From a7387f0047a1bb67c31e947a251171834a3f9817 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Sun, 6 Jul 2025 18:20:11 -0700
Subject: [PATCH] fix std.io.Writer.VectorWrapper mechanism

previous implementation erroneously attempted to detect whether vector
API was used via the buffer pointer. This implementation has an explicit
flag.
---
 lib/std/io/Reader.zig | 15 +++++++++++----
 lib/std/io/Writer.zig |  3 +++
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/lib/std/io/Reader.zig b/lib/std/io/Reader.zig
index a9e790a437..914f27d3ef 100644
--- a/lib/std/io/Reader.zig
+++ b/lib/std/io/Reader.zig
@@ -328,17 +328,24 @@ pub fn readVecLimit(r: *Reader, data: []const []u8, limit: Limit) Error!usize {
         };
         var n = r.vtable.stream(r, &wrapper.writer, .limited(remaining)) catch |err| switch (err) {
             error.WriteFailed => {
+                assert(!wrapper.used);
                 if (wrapper.writer.buffer.ptr == first.ptr) {
                     remaining -= wrapper.writer.end;
                 } else {
+                    assert(wrapper.writer.end <= r.buffer.len);
                     r.end = wrapper.writer.end;
                 }
                 break;
             },
             else => |e| return e,
         };
-        if (wrapper.writer.buffer.ptr != first.ptr) {
-            r.end = n;
+        if (!wrapper.used) {
+            if (wrapper.writer.buffer.ptr == first.ptr) {
+                remaining -= n;
+            } else {
+                assert(n <= r.buffer.len);
+                r.end = n;
+            }
             break;
         }
         if (n < first.len) {
@@ -355,6 +362,7 @@ pub fn readVecLimit(r: *Reader, data: []const []u8, limit: Limit) Error!usize {
             remaining -= mid.len;
             n -= mid.len;
         }
+        assert(n <= r.buffer.len);
         r.end = n;
         break;
     }
@@ -596,12 +604,11 @@ pub fn readSliceShort(r: *Reader, buffer: []u8) ShortError!usize {
         };
         const n = r.vtable.stream(r, &wrapper.writer, .unlimited) catch |err| switch (err) {
             error.WriteFailed => {
-                if (wrapper.writer.buffer.ptr != remaining.ptr) {
+                if (!wrapper.used) {
                     assert(r.seek == 0);
                     r.seek = remaining.len;
                     r.end = wrapper.writer.end;
                     @memcpy(remaining, r.buffer[0..remaining.len]);
-                    return buffer.len;
                 }
                 return buffer.len;
             },
diff --git a/lib/std/io/Writer.zig b/lib/std/io/Writer.zig
index a5cb0aded1..b1d1f78064 100644
--- a/lib/std/io/Writer.zig
+++ b/lib/std/io/Writer.zig
@@ -390,12 +390,15 @@ pub const WritableVectorIterator = struct {
 pub const VectorWrapper = struct {
     writer: Writer,
     it: WritableVectorIterator,
+    /// Tracks whether the "writable vector" API was used.
+    used: bool = false,
     pub const vtable: VTable = .{ .drain = fixedDrain };
 };
 
 pub fn writableVectorIterator(w: *Writer) Error!WritableVectorIterator {
     if (w.vtable == &VectorWrapper.vtable) {
         const wrapper: *VectorWrapper = @fieldParentPtr("writer", w);
+        wrapper.used = true;
         return wrapper.it;
     }
     return .{ .first = try writableSliceGreedy(w, 1) };