From 00b7eab9faeac851f961ea6cde9b8cdc2b5acaa2 Mon Sep 17 00:00:00 2001
From: robobun <117481402+robobun@users.noreply.github.com>
Date: Mon, 11 May 2026 09:37:05 +0000
Subject: [PATCH] Fix capacity under-allocation in ESMConditions.init

The expression `if (allow_addons) 1 else 0 + conditions.len` parses as
`if (allow_addons) 1 else (0 + conditions.len)`, so when allow_addons is
true (the default), conditions.len was dropped from the capacity
calculation and putAssumeCapacity would overflow the map when enough
user conditions were passed to Bun.build.
---
 src/bundler/options.zig            |  7 ++++---
 test/bundler/bun-build-api.test.ts | 21 +++++++++++++++++++++
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/src/bundler/options.zig b/src/bundler/options.zig
index daea5ced59a3..e2cca4ba92a5 100644
--- a/src/bundler/options.zig
+++ b/src/bundler/options.zig
@@ -1087,9 +1087,10 @@ pub const ESMConditions = struct {
         var require_condition_map = ConditionsMap.init(allocator);
         var style_condition_map = ConditionsMap.init(allocator);
 
-        try default_condition_amp.ensureTotalCapacity(defaults.len + 2 + if (allow_addons) 1 else 0 + conditions.len);
-        try import_condition_map.ensureTotalCapacity(defaults.len + 2 + if (allow_addons) 1 else 0 + conditions.len);
-        try require_condition_map.ensureTotalCapacity(defaults.len + 2 + if (allow_addons) 1 else 0 + conditions.len);
+        const addons: usize = if (allow_addons) 1 else 0;
+        try default_condition_amp.ensureTotalCapacity(defaults.len + 2 + addons + conditions.len);
+        try import_condition_map.ensureTotalCapacity(defaults.len + 2 + addons + conditions.len);
+        try require_condition_map.ensureTotalCapacity(defaults.len + 2 + addons + conditions.len);
         try style_condition_map.ensureTotalCapacity(defaults.len + 2 + conditions.len);
 
         import_condition_map.putAssumeCapacity("import", {});
diff --git a/test/bundler/bun-build-api.test.ts b/test/bundler/bun-build-api.test.ts
index c1112743f02c..07de7d2d4868 100644
--- a/test/bundler/bun-build-api.test.ts
+++ b/test/bundler/bun-build-api.test.ts
@@ -649,6 +649,27 @@ describe("Bun.build", () => {
       expect(await html?.text()).toContain("<meta name='injected-by-plugin' content='true'>");
     },
   );
+
+  test.concurrent("many custom conditions does not crash", async () => {
+    const dir = tempDirWithFiles("bun-build-api-many-conditions", {
+      "entry.js": "export const x = 1;\n",
+    });
+    const conditions = Array.from({ length: 64 }, (_, i) => "cond" + i);
+    await using proc = Bun.spawn({
+      cmd: [
+        bunExe(),
+        "-e",
+        `const r = await Bun.build({ entrypoints: [${JSON.stringify(join(dir, "entry.js"))}], conditions: ${JSON.stringify(conditions)} }); if (!r.success) throw new AggregateError(r.logs); console.log("ok");`,
+      ],
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+    const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+    expect(stderr).toBe("");
+    expect(stdout).toBe("ok\n");
+    expect(exitCode).toBe(0);
+  });
 });
 
 test.concurrent("macro with nested object", async () => {