install: isolated linker honors active bun link

src/install/PackageManager.zig

@@ -95,6 +95,16 @@ global_link_dir: ?std.fs.Dir = null,
 global_dir: ?std.fs.Dir = null,
 global_link_dir_path: string = "",
 
+/// Names of packages registered via `bun link`, read from
+/// `<globalLinkDir>/` once per install. Populated on the main thread
+/// before any install worker starts; after that it's read-only and can
+/// be accessed lock-free from worker threads. Lookups return early
+/// when the set is empty (no active links) — the common case on dev
+/// machines without `bun link` configured, and unconditional on CI.
+/// See `populateLinkedNamesCache` / `linkedPackagePath`.
+linked_names: bun.StringHashMapUnmanaged(void) = .{},
+linked_names_populated: bool = false,
+
 onWake: WakeHandler = .{},
 ci_mode: bun.LazyBool(computeIsContinuousIntegration, @This(), "ci_mode") = .{},
 
@@ -1177,6 +1187,8 @@ pub const getTemporaryDirectory = directories.getTemporaryDirectory;
 pub const globalLinkDir = directories.globalLinkDir;
 pub const globalLinkDirAndPath = directories.globalLinkDirAndPath;
 pub const globalLinkDirPath = directories.globalLinkDirPath;
+pub const linkedPackagePath = directories.linkedPackagePath;
+pub const populateLinkedNamesCache = directories.populateLinkedNamesCache;
 pub const isFolderInCache = directories.isFolderInCache;
 pub const pathForCachedNPMPath = directories.pathForCachedNPMPath;
 pub const pathForResolution = directories.pathForResolution;

src/install/PackageManager/PackageManagerDirectories.zig

@@ -410,6 +410,105 @@ pub fn globalLinkDirAndPath(this: *PackageManager) struct { std.fs.Dir, []const
     return .{ dir, this.global_link_dir_path };
 }
 
+/// Read the global link dir once and populate
+/// `this.linked_names` with every registered package name (including
+/// scoped names as `@scope/name`). Must be called on the main thread
+/// before any install worker touches `linkedPackagePath`; after that
+/// the map is read-only and lock-free.
+///
+/// Safe to call repeatedly; subsequent calls are no-ops.
+pub fn populateLinkedNamesCache(this: *PackageManager) void {
+    if (this.linked_names_populated) return;
+    this.linked_names_populated = true;
+
+    const dir_path = this.globalLinkDirPath();
+    const root_fd = switch (bun.openDirForIteration(bun.FD.cwd(), dir_path)) {
+        .result => |fd| fd,
+        // Dir missing / unreadable → empty set. Every linkedPackagePath
+        // lookup will short-circuit to null with no further syscalls.
+        .err => return,
+    };
+    defer root_fd.close();
+
+    var iter = bun.DirIterator.iterate(root_fd, if (bun.Environment.isWindows) .u16 else .u8);
+    while (iter.next().unwrap() catch null) |entry| {
+        const name = entry.name.slice();
+        if (name.len == 0) continue;
+
+        // Scope dirs (`@scope`) contain the actual links nested one
+        // level deeper; flatten to `@scope/name` in the cache.
+        if (name[0] == '@' and entry.kind == .directory) {
+            if (comptime bun.Environment.isWindows) {
+                // WTF-16 name; skip scope flattening on Windows for now.
+                // Falls through to the lstat path in linkedPackagePath.
+                continue;
+            }
+            const scope_fd = switch (bun.openDirForIteration(root_fd, name)) {
+                .result => |fd| fd,
+                .err => continue,
+            };
+            defer scope_fd.close();
+
+            var scope_iter = bun.DirIterator.iterate(scope_fd, .u8);
+            while (scope_iter.next().unwrap() catch null) |scope_entry| {
+                const sub_name = scope_entry.name.slice();
+                if (sub_name.len == 0) continue;
+                const full = std.fmt.allocPrint(this.allocator, "{s}/{s}", .{ name, sub_name }) catch continue;
+                this.linked_names.put(this.allocator, full, {}) catch continue;
+            }
+            continue;
+        }
+
+        if (comptime bun.Environment.isWindows) continue;
+        const dup = this.allocator.dupe(u8, name) catch continue;
+        this.linked_names.put(this.allocator, dup, {}) catch continue;
+    }
+}
+
+/// If `<globalLinkDir>/<pkg_name>` exists (typically a symlink created by
+/// `bun link` from the producer dir), write its absolute path into `buf` and
+/// return it. Otherwise `null`. Scoped names (`@scope/name`) are handled
+/// because `joinAbsStringBufZ` preserves the `/`.
+///
+/// Performance: when `linked_names` has been populated (via
+/// `populateLinkedNamesCache` at install start), this is a single
+/// hashmap check with no syscalls. When the cache is empty (no active
+/// links on this machine), it returns immediately. Falls back to the
+/// per-call `lstat` when the cache has not been populated (e.g. on
+/// Windows, or if a caller runs outside the isolated-install flow).
+pub fn linkedPackagePath(
+    this: *PackageManager,
+    pkg_name: []const u8,
+    buf: *bun.PathBuffer,
+) ?[:0]const u8 {
+    if (pkg_name.len == 0) return null;
+
+    const use_cache = this.linked_names_populated and !bun.Environment.isWindows;
+    if (use_cache) {
+        if (this.linked_names.count() == 0) return null;
+        if (!this.linked_names.contains(pkg_name)) return null;
+        const dir_path = this.globalLinkDirPath();
+        return bun.path.joinAbsStringBufZ(dir_path, buf, &.{pkg_name}, .auto);
+    }
+
+    const dir_path = this.globalLinkDirPath();
+    const joined = bun.path.joinAbsStringBufZ(dir_path, buf, &.{pkg_name}, .auto);
+    if (comptime bun.Environment.isWindows) {
+        const attrs = bun.sys.getFileAttributes(joined) orelse return null;
+        return if (attrs.is_directory or attrs.is_reparse_point) joined else null;
+    }
+    return switch (bun.sys.lstat(joined)) {
+        .result => |st| brk: {
+            const mode: u32 = @intCast(st.mode);
+            if (std.posix.S.ISDIR(mode) or std.posix.S.ISLNK(mode)) {
+                break :brk joined;
+            }
+            break :brk null;
+        },
+        .err => null,
+    };
+}
+
 pub fn pathForCachedNPMPath(
     this: *PackageManager,
     buf: *bun.PathBuffer,

