+					if (!stat(argv[0], &st2)) {
+						if (st2.st_size < st1.st_size) {
+							fprintf(stderr, "%s: file truncated\n", argv[0]);
+							rewind(fp);
+						}
+						st1 = st2;
+					}
+					sleep(1);
 				}
 			}
 			fclose(fp);