Add a tor_ftruncate to replace ftruncate.

(Windows doesn't have ftruncate, and some ftruncates do not move the
file pointer to the start of the file.)

src/common/compat.c

@@ -1004,6 +1004,23 @@ tor_fd_setpos(int fd, off_t pos)
 #endif
 }
 
+/** Replacement for ftruncate(fd, 0): move to the front of the file and remove
+ * all the rest of the file. Return -1 on error, 0 on success. */
+int
+tor_ftruncate(int fd)
+{
+  /* Rumor has it that some versions of ftruncate do not move the file pointer.
+   */
+  if (tor_fd_setpos(fd, 0) < 0)
+    return -1;
+
+#ifdef _WIN32
+  return _chsize(fd, 0);
+#else
+  return ftruncate(fd, 0);
+#endif
+}
+
 #undef DEBUG_SOCKET_COUNTING
 #ifdef DEBUG_SOCKET_COUNTING
 /** A bitarray of all fds that should be passed to tor_socket_close(). Only

src/common/compat.h

@@ -408,6 +408,7 @@ void tor_lockfile_unlock(tor_lockfile_t *lockfile);
 off_t tor_fd_getpos(int fd);
 int tor_fd_setpos(int fd, off_t pos);
 int tor_fd_seekend(int fd);
+int tor_ftruncate(int fd);
 
 #ifdef _WIN32
 #define PATH_SEPARATOR "\\"

src/common/log.c

@@ -1305,6 +1305,10 @@ switch_logs_debug(void)
 void
 truncate_logs(void)
 {
-  for (logfile_t *lf = logfiles; lf; lf = lf->next)
-    ftruncate(lf->fd, 0);
+  for (logfile_t *lf = logfiles; lf; lf = lf->next) {
+    if (lf->fd >= 0) {
+      tor_ftruncate(lf->fd);
+    }
+  }
 }
+