From 2ec5523d5a70db80bac8563e1f06d3b164f53db3 Mon Sep 17 00:00:00 2001
From: Ryan Liptak <squeek502@hotmail.com>
Date: Tue, 15 Sep 2026 15:18:34 -0700
Subject: [PATCH] Reader.allocRemaining: only error with StreamTooLong when
 limit is exceeded

Follow up to #36708 and #36692. The intended behavior of this function was what was previously documented: that StreamTooLong was only returned when the limit was *exceeded*. However, that was never implemented.

This commit implements the intended behavior, with the added caveat that `limit + 1` bytes will be discarded if the limit is exceeded.
---
 lib/std/Io/Reader.zig | 37 +++++++++++++++++++++++++++++++++----
 1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/lib/std/Io/Reader.zig b/lib/std/Io/Reader.zig
index da1894cc83b5e01e685cfce8cd96242ec943dcc1..372a7890b734638cf9fd7d4a8f99ca5a29c13c01 100644
--- a/lib/std/Io/Reader.zig
+++ b/lib/std/Io/Reader.zig
@@ -287,16 +287,20 @@ pub const LimitedAllocError = Allocator.Error || ShortError || error{StreamTooLo
 /// Transfers all bytes from the current position to the end of the stream, up
 /// to `limit`, returning them as a caller-owned allocated slice.
 ///
-/// If `limit` is reached or exceeded, `error.StreamTooLong` is returned
-/// instead. In such case, the next byte that would be read will be the first
-/// one to exceed `limit`, and all preceeding bytes have been discarded.
+/// If `limit` is exceeded, `error.StreamTooLong` is returned instead.
+/// In such case, the next byte that would be read will be one byte past the
+/// first one to exceed `limit`, and all preceeding bytes have been discarded.
 ///
 /// See also:
 /// * `appendRemaining`
 pub fn allocRemaining(r: *Reader, gpa: Allocator, limit: Limit) LimitedAllocError![]u8 {
     var buffer: ArrayList(u8) = .empty;
     defer buffer.deinit(gpa);
-    try appendRemaining(r, gpa, &buffer, limit);
+
+    try appendRemaining(r, gpa, &buffer, switch (limit) {
+        .unlimited => limit,
+        .nothing, _ => .limited(@backingInt(limit) + 1),
+    });
     return buffer.toOwnedSlice(gpa);
 }
 
@@ -1894,6 +1898,31 @@ test "takeStruct and peekStruct packed" {
     try testing.expectError(error.EndOfStream, r.takeStruct(S, .little));
 }
 
+test allocRemaining {
+    {
+        var r: Reader = .fixed("abc");
+        const str = try r.allocRemaining(testing.allocator, .unlimited);
+        defer testing.allocator.free(str);
+        try testing.expectEqualStrings("abc", str);
+    }
+    {
+        var r: Reader = .fixed("abc");
+        const str = try r.allocRemaining(testing.allocator, .limited(5));
+        defer testing.allocator.free(str);
+        try testing.expectEqualStrings("abc", str);
+    }
+    {
+        var r: Reader = .fixed("abc");
+        const str = try r.allocRemaining(testing.allocator, .limited(3));
+        defer testing.allocator.free(str);
+        try testing.expectEqualStrings("abc", str);
+    }
+    {
+        var r: Reader = .fixed("abcd");
+        try testing.expectError(error.StreamTooLong, r.allocRemaining(testing.allocator, .limited(3)));
+    }
+}
+
 /// Provides a `Reader` implementation by passing data from an underlying
 /// reader through `Hasher.update`.
 ///
-- 
2.54.0