From a76775b50a65fd0ea0fd17d6ef3c42058df13997 Mon Sep 17 00:00:00 2001
From: Ryan Liptak <squeek502@hotmail.com>
Date: Fri, 17 Jun 2022 22:05:41 -0700
Subject: [PATCH] Fix stack traces with non-null `first_address` on Windows

Before this commit, the passed in length would always be given to the RtlCaptureStackBackTrace call. Now we always give the length of the actual buffer we're using (the addr_buf_stack size of 32 or the passed in length if it's larger than 32; this matches what the doc comment says the function was meant to be doing as well).

This was causing empty stack traces for things like the GeneralPurposeAllocator leak checking.

Fixes #6687
---
 lib/std/debug.zig | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/lib/std/debug.zig b/lib/std/debug.zig
index af5d54ae62..54c614f270 100644
--- a/lib/std/debug.zig
+++ b/lib/std/debug.zig
@@ -180,11 +180,10 @@ pub fn dumpStackTraceFromBase(bp: usize, ip: usize) void {
 pub fn captureStackTrace(first_address: ?usize, stack_trace: *std.builtin.StackTrace) void {
     if (native_os == .windows) {
         const addrs = stack_trace.instruction_addresses;
-        const u32_addrs_len = @intCast(u32, addrs.len);
         const first_addr = first_address orelse {
             stack_trace.index = windows.ntdll.RtlCaptureStackBackTrace(
                 0,
-                u32_addrs_len,
+                @intCast(u32, addrs.len),
                 @ptrCast(**anyopaque, addrs.ptr),
                 null,
             );
@@ -192,7 +191,7 @@ pub fn captureStackTrace(first_address: ?usize, stack_trace: *std.builtin.StackT
         };
         var addr_buf_stack: [32]usize = undefined;
         const addr_buf = if (addr_buf_stack.len > addrs.len) addr_buf_stack[0..] else addrs;
-        const n = windows.ntdll.RtlCaptureStackBackTrace(0, u32_addrs_len, @ptrCast(**anyopaque, addr_buf.ptr), null);
+        const n = windows.ntdll.RtlCaptureStackBackTrace(0, @intCast(u32, addr_buf.len), @ptrCast(**anyopaque, addr_buf.ptr), null);
         const first_index = for (addr_buf[0..n]) |addr, i| {
             if (addr == first_addr) {
                 break i;
@@ -201,7 +200,8 @@ pub fn captureStackTrace(first_address: ?usize, stack_trace: *std.builtin.StackT
             stack_trace.index = 0;
             return;
         };
-        const slice = addr_buf[first_index..n];
+        const end_index = math.min(first_index + addrs.len, n);
+        const slice = addr_buf[first_index..end_index];
         // We use a for loop here because slice and addrs may alias.
         for (slice) |addr, i| {
             addrs[i] = addr;