fix(builtin): make linker aspect run in constant time per target

Previously, for every transitive dependency of a node binary target, the
module_mappings_aspect had a runtime quadratic in the number of
mappings: It traverses the mappings of all direct dependencies in a loop
and called _link_mapping in its body, which again traverses the
mappings.

With this commit, the LinkerPackageMappingInfo provider is restructured
to hold depsets of mapping entries and node_modules root paths. In this
way the aspect invocations for the individual targets never traverse the
(partially constructed) mappings.
Instead, write_node_modules_manifest flattens the depset only after the
aspect has run. If this should ever become a performance bottleneck
again, the flattening could be moved into the execution phase by
letting a dedicated binary consume the depset wrappen in an Args object.

In an experiment with a single medium-sized package.json and a Node
binary depending on all targets in the corresponding npm repository,
this commit reduces the analysis wall time spent in the aspect
implementation from ~30s to ~70ms.

internal/linker/link_node_modules.bzl

@@ -21,7 +21,17 @@ def _debug(vars, *args):
 LinkerPackageMappingInfo = provider(
     doc = """Provider capturing package mappings for the linker to consume.""",
     fields = {
-        "mappings": "Dictionary of mappings. Maps package names to an exec path.",
+        "mappings": """Depset of structs with mapping info.
+
+Each struct has the following fields:
+  package_name: The name of the package.
+  package_path: The root path of the node_modules under which this package should be linked.
+  link_path: The exec path under which the package is available.
+
+Note: The depset may contain multiple entries per (package_name, package_path) pair.
+Consumers should handle these duplicated appropriately. The depset uses topological order to ensure
+that a target's mappings come before possibly conflicting mappings of its dependencies.""",
+        "node_modules_roots": "Depset of node_module roots.",
     },
 )
 
@@ -40,39 +50,54 @@ def add_arg(args, arg):
     else:
         args.add(arg)
 
-def _link_mapping(label, mappings, k, v):
-    # Check that two package name mapping do not map to two different source paths
-    k_segs = k.split(":")
-    package_name = k_segs[0]
-    package_path = k_segs[1] if len(k_segs) > 1 else ""
-    link_path = v
-
-    for iter_key, iter_values in mappings.items():
-        # Map key is of format "package_name:package_path"
-        # Map values are of format [deprecated, link_path]
-        iter_segs = iter_key.split(":")
-        iter_package_name = iter_segs[0]
-        iter_package_path = iter_segs[1] if len(iter_segs) > 1 else ""
-        iter_source_path = iter_values
-        if package_name == iter_package_name and package_path == iter_package_path:
-            # If we're linking to the output tree be tolerant of linking to different
-            # output trees since we can have "static" links that come from cfg="exec" binaries.
-            # In the future when we static link directly into runfiles without the linker
-            # we can remove this logic.
-            link_path_segments = link_path.split("/")
-            iter_source_path_segments = iter_source_path.split("/")
-            bin_links = len(link_path_segments) >= 3 and len(iter_source_path_segments) >= 3 and link_path_segments[0] == "bazel-out" and iter_source_path_segments[0] == "bazel-out" and link_path_segments[2] == "bin" and iter_source_path_segments[2] == "bin"
-            if bin_links:
-                compare_link_path = "/".join(link_path_segments[3:]) if len(link_path_segments) > 3 else ""
-                compare_iter_source_path = "/".join(iter_source_path_segments[3:]) if len(iter_source_path_segments) > 3 else ""
-            else:
-                compare_link_path = link_path
-                compare_iter_source_path = iter_source_path
-            if compare_link_path != compare_iter_source_path:
-                msg = "conflicting mapping at '%s': '%s' and '%s' map to conflicting %s and %s" % (label, k, iter_key, compare_link_path, compare_iter_source_path)
-                fail(msg)
-
-    return True
+def _detect_conflicts(module_sets, mapping):
+    """Verifies that the new mapping does not conflict with existing mappings in module_sets."""
+    if mapping.package_path not in module_sets:
+        return
+    existing_link_path = module_sets[mapping.package_path].get(mapping.package_name)
+    if existing_link_path == None:
+        return
+
+    # If we're linking to the output tree be tolerant of linking to different
+    # output trees since we can have "static" links that come from cfg="exec" binaries.
+    # In the future when we static link directly into runfiles without the linker
+    # we can remove this logic.
+    link_path_segments = mapping.link_path.split("/")
+    existing_link_path_segments = existing_link_path.split("/")
+    bin_links = len(link_path_segments) >= 3 and len(existing_link_path_segments) >= 3 and link_path_segments[0] == "bazel-out" and existing_link_path_segments[0] == "bazel-out" and link_path_segments[2] == "bin" and existing_link_path_segments[2] == "bin"
+    if bin_links:
+        compare_link_path = "/".join(link_path_segments[3:]) if len(link_path_segments) > 3 else ""
+        compare_existing_link_path = "/".join(existing_link_path_segments[3:]) if len(existing_link_path_segments) > 3 else ""
+    else:
+        compare_link_path = mapping.link_path
+        compare_existing_link_path = existing_link_path
+    if compare_link_path != compare_existing_link_path:
+        msg = "conflicting mapping: '%s' (package path: %s) maps to conflicting paths '%s' and '%s'" % (mapping.package_name, mapping.package_path, compare_link_path, compare_existing_link_path)
+        fail(msg)
+
+def _flatten_to_module_set(mappings_depset):
+    """Convert a depset of mapping to a module sets (modules per package package_path).
+
+    The returned dictionary has the following structure:
+    {
+        "package_path": {
+            "package_name": "link_path",
+            ...
+        },
+        ...
+    }
+    """
+
+    # FIXME: Flattens a depset during the analysis phase. Ideally, this would be done during the
+    # execution phase using an Args object.
+    flattens_mappings = mappings_depset.to_list()
+    module_sets = {}
+    for mapping in flattens_mappings:
+        _detect_conflicts(module_sets, mapping)
+        if mapping.package_path not in module_sets:
+            module_sets[mapping.package_path] = {}
+        module_sets[mapping.package_path][mapping.package_name] = mapping.link_path
+    return module_sets
 
