mmap_seek: print error code and text on failure

If lseek() returns an unexpected error, it's useful to know the error
code to help connect it to the trouble spot inside the module.

Since the two seek functions should be basically identical, lift them
into a single generic function.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Robert Evans <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
Closes openzfs#17843

Author: robn

tests/zfs-tests/cmd/mmap_seek.c

@@ -47,25 +47,34 @@
 #endif
 
 static void
+seek_expect(int fd, off_t offset, int whence, off_t expect_offset)
+{
+	errno = 0;
+	off_t seek_offset = lseek(fd, offset, whence);
+	if (seek_offset == expect_offset)
+		return;
+
+	int err = errno;
+	fprintf(stderr, "lseek(fd, %ld, SEEK_%s) = %ld (expected %ld)",
+	    offset, (whence == SEEK_DATA ? "DATA" : "HOLE"),
+	    seek_offset, expect_offset);
+	if (err != 0)
+		fprintf(stderr, " (errno %d [%s])\n", err, strerror(err));
+	else
+		fputc('\n', stderr);
+	exit(2);
+}
+
+static inline void
 seek_data(int fd, off_t offset, off_t expected)
 {
-	off_t data_offset = lseek(fd, offset, SEEK_DATA);
-	if (data_offset != expected) {
-		fprintf(stderr, "lseek(fd, %d, SEEK_DATA) = %d (expected %d)\n",
-		    (int)offset, (int)data_offset, (int)expected);
-		exit(2);
-	}
+	seek_expect(fd, offset, SEEK_DATA, expected);
 }
 
-static void
+static inline void
 seek_hole(int fd, off_t offset, off_t expected)
 {
-	off_t hole_offset = lseek(fd, offset, SEEK_HOLE);
-	if (hole_offset != expected) {
-		fprintf(stderr, "lseek(fd, %d, SEEK_HOLE) = %d (expected %d)\n",
-		    (int)offset, (int)hole_offset, (int)expected);
-		exit(2);
-	}
+	seek_expect(fd, offset, SEEK_HOLE, expected);
 }
 
 int