I think this is all fixed now.
Try not to introduce more `using std::type` aliases. When reader sees `mutex` in a function, it is not immediately clear if it is `std::mutex` or Bazel's own custom mutex implementation.

`using MagicMap = std::unordered_map<std::string, MagicItem>` that saves significant amount of typing is strongly encouraged. Done. Thanks for the advice!
Bazel server: ensure file streams are closed
### Description of the problem / feature request:

The Bazel server (the Java code) sometimes opens streams without closing them, relying on the garbage collector to finalize the InputStream and close the file. This leads to Bazel holding on to files and (on Windows) being unable to delete them later.

### Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

Examples:
https://github.com/bazelbuild/bazel/blob/52035c1c2d0ad494fb4318a6c10a6a46749ea6a2/src/main/java/com/google/devtools/build/lib/vfs/FileSystem.java#L270-L276
https://github.com/bazelbuild/bazel/blob/52035c1c2d0ad494fb4318a6c10a6a46749ea6a2/src/main/java/com/google/devtools/build/lib/analysis/actions/LauncherFileWriteAction.java#L78
https://github.com/bazelbuild/bazel/blob/52035c1c2d0ad494fb4318a6c10a6a46749ea6a2/src/main/java/com/google/devtools/build/lib/bazel/rules/android/AndroidSdkRepositoryFunction.java#L314
https://github.com/bazelbuild/bazel/blob/52035c1c2d0ad494fb4318a6c10a6a46749ea6a2/src/main/java/com/google/devtools/build/lib/bazel/rules/android/SdkMavenRepository.java#L203

Correct usage would be try-with-resources:
https://github.com/bazelbuild/bazel/blob/52035c1c2d0ad494fb4318a6c10a6a46749ea6a2/src/main/java/com/google/devtools/build/lib/exec/BinTools.java#L242

### What's the output of `bazel info release`?

release 0.15.0
src/main/cpp/blaze.cc src/main/cpp/blaze_util_platform.h src/main/cpp/blaze_util_posix.cc src/main/cpp/blaze_util_windows.cc
src/main/cpp/blaze.cc src/main/cpp/blaze_util_platform.h src/main/cpp/blaze_util_posix.cc src/main/cpp/blaze_util_windows.cc
diff --git a/src/main/cpp/blaze.cc b/src/main/cpp/blaze.cc
index 09760fcba3c7f4..c50503379cbddb 100644
--- a/src/main/cpp/blaze.cc
+++ b/src/main/cpp/blaze.cc
@@ -878,8 +878,9 @@ static void StartServerAndConnect(const WorkspaceLayout *workspace_layout,
 // A PureZipExtractorProcessor to extract the files from the blaze zip.
 class ExtractBlazeZipProcessor : public PureZipExtractorProcessor {
  public:
-  explicit ExtractBlazeZipProcessor(const string &embedded_binaries)
-      : embedded_binaries_(embedded_binaries) {}
+  explicit ExtractBlazeZipProcessor(const string &embedded_binaries,
+                                    blaze::embedded_binaries::Dumper* dumper)
+      : embedded_binaries_(embedded_binaries), dumper_(dumper) {}
 
   bool AcceptPure(const char *filename,
                   const devtools_ijar::u4 attr) const override {
@@ -892,29 +893,13 @@ class ExtractBlazeZipProcessor : public PureZipExtractorProcessor {
 
