From df46ee61c4a853c0cf43007306bdc44e578d0702 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Fri, 8 Aug 2025 16:43:24 -0700
Subject: [PATCH] std.Io.Writer.Allocating: configurable bump amount

---
 lib/std/Io/Writer.zig                 | 9 ++++++---
 lib/std/compress/flate/Decompress.zig | 9 +++++++++
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/lib/std/Io/Writer.zig b/lib/std/Io/Writer.zig
index f27e42ab47..997dd1e09f 100644
--- a/lib/std/Io/Writer.zig
+++ b/lib/std/Io/Writer.zig
@@ -2497,6 +2497,10 @@ pub fn Hashing(comptime Hasher: type) type {
 pub const Allocating = struct {
     allocator: Allocator,
     writer: Writer,
+    /// Every call to `drain` ensures at least this amount of unused capacity
+    /// before it returns. This prevents an infinite loop in interface logic
+    /// that calls `drain`.
+    minimum_unused_capacity: usize = 1,
 
     pub fn init(allocator: Allocator) Allocating {
         return .{
@@ -2607,14 +2611,13 @@ pub const Allocating = struct {
         const gpa = a.allocator;
         const pattern = data[data.len - 1];
         const splat_len = pattern.len * splat;
+        const bump = a.minimum_unused_capacity;
         var list = a.toArrayList();
         defer setArrayList(a, list);
         const start_len = list.items.len;
-        // Even if we append no data, this function needs to ensure there is more
-        // capacity in the buffer to avoid infinite loop, hence the +1 in this loop.
         assert(data.len != 0);
         for (data) |bytes| {
-            list.ensureUnusedCapacity(gpa, bytes.len + splat_len + 1) catch return error.WriteFailed;
+            list.ensureUnusedCapacity(gpa, bytes.len + splat_len + bump) catch return error.WriteFailed;
             list.appendSliceAssumeCapacity(bytes);
         }
         if (splat == 0) {
diff --git a/lib/std/compress/flate/Decompress.zig b/lib/std/compress/flate/Decompress.zig
index 625fce37d3..1908dda44f 100644
--- a/lib/std/compress/flate/Decompress.zig
+++ b/lib/std/compress/flate/Decompress.zig
@@ -73,7 +73,12 @@ const indirect_vtable: Reader.VTable = .{
     .readVec = readVec,
 };
 
+/// `input` buffer is asserted to be at least 10 bytes, or EOF before then.
+///
+/// If `buffer` is provided then asserted to have `flate.max_window_len`
+/// capacity, as well as `flate.history_len` unused capacity on every write.
 pub fn init(input: *Reader, container: Container, buffer: []u8) Decompress {
+    if (buffer.len != 0) assert(buffer.len >= flate.max_window_len);
     return .{
         .reader = .{
             .vtable = if (buffer.len == 0) &direct_vtable else &indirect_vtable,
@@ -234,6 +239,8 @@ fn decodeSymbol(self: *Decompress, decoder: anytype) !Symbol {
 }
 
 fn streamDirect(r: *Reader, w: *Writer, limit: std.Io.Limit) Reader.StreamError!usize {
+    assert(w.buffer.len >= flate.max_window_len);
+    assert(w.unusedCapacityLen() >= flate.history_len);
     const d: *Decompress = @alignCast(@fieldParentPtr("reader", r));
     return streamFallible(d, w, limit);
 }
@@ -1246,6 +1253,7 @@ test "zlib should not overshoot" {
 fn testFailure(container: Container, in: []const u8, expected_err: anyerror) !void {
     var reader: Reader = .fixed(in);
     var aw: Writer.Allocating = .init(testing.allocator);
+    aw.minimum_unused_capacity = flate.history_len;
     try aw.ensureUnusedCapacity(flate.max_window_len);
     defer aw.deinit();
 
@@ -1257,6 +1265,7 @@ fn testFailure(container: Container, in: []const u8, expected_err: anyerror) !vo
 fn testDecompress(container: Container, compressed: []const u8, expected_plain: []const u8) !void {
     var in: std.Io.Reader = .fixed(compressed);
     var aw: std.Io.Writer.Allocating = .init(testing.allocator);
+    aw.minimum_unused_capacity = flate.history_len;
     try aw.ensureUnusedCapacity(flate.max_window_len);
     defer aw.deinit();