src/install/PackageManager/patchPackage.zig

@@ -835,6 +835,7 @@ fn overwritePackageInNodeModulesFolder(
         .fromStdDir(cached_package_folder),
         src_path,
         dest_subpath,
+        &.{},
         ignore_directories,
     );
     defer copier.deinit();

src/install/isolated_install.zig

@@ -10,6 +10,15 @@ pub fn installIsolatedPackages(
 ) OOM!PackageInstall.Summary {
     bun.analytics.Features.isolated_bun_install += 1;
 
+    // Populate the linked-names cache once, on the main thread, before
+    // any install worker calls `manager.linkedPackagePath()`. Replaces a
+    // per-dependency `lstat` with a single readdir of the global link
+    // dir; short-circuits every subsequent lookup when nothing is
+    // linked (the common case on CI and dev machines without active
+    // links). See populateLinkedNamesCache in
+    // PackageManager/PackageManagerDirectories.zig.
+    manager.populateLinkedNamesCache();
+
     const lockfile = manager.lockfile;
 
     const store: Store = store: {
@@ -1577,6 +1586,17 @@ pub fn installIsolatedPackages(
                     // write the new project-local tree through the link into
                     // the shared cache). Treat the stale link as
                     // needs-install so `link_package` detaches and rebuilds.
+                    // An active `bun link` for this package name means the
+                    // producer dir is the source of truth — override the
+                    // cache-based materialization regardless of whether the
+                    // store dir already exists. The existence check below
+                    // would otherwise short-circuit and leave the body stale.
+                    // Only fires when the user didn't opt into the
+                    // symlink-only backend.
+                    var link_buf: bun.PathBuffer = undefined;
+                    const has_active_link = PackageInstall.supported_method != .symlink and
+                        manager.linkedPackagePath(pkg_name.slice(string_buf), &link_buf) != null;
+
                     const has_stale_gvs_link = !uses_global_store and stale: {
                         if (installer.global_store_path == null) break :stale false;
                         var local: bun.Path(.{ .sep = .auto }) = .initTopLevelDir();
@@ -1599,6 +1619,7 @@ pub fn installIsolatedPackages(
                         // should still take the cheap symlink-only path.
                         (is_new_bun_modules and !uses_global_store) or
                         has_stale_gvs_link or
+                        has_active_link or
                         patch_info == .remove or
                         needs_install: {
                             var store_path: bun.AbsPath(.{}) = .initTopLevelDir();
@@ -1657,6 +1678,15 @@ pub fn installIsolatedPackages(
                         continue;
                     }
 
+                    // `link_package` will source from the producer dir via
+                    // `linkedPackagePath`; skip the cache-fetch dance entirely
+                    // (mirrors how `.folder` is handled — no registry traffic
+                    // needed when the body comes from an on-disk producer).
+                    if (has_active_link) {
+                        installer.startTask(entry_id);
+                        continue;
+                    }
+
                     var pkg_cache_dir_subpath: bun.RelPath(.{ .sep = .auto }) = .from(switch (pkg_res_tag) {
                         .npm => manager.cachedNPMPackageFolderName(pkg_name.slice(string_buf), pkg_res.value.npm.version, patch_info.contentsHash()),
                         .git => manager.cachedGitFolderName(&pkg_res.value.git, patch_info.contentsHash()),

src/install/isolated_install/FileCopier.zig

@@ -7,6 +7,7 @@ pub const FileCopier = struct {
         src_dir: FD,
         src_path: bun.AbsPath(.{ .sep = .auto, .unit = .os }),
         dest_subpath: bun.Path(.{ .sep = .auto, .unit = .os }),
+        skip_filenames: []const bun.OSPathSlice,
         skip_dirnames: []const bun.OSPathSlice,
     ) OOM!FileCopier {
         return .{
@@ -16,7 +17,7 @@ pub const FileCopier = struct {
                 var w = try Walker.walk(
                     src_dir,
                     bun.default_allocator,
-                    &.{},
+                    skip_filenames,
                     skip_dirnames,
                 );
                 w.resolve_unknown_entry_types = true;

src/install/isolated_install/Hardlinker.zig

@@ -9,6 +9,7 @@ pub fn init(
     folder_dir: FD,
     src: bun.AbsPath(.{ .sep = .auto, .unit = .os }),
     dest: bun.Path(.{ .sep = .auto, .unit = .os }),
+    skip_filenames: []const bun.OSPathSlice,
     skip_dirnames: []const bun.OSPathSlice,
 ) OOM!Hardlinker {
     return .{
@@ -19,7 +20,7 @@ pub fn init(
             var w = try Walker.walk(
                 folder_dir,
                 bun.default_allocator,
-                &.{},
+                skip_filenames,
                 skip_dirnames,
             );
             w.resolve_unknown_entry_types = true;