 def write_node_modules_manifest(ctx, extra_data = [], mnemonic = None, link_workspace_root = False):
     """Writes a manifest file read by the linker, containing info about resolving runtime dependencies
@@ -85,7 +110,6 @@ def write_node_modules_manifest(ctx, extra_data = [], mnemonic = None, link_work
             If source files need to be required then they can be copied to the bin_dir with copy_to_bin.
     """
 
-    mappings = {ctx.workspace_name: ctx.bin_dir.path} if link_workspace_root else {}
     node_modules_roots = {}
 
     # Look through data/deps attributes to find the root directories for the third-party node_modules;
@@ -100,38 +124,33 @@ def write_node_modules_manifest(ctx, extra_data = [], mnemonic = None, link_work
                     fail("All npm dependencies at the path '%s' must come from a single workspace. Found '%s' and '%s'." % (path, other_workspace, workspace))
             node_modules_roots[path] = workspace
 
-    # Look through data/deps attributes to find first party deps to link
+    direct_mappings = []
+    direct_node_modules_roots = []
+    if link_workspace_root:
+        direct_mappings.append(struct(
+            package_name = ctx.workspace_name,
+            package_path = "",
+            link_path = ctx.bin_dir.path,
+        ))
+        direct_node_modules_roots.append("")
+
+    transitive_mappings = []
+    transitive_node_modules_roots = []
     for dep in extra_data + getattr(ctx.attr, "data", []) + getattr(ctx.attr, "deps", []):
         if not LinkerPackageMappingInfo in dep:
             continue
+        transitive_mappings.append(dep[LinkerPackageMappingInfo].mappings)
+        transitive_node_modules_roots.append(dep[LinkerPackageMappingInfo].node_modules_roots)
 
