gui: harden the restart relaunch (ultra edge cases)

Follow-up to the Restart Now button. Addresses the edge cases the review
flagged, and drops the fragile batch layer entirely:

- Windows no longer writes a .bat and shells through cmd (which re-parsed
  the arguments and could leave a stray file on failure). It builds the
  command line once, quoted correctly (only backslashes before a quote are
  doubled, so ordinary paths are not mangled), and CreateProcess launches
  the new instance directly.
- The delay moved into the new instance: a -restartwait flag makes it wait
  ~2.5s before it opens the network port and the block index, so the old
  process is gone first. No shell delay hack. Verified: -restartwait adds
  the wait (0.16s -> 2.67s).
- Linux re-execs /proc/self/exe (absolute) and only falls back to argv[0],
  which may be relative or a bare PATH name that execv cannot resolve.
- This instance now exits only if the relaunch actually started; on failure
  it keeps running and tells the user to restart manually, so a failed
  relaunch never leaves them with no node.

GUI/process wiring only; wallet code unchanged, suites still green.

1 files changed, 8 insertions, 0 deletions

diff --git a/src/main_gui.cpp b/src/main_gui.cpp
index 6cd8b75..55ca8c6 100644
--- a/src/main_gui.cpp
+++ b/src/main_gui.cpp
@@ -509,6 +509,14 @@ int main(int argc, char* argv[])
 
     ParseStartupArguments(argc, argv);
 
+#ifdef _WIN32
+    // Set by the GUI "Restart Now" button: give the previous instance a moment
+    // to release the network port and the block-index files before this one
+    // opens them, so a relaunch does not race the process it is replacing.
+    if (arg(argc, argv, "/restartwait") || arg(argc, argv, "-restartwait"))
+        Sleep(2500);
+#endif
+
     // Hidden self-test helper. It must run before LoadWallet(): several
     // storage-sanity scenarios deliberately make wallet.dat unsafe to load.
     string strSelfTestMutateWallet =