From 73a65674e4d464c2588eb752da0aa4869c671c0a Mon Sep 17 00:00:00 2001
From: Matthew Lugg <mlugg@mlugg.co.uk>
Date: Thu, 13 Aug 2026 12:14:24 +0100
Subject: [PATCH] link.MappedFile: clarify some logic

No functional changes, just a small refactor so I can properly
understand what's happening in this logic.
---
 src/link/MappedFile.zig | 28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/src/link/MappedFile.zig b/src/link/MappedFile.zig
index fa8fe9e3936b87d4773d3e3b2abebe0b14ef017f..38636f01145e57f7deb260c919e0ff180c900bbf 100644
--- a/src/link/MappedFile.zig
+++ b/src/link/MappedFile.zig
@@ -856,19 +856,20 @@ fn resizeNode(
         ni.setLocationAssumeCapacity(mf, old_offset, new_size);
         return;
     }
-    if (is_linux and !mf.flags.fallocate_insert_range_unsupported and
-        node.flags.alignment.order(mf.flags.block_size).compare(.gte))
     insert_range: {
+        if (!is_linux) break :insert_range;
+        if (mf.flags.fallocate_insert_range_unsupported) break :insert_range;
+
+        // We need the node to be aligned to `mf.flags.block_size` in the file in order to use this
+        // fast path. It is not sufficient to check `node.flags.alignment`, because that doesn't
+        // necessarily mean that all *parent* nodes are equally aligned; instead we must compute the
+        // actual file offset.
         const range_file_offset = ni.fileLocation(mf, false).offset + old_size;
         const range_size = node.flags.alignment.forward(
             @intCast(requested_size +| requested_size / growth_factor),
         ) - old_size;
-
-        // If this node is being realigned, its current state might not
-        // meet the requirements for fallocate
-        if (!mf.flags.block_size.check(@intCast(range_file_offset)) or
-            !mf.flags.block_size.check(@intCast(range_size)))
-            break :insert_range;
+        if (!mf.flags.block_size.check(@intCast(range_file_offset))) break :insert_range;
+        if (!mf.flags.block_size.check(@intCast(range_size))) break :insert_range;
 
         mf.memory_map.write(io) catch |err| switch (err) {
             error.WouldBlock => return error.Unexpected, // file was not opened as non-blocking
@@ -1208,9 +1209,11 @@ fn moveRange(mf: *MappedFile, old_file_offset: u64, new_file_offset: u64, size:
     // make a copy of this node at the new location
     try mf.copyRange(old_file_offset, new_file_offset, size);
     // delete the copy of this node at the old location
-    if (is_linux and !mf.flags.fallocate_punch_hole_unsupported and
-        size >= mf.flags.block_size.toByteUnits() * 2 - 1) while (true)
-        switch (linux.errno(linux.fallocate(
+    if (is_linux and
+        !mf.flags.fallocate_punch_hole_unsupported and
+        size >= mf.flags.block_size.toByteUnits() * 2 - 1)
+    {
+        while (true) switch (linux.errno(linux.fallocate(
             mf.memory_map.file.handle,
             linux.FALLOC.FL_PUNCH_HOLE | linux.FALLOC.FL_KEEP_SIZE,
             @intCast(old_file_offset),
@@ -1224,13 +1227,14 @@ fn moveRange(mf: *MappedFile, old_file_offset: u64, new_file_offset: u64, size:
             .NOSPC => return error.NoSpaceLeft,
             .NOSYS, .OPNOTSUPP => {
                 mf.flags.fallocate_punch_hole_unsupported = true;
-                break;
+                break; // fall back to slow path
             },
             .PERM => return error.PermissionDenied,
             .SPIPE => return error.Unseekable,
             .TXTBSY => return error.FileBusy,
             else => |e| return std.posix.unexpectedErrno(e),
         };
+    }
     @memset(mf.memory_map.memory[@intCast(old_file_offset)..][0..@intCast(size)], 0);
 }
 
-- 
2.54.0