-        for k, v in dep[LinkerPackageMappingInfo].mappings.items():
-            map_key_split = k.split(":")
-            package_name = map_key_split[0]
-            package_path = map_key_split[1] if len(map_key_split) > 1 else ""
-            if package_path not in node_modules_roots:
-                node_modules_roots[package_path] = ""
-            if _link_mapping(dep.label, mappings, k, v):
-                _debug(ctx.var, "Linking %s: %s" % (k, v))
-                mappings[k] = v
-
-    # Convert mappings to a module sets (modules per package package_path)
-    # {
-    #     "package_path": {
-    #             "package_name": "link_path",
-    #             ...
-    #     },
-    #     ...
-    # }
-    module_sets = {}
-    for k, v in mappings.items():
-        map_key_split = k.split(":")
-        package_name = map_key_split[0]
-        package_path = map_key_split[1] if len(map_key_split) > 1 else ""
-        link_path = v
-        if package_path not in module_sets:
-            module_sets[package_path] = {}
-        module_sets[package_path][package_name] = link_path
+    mappings = depset(direct_mappings, transitive = transitive_mappings, order = "topological")
+    module_sets = _flatten_to_module_set(mappings)
+
+    # FIXME: Flattens a depset during the analysis phase. Ideally, this would be done during the
+    # execution phase using an Args object.
+    linker_node_modules_roots = depset(direct_node_modules_roots, transitive = transitive_node_modules_roots).to_list()
+    for node_modules_root in linker_node_modules_roots:
+        if node_modules_root not in node_modules_roots:
+            node_modules_roots[node_modules_root] = ""
 
     # Write the result to a file, and use the magic node option --bazel_node_modules_manifest
     # The launcher.sh will peel off this argument and pass it to the linker rather than the program.
@@ -148,58 +167,60 @@ def write_node_modules_manifest(ctx, extra_data = [], mnemonic = None, link_work
     ctx.actions.write(modules_manifest, str(content))
     return modules_manifest
 
-def _get_module_mappings(target, ctx):
-    """Gathers module mappings from LinkablePackageInfo which maps "package_name:package_path" to link_path.
+def _get_linker_package_mapping_info(target, ctx):
+    """Transitively gathers module mappings and node_modules roots from LinkablePackageInfo.
 
     Args:
       target: target
       ctx: ctx
 
     Returns:
-        Returns module mappings of shape:
-        { "package_name:package_path": link_path, ... }
+      A LinkerPackageMappingInfo provider that contains the mappings and roots for the current
+      target and all its transitive dependencies.
     """
-    mappings = {}
 
-    # Propagate transitive mappings
+    transitive_mappings = []
+    transitive_node_modules_roots = []
     for name in _MODULE_MAPPINGS_DEPS_NAMES:
         for dep in getattr(ctx.rule.attr, name, []):
             if not LinkerPackageMappingInfo in dep:
                 continue
+            transitive_mappings.append(dep[LinkerPackageMappingInfo].mappings)
+            transitive_node_modules_roots.append(dep[LinkerPackageMappingInfo].node_modules_roots)
 
-            for k, v in dep[LinkerPackageMappingInfo].mappings.items():
-                if _link_mapping(target.label, mappings, k, v):
-                    _debug(ctx.var, "target %s propagating module mapping %s: %s" % (dep.label, k, v))
-                    mappings[k] = v
+    direct_mappings = []
+    direct_node_modules_roots = []
 
     # Look for LinkablePackageInfo mapping in this node
+    # LinkablePackageInfo may be provided without a package_name so check for that case as well
     if not LinkablePackageInfo in target:
-        # No mappings contributed here, short-circuit with the transitive ones we collected
         _debug(ctx.var, "No LinkablePackageInfo for", target.label)
-        return mappings
-
-    linkable_package_info = target[LinkablePackageInfo]
-
-    # LinkablePackageInfo may be provided without a package_name so check for that case as well
-    if not linkable_package_info.package_name:
-        # No mappings contributed here, short-circuit with the transitive ones we collected
+    elif not target[LinkablePackageInfo].package_name:
         _debug(ctx.var, "No package_name in LinkablePackageInfo for", target.label)
-        return mappings
-
-    package_path = linkable_package_info.package_path if hasattr(linkable_package_info, "package_path") else ""
-    map_key = "%s:%s" % (linkable_package_info.package_name, package_path)
-    map_value = linkable_package_info.path
-
-    if _link_mapping(target.label, mappings, map_key, map_value):
-        _debug(ctx.var, "target %s adding module mapping %s: %s" % (target.label, map_key, map_value))
-        mappings[map_key] = map_value
-
-    # Returns mappings of shape:
-    # {
-    #   "package_name:package_path": link_path,
-    #   ...
-    # }
-    return mappings
+    else:
+        linkable_package_info = target[LinkablePackageInfo]
+        package_path = linkable_package_info.package_path if hasattr(linkable_package_info, "package_path") else ""
+        direct_mappings.append(
+            struct(
+                package_name = linkable_package_info.package_name,
+                package_path = package_path,
+                link_path = linkable_package_info.path,
+            ),
+        )
+        _debug(ctx.var, "target %s (package path: %s) adding module mapping %s: %s" % (
+            target.label,
+            package_path,
+            linkable_package_info.package_name,
+            linkable_package_info.path,
+        ))
+        direct_node_modules_roots.append(package_path)
+
+    mappings = depset(direct_mappings, transitive = transitive_mappings, order = "topological")
+    node_modules_roots = depset(direct_node_modules_roots, transitive = transitive_node_modules_roots)
+    return LinkerPackageMappingInfo(
+        mappings = mappings,
+        node_modules_roots = node_modules_roots,
+    )
 