   void Process(const char *filename, const devtools_ijar::u4 attr,
                const devtools_ijar::u1 *data, const size_t size) override {
-    string path = blaze_util::JoinPath(embedded_binaries_, filename);
-    string dirname = blaze_util::Dirname(path);
-    // Performance optimization: memoize the paths we already created a
-    // directory for, to spare a stat in attempting to recreate an already
-    // existing directory. This optimization alone shaves off seconds from the
-    // extraction time on Windows.
-    if (created_directories_.insert(dirname).second) {
-      if (!blaze_util::MakeDirectories(dirname, 0777)) {
-        BAZEL_DIE(blaze_exit_code::INTERNAL_ERROR)
-            << "couldn't create '" << path << "': " << GetLastErrorString();
-      }
-    }
-
-    if (!blaze_util::WriteFile(data, size, path, 0755)) {
-      BAZEL_DIE(blaze_exit_code::LOCAL_ENVIRONMENTAL_ERROR)
-          << "Failed to write zipped file '" << path
-          << "': " << GetLastErrorString();
-    }
+    dumper_->Dump(
+        data, size, blaze_util::JoinPath(embedded_binaries_, filename));
   }
 
  private:
   const string embedded_binaries_;
-  set<string> created_directories_;
+  blaze::embedded_binaries::Dumper* dumper_;
 };
 
 // Actually extracts the embedded data files into the tree whose root
@@ -923,7 +908,16 @@ static void ActuallyExtractData(const string &argv0,
                                 const string &embedded_binaries) {
   std::string install_md5;
   GetInstallKeyFileProcessor install_key_processor(&install_md5);
-  ExtractBlazeZipProcessor extract_blaze_processor(embedded_binaries);
+
+  std::string error;
+  std::unique_ptr<blaze::embedded_binaries::Dumper> dumper(
+      blaze::embedded_binaries::Create(&error));
+  if (dumper == nullptr) {
+    BAZEL_DIE(blaze_exit_code::LOCAL_ENVIRONMENTAL_ERROR) << error;
+  }
+  ExtractBlazeZipProcessor extract_blaze_processor(embedded_binaries,
+                                                   dumper.get());
+
   CompoundZipProcessor processor({&extract_blaze_processor,
                                   &install_key_processor});
   if (!blaze_util::MakeDirectories(embedded_binaries, 0777)) {
@@ -948,6 +942,11 @@ static void ActuallyExtractData(const string &argv0,
         << " as a zip file: " << extractor->GetError();
   }
 
+  if (!dumper->Finish(&error)) {
+    BAZEL_DIE(blaze_exit_code::LOCAL_ENVIRONMENTAL_ERROR)
+        << "Failed to extract embedded binaries: " << error;
+  }
+
   if (install_md5 != globals->install_md5) {
     BAZEL_DIE(blaze_exit_code::LOCAL_ENVIRONMENTAL_ERROR)
         << "The " << globals->options->product_name << " binary at " << argv0
diff --git a/src/main/cpp/blaze_util_platform.h b/src/main/cpp/blaze_util_platform.h
index 29596df57bdaf9..24a28eb6e7db06 100644
--- a/src/main/cpp/blaze_util_platform.h
+++ b/src/main/cpp/blaze_util_platform.h
@@ -25,6 +25,50 @@
 
 namespace blaze {
 
+namespace embedded_binaries {
+
+// Dumps embedded binaries that were extracted from the Bazel zip to disk.
+// The platform-specific implementations may use multi-threaded I/O.
+class Dumper {
+ public:
+  // Requests to write the `data` of `size` bytes to disk under `path`.
+  // The actual writing may happen asynchronously.
+  // `path` must be an absolute path. All of its parent directories will be
+  // created.
+  // The caller retains ownership of `data` and may release it immediately after
+  // this method returns.
+  // Callers may call this method repeatedly, but only from the same thread
+  // (this method is not thread-safe).
+  // If writing fails, this method sets a flag in the `Dumper`, and `Finish`
+  // will return false. Subsequent `Dump` calls will have no effect.
+  virtual void Dump(const void* data, const size_t size,
+                    const std::string& path) = 0;
+
+  // Finishes dumping data.
+  //
+  // This method may block in case the Dumper is asynchronous and some async
+  // writes are still in progress.
+  // Subsequent `Dump` calls after this method have no effect.
+  //
+  // Returns true if there were no errors in any of the `Dump` calls.
+  // Returns false if any of the `Dump` calls failed, and if `error` is not
+  // null then puts an error message in `error`.
+  virtual bool Finish(std::string* error) = 0;
+
+  // Destructor. Subclasses should make sure it calls `Finish(nullptr)`.
+  virtual ~Dumper() {}
+
+ protected:
+  Dumper() {}
+};
+
+// Creates a new Dumper. The caller takes ownership of the returned object.
+// Returns nullptr upon failure and puts an error message in `error` (if `error`
+// is not nullptr).
+Dumper* Create(std::string* error = nullptr);
+
+}  // namespace embedded_binaries
+
 struct GlobalVariables;
 
