Reuse swiftmodule for incremental builds

By doing this, for some parts of the build graph, the swiftmodule doesn't have to be recreated. Here is an example with Swift 5.5:

Before:
```console
remark: Incremental compilation: Enabling incremental cross-module building
remark: Incremental compilation: May skip current input:  {compile: A.swift.o <= A.swift}
remark: Incremental compilation: Skipping input:  {compile: A.swift.o <= A.swift}
remark: Incremental compilation: Not skipping job: Merging module A; Missing output 'bazel-out/darwin-fastbuild/bin/A.swiftmodule'
remark: Starting Merging module A
remark: Finished Merging module A
remark: Skipped Compiling A.swift
```

After:
```console
remark: Incremental compilation: Enabling incremental cross-module building
remark: Incremental compilation: May skip current input:  {compile: A.swift.o <= A.swift}
remark: Incremental compilation: Skipping input:  {compile: A.swift.o <= A.swift}
remark: Incremental compilation: Skipping job: Merging module A; oldest output is current 'bazel-out/darwin-fastbuild/bin/A.swiftmodule'
remark: Skipped Compiling A.swift
```

Shout out to Cody Vandermyn (@codeman9) for his initial investigation and PoC on this.

tools/common/file_system.cc

@@ -22,6 +22,7 @@
 
 #ifdef __APPLE__
 #include <copyfile.h>
+#include <removefile.h>
 #else
 #include <fcntl.h>
 #include <sys/sendfile.h>
@@ -37,6 +38,14 @@ std::string GetCurrentDirectory() {
   return cwd;
 }
 
+bool FileExists(const std::string &path) {
+  return access(path.c_str(), 0) == 0;
+}
+
+bool RemoveFile(const std::string &path) {
+  return removefile(path.c_str(), nullptr, 0);
+}
+
 bool CopyFile(const std::string &src, const std::string &dest) {
 #ifdef __APPLE__
   // The `copyfile` function with `COPYFILE_ALL` mode preserves permissions and

tools/common/file_system.h

@@ -20,6 +20,12 @@
 // Gets the path to the current working directory.
 std::string GetCurrentDirectory();
 
+// Returns true if something exists at path.
+bool FileExists(const std::string &path);
+
+// Removes the file at path. Returns true if successful.
+bool RemoveFile(const std::string &path);
+
 // Copies the file at src to dest. Returns true if successful.
 bool CopyFile(const std::string &src, const std::string &dest);
 

tools/worker/output_file_map.cc

@@ -57,6 +57,7 @@ void OutputFileMap::WriteToPath(const std::string &path) {
 void OutputFileMap::UpdateForIncremental(const std::string &path) {
   nlohmann::json new_output_file_map;
   std::map<std::string, std::string> incremental_outputs;
+  std::map<std::string, std::string> incremental_inputs;
 
   // The empty string key is used to represent outputs that are for the whole
   // module, rather than for a particular source file.
@@ -111,6 +112,15 @@ void OutputFileMap::UpdateForIncremental(const std::string &path) {
     new_output_file_map[src] = src_map;
   }
 
+  auto swiftmodule_path = ReplaceExtension(path, ".swiftmodule", /*all_extensions=*/true);
+  auto copied_swiftmodule_path = MakeIncrementalOutputPath(swiftmodule_path);
+  incremental_inputs[swiftmodule_path] = copied_swiftmodule_path;
+
+  auto swiftdoc_path = ReplaceExtension(path, ".swiftdoc", /*all_extensions=*/true);
+  auto copied_swiftdoc_path = MakeIncrementalOutputPath(swiftdoc_path);
+  incremental_inputs[swiftdoc_path] = copied_swiftdoc_path;
+
   json_ = new_output_file_map;
   incremental_outputs_ = incremental_outputs;
+  incremental_inputs_ = incremental_inputs;
 }

tools/worker/output_file_map.h

@@ -39,6 +39,14 @@ class OutputFileMap {
     return incremental_outputs_;
   }
 
+  // A map containing expected output files that will be generated in the
+  // non-incremental storage area, but need to be copied back at the start of
+  // the next compile. The key is the original object path; the corresponding
+  // value is its location in the incremental storage area.
+  const std::map<std::string, std::string> incremental_inputs() const {
+    return incremental_inputs_;
+  }
+
   // Reads the output file map from the JSON file at the given path, and updates
   // it to support incremental builds.
   void ReadFromPath(const std::string &path);
@@ -53,6 +61,7 @@ class OutputFileMap {
 
   nlohmann::json json_;
   std::map<std::string, std::string> incremental_outputs_;
+  std::map<std::string, std::string> incremental_inputs_;
 };
 
