fail builds with duplicate module names

Summary: When compiling Swift with explicit modules, its possible for the module map to contain multiple entries with the same name. This should be added as a compiler warning, but until that is upstreamed add a validation check to the Swift wrapper to fix any existing cases.

Reviewed By: milend

Differential Revision: D67203279

fbshipit-source-id: 97563ddbf68a7341bdfd45d3fac7c4f3fde73df0

prelude/apple/swift/swift_module_map.bzl

@@ -15,5 +15,6 @@ def write_swift_module_map_with_deps(
     return ctx.actions.write_json(
         module_name + ".swift_module_map.json",
         all_deps.project_as_json("swift_module_map"),
+        pretty = True,
         with_inputs = True,
     )

prelude/apple/tools/swift_exec.py

@@ -6,6 +6,7 @@
 # License, Version 2.0 found in the LICENSE-APACHE file in the root directory
 # of this source tree.
 
+import json
 import os
 import subprocess
 import sys
@@ -14,6 +15,42 @@
 _FILE_WRITE_FAILURE_MARKER = "could not write"
 
 
+def expand_argsfile(command):
+    for arg in command:
+        if arg.startswith("@"):
+            with open(arg[1:]) as f:
+                return f.read().splitlines()
+
+    return []
+
+
+def validate_swift_module_map(path):
+    seen_clang_modules = set()
+    seen_swift_modules = set()
+    with open(path, "r") as f:
+        j = json.load(f)
+
+    for entry in j:
+        is_clang_module = "clangModulePath" in entry
+        module_name = entry["moduleName"]
+        if is_clang_module:
+            if module_name in seen_clang_modules:
+                print(
+                    f"Duplicate clang module {module_name} found in {path}",
+                    file=sys.stderr,
+                )
+                sys.exit(1)
+            seen_clang_modules.add(module_name)
+        else:
+            if module_name in seen_swift_modules:
+                print(
+                    f"Duplicate Swift module {module_name} found in {path}",
+                    file=sys.stderr,
+                )
+                sys.exit(1)
+            seen_swift_modules.add(module_name)
+
+
 def main():
     env = os.environ.copy()
     if "INSIDE_RE_WORKER" in env and _RE_TMPDIR_ENV_VAR in env:
@@ -35,6 +72,18 @@ def main():
 
     command = sys.argv[1:]
 
+    # T209485965: swift module maps allow for duplicate entries
+    # and the first one will be picked. Until we have a compiler
+    # warning for this case we need to validate here.
+    expanded_args = expand_argsfile(command)
+    for i in range(len(expanded_args)):
+        if expanded_args[i] == "-explicit-swift-module-map-file":
+            if expanded_args[i + 1] == "-Xfrontend":
+                validate_swift_module_map(expanded_args[i + 2])
+            else:
+                validate_swift_module_map(expanded_args[i + 1])
+            break
+
     # Use relative paths for debug information and index information,
     # so we generate relocatable files.
     #