 class SignalHandler {
diff --git a/src/main/cpp/blaze_util_posix.cc b/src/main/cpp/blaze_util_posix.cc
index 0a3daaab81ef86..b24aba14ed871a 100644
--- a/src/main/cpp/blaze_util_posix.cc
+++ b/src/main/cpp/blaze_util_posix.cc
@@ -36,6 +36,8 @@
 
 #include <cassert>
 #include <cinttypes>
+#include <set>
+#include <string>
 
 #include "src/main/cpp/blaze_util.h"
 #include "src/main/cpp/global_variables.h"
@@ -54,9 +56,74 @@ namespace blaze {
 using blaze_exit_code::INTERNAL_ERROR;
 using blaze_util::GetLastErrorString;
 
+using std::set;
 using std::string;
 using std::vector;
 
+namespace embedded_binaries {
+
+class PosixDumper : public Dumper {
+ public:
+  static PosixDumper* Create(string* error);
+  ~PosixDumper() { Finish(nullptr); }
+  virtual void Dump(const void* data, const size_t size, const string& path)
+      override;
+  virtual bool Finish(string* error) override;
+
+ private:
+  PosixDumper() : was_error_(false) {}
+
+  set<string> dir_cache_;
+  string error_msg_;
+  bool was_error_;
+};
+
+Dumper* Create(string* error) {
+  return PosixDumper::Create(error);
+}
+
+PosixDumper* PosixDumper::Create(string* error) {
+  return new PosixDumper();
+}
+
+void PosixDumper::Dump(const void* data, const size_t size,
+                       const string& path) {
+  if (was_error_) {
+    return;
+  }
+
+  string dirname = blaze_util::Dirname(path);
+  // Performance optimization: memoize the paths we already created a
+  // directory for, to spare a stat in attempting to recreate an already
+  // existing directory.
+  if (dir_cache_.insert(dirname).second) {
+    if (!blaze_util::MakeDirectories(dirname, 0777)) {
+      was_error_ = true;
+      string msg = GetLastErrorString();
+      error_msg_ = string("couldn't create '") + path + "': " + msg;
+    }
+  }
+
+  if (was_error_) {
+    return;
+  }
+
+  if (!blaze_util::WriteFile(data, size, path, 0755)) {
+    was_error_ = true;
+    string msg = GetLastErrorString();
+    error_msg_ = string("Failed to write zipped file '") + path + "': " + msg;
+  }
+}
+
+bool PosixDumper::Finish(string* error) {
+  if (was_error_ && error) {
+    *error = error_msg_;
+  }
+  return !was_error_;
+}
+
+}  // namespace embedded_binaries
+
 SignalHandler SignalHandler::INSTANCE;
 
 // The number of the last received signal that should cause the client
diff --git a/src/main/cpp/blaze_util_windows.cc b/src/main/cpp/blaze_util_windows.cc
index 3751a1d0b1cd41..b6730871c6115d 100644
--- a/src/main/cpp/blaze_util_windows.cc
+++ b/src/main/cpp/blaze_util_windows.cc
@@ -27,10 +27,13 @@
 #include <shlobj.h>        // SHGetKnownFolderPath
 
 #include <algorithm>
+#include <atomic>
 #include <cstdio>
 #include <cstdlib>
+#include <mutex>  // NOLINT
+#include <set>
 #include <sstream>
-#include <thread>  // NOLINT (to slience Google-internal linter)
+#include <thread>  // NOLINT (to silence Google-internal linter)
 #include <type_traits>  // static_assert
 #include <vector>
 