 #endif  // BUILD_BAZEL_RULES_SWIFT_TOOLS_WORKER_OUTPUT_FILE_MAP_H_

tools/worker/work_processor.cc

@@ -12,8 +12,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "tools/worker/work_processor.h"
-
 #include <google/protobuf/text_format.h>
 #include <sys/stat.h>
 
@@ -28,6 +26,7 @@
 #include "tools/common/temp_file.h"
 #include "tools/worker/output_file_map.h"
 #include "tools/worker/swift_runner.h"
+#include "tools/worker/work_processor.h"
 
 namespace {
 
@@ -38,6 +37,15 @@ static bool ArgumentEnablesWMO(const std::string &arg) {
          arg == "-force-single-frontend-invocation";
 }
 
+static void FinalizeWorkRequest(const blaze::worker::WorkRequest &request,
+                                blaze::worker::WorkResponse *response,
+                                int exit_code,
+                                const std::ostringstream &output) {
+  response->set_exit_code(exit_code);
+  response->set_output(output.str());
+  response->set_request_id(request.request_id());
+}
+
 };  // end namespace
 
 WorkProcessor::WorkProcessor(const std::vector<std::string> &args) {
@@ -111,6 +119,8 @@ void WorkProcessor::ProcessWorkRequest(
   processed_args.push_back("@" + params_file->GetPath());
   params_file_stream.close();
 
+  std::ostringstream stderr_stream;
+
   if (!is_wmo) {
     for (const auto &expected_object_pair :
          output_file_map.incremental_outputs()) {
@@ -119,15 +129,30 @@ void WorkProcessor::ProcessWorkRequest(
       // incremental storage area.
       auto dir_path = Dirname(expected_object_pair.second);
       if (!MakeDirs(dir_path, S_IRWXU)) {
-        std::cerr << "Could not create directory " << dir_path << " (errno "
-                  << errno << ")\n";
+        stderr_stream << "swift_worker: Could not create directory " << dir_path
+                      << " (errno " << errno << ")\n";
+        FinalizeWorkRequest(request, response, EXIT_FAILURE, stderr_stream);
+      }
+    }
+
+    // Copy some input files from the incremental storage area to the locations
+    // where Bazel will generate them.
+    for (const auto &expected_object_pair :
+         output_file_map.incremental_inputs()) {
+      if (FileExists(expected_object_pair.second)) {
+        if (!CopyFile(expected_object_pair.second,
+                      expected_object_pair.first)) {
+          stderr_stream << "swift_worker: Could not copy "
+                        << expected_object_pair.second << " to "
+                        << expected_object_pair.first << " (errno " << errno
+                        << ")\n";
+          FinalizeWorkRequest(request, response, EXIT_FAILURE, stderr_stream);
+        }
       }
     }
   }
 
-  std::ostringstream stderr_stream;
   SwiftRunner swift_runner(processed_args, /*force_response_file=*/true);
-
   int exit_code = swift_runner.Run(&stderr_stream, /*stdout_to_stderr=*/true);
 
   if (!is_wmo) {
@@ -136,14 +161,34 @@ void WorkProcessor::ProcessWorkRequest(
     for (const auto &expected_object_pair :
          output_file_map.incremental_outputs()) {
       if (!CopyFile(expected_object_pair.second, expected_object_pair.first)) {
-        std::cerr << "Could not copy " << expected_object_pair.second << " to "
-                  << expected_object_pair.first << " (errno " << errno << ")\n";
-        exit_code = EXIT_FAILURE;
+        stderr_stream << "swift_worker: Could not copy "
+                      << expected_object_pair.second << " to "
+                      << expected_object_pair.first << " (errno " << errno
+                      << ")\n";
+        FinalizeWorkRequest(request, response, EXIT_FAILURE, stderr_stream);
       }
     }
-  }
 
-  response->set_exit_code(exit_code);
-  response->set_output(stderr_stream.str());
-  response->set_request_id(request.request_id());
+    // Copy the replaced input files back to the incremental storage for the
+    // next run.
+    for (const auto &expected_object_pair :
+         output_file_map.incremental_inputs()) {
+      if (FileExists(expected_object_pair.first)) {
+        if (FileExists(expected_object_pair.second)) {
+          // CopyFile fails if the file already exists
+          RemoveFile(expected_object_pair.second);
+        }
+        if (!CopyFile(expected_object_pair.first,
+                      expected_object_pair.second)) {
+          stderr_stream << "swift_worker: Could not copy "
+                        << expected_object_pair.first << " to "
+                        << expected_object_pair.second << " (errno " << errno
+                        << ")\n";
+          FinalizeWorkRequest(request, response, EXIT_FAILURE, stderr_stream);
+        }
+      }
+    }
+
+    FinalizeWorkRequest(request, response, exit_code, stderr_stream);
+  }
 }