From 4fa6521c562ebf09343353717fb7c927df1eee86 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Tue, 21 May 2024 12:18:21 +0200
Subject: Add notion of thread-specific current working directory

---
 libbutl/builtin.cxx |  2 +-
 libbutl/path.cxx    | 27 +++++++++++++++++++++++++++
 libbutl/path.hxx    | 22 +++++++++++++++++++++-
 libbutl/process.cxx | 20 +++++++++++++++++++-
 libbutl/utility.hxx |  2 ++
 5 files changed, 70 insertions(+), 3 deletions(-)

diff --git a/libbutl/builtin.cxx b/libbutl/builtin.cxx
index 2755bf1..a5861d4 100644
--- a/libbutl/builtin.cxx
+++ b/libbutl/builtin.cxx
@@ -243,7 +243,7 @@ namespace butl
   // completed using the current directory if it is relative. Fail if
   // std::system_error is thrown by the underlying function call.
   //
-  dir_path
+  static dir_path
   current_directory (const dir_path& wd, const function<error_record ()>& fail)
   {
     try
diff --git a/libbutl/path.cxx b/libbutl/path.cxx
index bd66f13..909971b 100644
--- a/libbutl/path.cxx
+++ b/libbutl/path.cxx
@@ -26,6 +26,8 @@
 #include <atomic>
 #include <cstring> // strcpy()
 
+#include <libbutl/ft/lang.hxx> // thread_local
+
 #include <libbutl/utility.hxx> // throw_*_error()
 #include <libbutl/process.hxx> // process::current_id()
 
@@ -55,10 +57,21 @@ namespace butl
   // char
   //
 
+  static
+#ifdef __cpp_thread_local
+  thread_local
+#else
+  __thread
+#endif
+  const path_traits<char>::string_type* current_directory_ = nullptr;
+
   template <>
   LIBBUTL_SYMEXPORT path_traits<char>::string_type path_traits<char>::
   current_directory ()
   {
+    if (const auto* twd = current_directory_)
+      return *twd;
+
 #ifdef _WIN32
     char cwd[_MAX_PATH];
     if (_getcwd (cwd, _MAX_PATH) == 0)
@@ -98,6 +111,20 @@ namespace butl
 #endif
   }
 
+  template <>
+  LIBBUTL_SYMEXPORT const path_traits<char>::string_type* path_traits<char>::
+  thread_current_directory ()
+  {
+    return current_directory_;
+  }
+
+  template <>
+  LIBBUTL_SYMEXPORT void path_traits<char>::
+  thread_current_directory (const string_type* twd)
+  {
+    current_directory_ = twd;
+  }
+
 #ifndef _WIN32
   static const small_vector<string, 4> tmp_vars (
     {"TMPDIR", "TMP", "TEMP", "TEMPDIR"});
diff --git a/libbutl/path.hxx b/libbutl/path.hxx
index a691812..7d8b862 100644
--- a/libbutl/path.hxx
+++ b/libbutl/path.hxx
@@ -451,11 +451,31 @@ namespace butl
     // Get/set current working directory. Throw std::system_error to report
     // underlying OS errors.
     //
+    // The curren_directory() accessor (as well as the relevant process
+    // startup functions) have a notion of a "thread working directory" which
+    // is implemented as a thread-specific override that can be added/removed
+    // with thread_current_directory() below.
+    //
+    // Note that the current_directory() modifier always sets the process-wide
+    // working directory.
+    //
+    // See also thread_env().
+    //
     static string_type
     current_directory ();
 
     static void
-    current_directory (string_type const&);
+    current_directory (const string_type&);
+
+    // Get/set thread working directory override. Note that the passed
+    // pointed-to string should be valid (and immutable) for as long as the
+    // override is in effect.
+    //
+    static const string_type*
+    thread_current_directory ();
+
+    static void
+    thread_current_directory (const string_type*);
 
     // Return the user home directory. Throw std::system_error to report
     // underlying OS errors.
diff --git a/libbutl/process.cxx b/libbutl/process.cxx
index e416807..1b8da98 100644
--- a/libbutl/process.cxx
+++ b/libbutl/process.cxx
@@ -262,7 +262,7 @@ namespace butl
   {
     // Note that there is a similar version for Win32.
 
-    typedef path::traits_type traits;
+    using traits = path::traits_type;
 
     size_t fn (strlen (f));
 
@@ -454,6 +454,15 @@ namespace butl
     else if (err == -2)
       in_efd.out = open_null ();
 
+    // If there is no user-supplied CWD and we have thread-specific override,
+    // use that instead of defaulting to the process-wide value.
+    //
+    if (cwd == nullptr || *cwd == '\0')
+    {
+      if (const string* twd = path::traits_type::thread_current_directory ())
+        cwd = twd->c_str ();
+    }
+
     const char* const* tevars (thread_env ());
 
     // The posix_spawn()-based implementation.
@@ -1392,6 +1401,15 @@ namespace butl
       throw process_error (m == nullptr ? last_error_msg () : m);
     };
 
+    // If there is no user-supplied CWD and we have thread-specific override,
+    // use that instead of defaulting to the process-wide value.
+    //
+    if (cwd == nullptr || *cwd == '\0')
+    {
+      if (const string* twd = path::traits_type::thread_current_directory ())
+        cwd = twd->c_str ();
+    }
+
     // (Un)set the environment variables for the child process.
     //
     // Note that we can not do it incrementally, as for POSIX implementation.
diff --git a/libbutl/utility.hxx b/libbutl/utility.hxx
index 779a0aa..9eb052d 100644
--- a/libbutl/utility.hxx
+++ b/libbutl/utility.hxx
@@ -301,6 +301,8 @@ namespace butl
   // of overrides over the process environment (sets and unsets), the same as
   // for the process startup.
   //
+  // See also path_traits::thread_current_directory().
+  //
   extern
 #ifdef __cpp_thread_local
   thread_local
-- 
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