@@ -76,6 +79,203 @@ using std::string;
 using std::unique_ptr;
 using std::wstring;
 
+namespace embedded_binaries {
+
+class WindowsDumper : public Dumper {
+ public:
+  static WindowsDumper* Create(string* error);
+  ~WindowsDumper() { Finish(nullptr); }
+  virtual void Dump(const void* data, const size_t size,
+                    const string& path) override;
+  virtual bool Finish(string* error) override;
+
+ private:
+  WindowsDumper()
+      : threadpool_(NULL), cleanup_group_(NULL), was_error_(false) {}
+
+  PTP_POOL threadpool_;
+  PTP_CLEANUP_GROUP cleanup_group_;
+  TP_CALLBACK_ENVIRON threadpool_env_;
+  std::mutex dir_cache_lock_;
+  std::set<string> dir_cache_;
+  std::atomic_bool was_error_;
+  string error_msg_;
+};
+
+namespace {
+
+class DumpContext {
+ public:
+  DumpContext(unique_ptr<uint8_t[]> data, const size_t size, const string path,
+              std::mutex* dir_cache_lock, std::set<string>* dir_cache,
+              std::atomic_bool* was_error, string* error_msg);
+  void Run();
+
+ private:
+  void MaybeSignalError(const string& msg);
+
+  unique_ptr<uint8_t[]> data_;
+  const size_t size_;
+  const string path_;
+  std::mutex* dir_cache_lock_;
+  std::set<string>* dir_cache_;
+  std::atomic_bool* was_error_;
+  string* error_msg_;
+};
+
+VOID CALLBACK WorkCallback(
+    _Inout_     PTP_CALLBACK_INSTANCE Instance,
+    _Inout_opt_ PVOID                 Context,
+    _Inout_     PTP_WORK              Work);
+
+} // namespace
+
+Dumper* Create(string* error) {
+  return WindowsDumper::Create(error);
+}
+
+WindowsDumper* WindowsDumper::Create(string* error) {
+  unique_ptr<WindowsDumper> result(new WindowsDumper());
+
+  result->threadpool_ = CreateThreadpool(NULL);
+  if (result->threadpool_ == NULL) {
+    if (error) {
+      string msg = GetLastErrorString();
+      *error = "CreateThreadpool failed: " + msg;
+    }
+    return nullptr;
+  }
+
+  result->cleanup_group_ = CreateThreadpoolCleanupGroup();
+  if (result->cleanup_group_ == NULL) {
+    string msg = GetLastErrorString();
+    CloseThreadpool(result->threadpool_);
+    if (error) {
+      string msg = GetLastErrorString();
+      *error = "CreateThreadpoolCleanupGroup failed: " + msg;
+    }
+    return nullptr;
+  }
+
+  // I (@laszlocsomor) experimented with different thread counts and found that
+  // 8 threads provide a significant advantage over 1 thread, but adding more
+  // threads provides only marginal speedup.
+  SetThreadpoolThreadMaximum(result->threadpool_, 16);
+  SetThreadpoolThreadMinimum(result->threadpool_, 8);
+
+  InitializeThreadpoolEnvironment(&result->threadpool_env_);
+  SetThreadpoolCallbackPool(&result->threadpool_env_, result->threadpool_);
+  SetThreadpoolCallbackCleanupGroup(&result->threadpool_env_,
+                                    result->cleanup_group_, NULL);
+
+  return result.release();  // release pointer ownership
+}
+
+void WindowsDumper::Dump(const void* data, const size_t size,
+                         const string& path) {
+  if (was_error_) {
+    return;
+  }
+
+  unique_ptr<uint8_t[]> data_copy(new uint8_t[size]);
+  memcpy(data_copy.get(), data, size);
+  unique_ptr<DumpContext> ctx(
+      new DumpContext(std::move(data_copy), size, path, &dir_cache_lock_,
+                      &dir_cache_, &was_error_, &error_msg_));