 def _module_mappings_aspect_impl(target, ctx):
     # If the target explicitly provides mapping information, we will not propagate
@@ -209,7 +230,7 @@ def _module_mappings_aspect_impl(target, ctx):
         return []
 
     return [
-        LinkerPackageMappingInfo(mappings = _get_module_mappings(target, ctx)),
+        _get_linker_package_mapping_info(target, ctx),
     ]
 
 module_mappings_aspect = aspect(

internal/node/node.bzl

@@ -59,12 +59,10 @@ def _compute_node_modules_roots(ctx, data):
     # Add in roots for multi-linked npm deps
     for dep in data:
         if LinkerPackageMappingInfo in dep:
-            for k, v in dep[LinkerPackageMappingInfo].mappings.items():
-                map_key_split = k.split(":")
-                package_name = map_key_split[0]
-                package_path = map_key_split[1] if len(map_key_split) > 1 else ""
-                if package_path not in node_modules_roots:
-                    node_modules_roots[package_path] = ""
+            linker_node_modules_roots = dep[LinkerPackageMappingInfo].node_modules_roots.to_list()
+            for node_modules_root in linker_node_modules_roots:
+                if node_modules_root not in node_modules_roots:
+                    node_modules_roots[node_modules_root] = ""
 
     return node_modules_roots
 

internal/providers/node_runtime_deps_info.bzl

@@ -66,12 +66,10 @@ def _compute_node_modules_roots(ctx):
     # Add in roots for multi-linked npm deps
     for dep in deps:
         if LinkerPackageMappingInfo in dep:
-            for k, v in dep[LinkerPackageMappingInfo].mappings.items():
-                map_key_split = k.split(":")
-                package_name = map_key_split[0]
-                package_path = map_key_split[1] if len(map_key_split) > 1 else ""
-                if package_path not in node_modules_roots:
-                    node_modules_roots[package_path] = ""
+            linker_node_modules_roots = dep[LinkerPackageMappingInfo].node_modules_roots.to_list()
+            for node_modules_root in linker_node_modules_roots:
+                if node_modules_root not in node_modules_roots:
+                    node_modules_roots[node_modules_root] = ""
 
     return node_modules_roots
 

packages/esbuild/esbuild.bzl

@@ -37,10 +37,13 @@ def _esbuild_impl(ctx):
 
         # Collect the path alias mapping to resolve packages correctly
         if LinkerPackageMappingInfo in dep:
-            for key, value in dep[LinkerPackageMappingInfo].mappings.items():
-                # key is of format "package_name:package_path"
-                package_name = key.split(":")[0]
-                path_alias_mappings.update(generate_path_mapping(package_name, value.replace(ctx.bin_dir.path + "/", "")))
+            for mapping in dep[LinkerPackageMappingInfo].mappings.to_list():
+                path_alias_mappings.update(
+                    generate_path_mapping(
+                        mapping.package_name,
+                        mapping.link_path.replace(ctx.bin_dir.path + "/", ""),
+                    ),
+                )
 
     entry_points = desugar_entry_point_names(ctx.file.entry_point, ctx.files.entry_points)