+  PTP_WORK w = CreateThreadpoolWork(WorkCallback, ctx.get(), &threadpool_env_);
+  if (w == NULL) {
+    string err = GetLastErrorString();
+    if (!was_error_.exchange(true)) {
+      // Benign race condition: though we use no locks to access `error_msg_`,
+      // only one thread may ever flip `was_error_` from false to true and enter
+      // the body of this if-clause. Since `was_error_` is the same object as
+      // used by all other threads trying to write to `error_msg_` (see
+      // DumpContext::MaybeSignalError), using it provides adequate mutual
+      // exclusion to write `error_msg_`.
+      error_msg_ = string("WindowsDumper::Dump() couldn't submit work: ") + err;
+    }
+  } else {
+    ctx.release();  // release pointer ownership
+    SubmitThreadpoolWork(w);
+  }
+}
+
+bool WindowsDumper::Finish(string* error) {
+  if (threadpool_ == NULL) {
+    return true;
+  }
+  CloseThreadpoolCleanupGroupMembers(cleanup_group_, FALSE, NULL);
+  CloseThreadpoolCleanupGroup(cleanup_group_);
+  CloseThreadpool(threadpool_);
+  threadpool_ = NULL;
+  cleanup_group_ = NULL;
+  if (was_error_ && error) {
+    // No race condition reading `error_msg_`: all worker threads terminated
+    // by now.
+    *error = error_msg_;
+  }
+  return !was_error_;
+}
+
+namespace {
+
+DumpContext::DumpContext(
+    unique_ptr<uint8_t[]> data, const size_t size, const string path,
+    std::mutex* dir_cache_lock, std::set<string>* dir_cache,
+    std::atomic_bool* was_error, string* error_msg)
+  : data_(std::move(data)), size_(size), path_(path),
+    dir_cache_lock_(dir_cache_lock), dir_cache_(dir_cache),
+    was_error_(was_error), error_msg_(error_msg) {}
+
+void DumpContext::Run() {
+  string dirname = blaze_util::Dirname(path_);
+
+  bool success = true;
+  // Performance optimization: memoize the paths we already created a
+  // directory for, to spare a stat in attempting to recreate an already
+  // existing directory. This optimization alone shaves off seconds from the
+  // extraction time on Windows.
+  {
+    std::lock_guard<std::mutex> guard(*dir_cache_lock_);
+    if (dir_cache_->insert(dirname).second) {
+      success = blaze_util::MakeDirectories(dirname, 0777);
+    }
+  }
+
+  if (!success) {
+    MaybeSignalError(string("Couldn't create directory '") + dirname + "'");
+    return;
+  }
+
+  if (!blaze_util::WriteFile(data_.get(), size_, path_, 0755)) {
+    MaybeSignalError(string("Failed to write zipped file '") + path_ + "'");
+  }
+}
+
+void DumpContext::MaybeSignalError(const string& msg) {
+  if (!was_error_->exchange(true)) {
+    // Benign race condition: though we use no locks to access `error_msg_`,
+    // only one thread may ever flip `was_error_` from false to true and enter
+    // the body of this if-clause. Since `was_error_` is the same object as used
+    // by all other threads and by WindowsDumper::Dump(), using it provides
+    // adequate mutual exclusion to write `error_msg_`.
+    *error_msg_ = msg;
+  }
+}
+
+VOID CALLBACK WorkCallback(
+    _Inout_     PTP_CALLBACK_INSTANCE Instance,
+    _Inout_opt_ PVOID                 Context,
+    _Inout_     PTP_WORK              Work) {
+  unique_ptr<DumpContext> ctx(reinterpret_cast<DumpContext*>(Context));
+  ctx->Run();
+}
+
+
+} // namespace
+
+}  // namespace embedded_binaries
+
 SignalHandler SignalHandler::INSTANCE;
 
 class WindowsClock {
