Reland "[llvm][Support] Add support for executing a detached process (#81708)" (#83367)

Relands #81708, which was reverted by
f410f74cd5b26319b5796e0404c6a0f3b5cc00a5, now with a corrected unit
test. Origionally the test failed on Windows when run with lit as
`GetConsoleWindow` could not retrieve a window handle regardless of
whether `DetachProcess` was `true` or `false`. The test now uses
`GetStdHandle(STD_OUTPUT_HANDLE)` which does not rely on a console
window existing. Original commit message below.

Adds a new parameter, `bool DetachProcess` with a default option of
`false`, to `llvm::sys::ExecuteNoWait`, which, when set to `true`,
executes the specified program without a controlling terminal.

Functionality added so that the module build daemon can be run without a
controlling terminal.

1 files changed, 14 insertions, 4 deletions

diff --git a/llvm/lib/Support/Unix/Program.inc b/llvm/lib/Support/Unix/Program.inc
index 5d9757b..2742734 100644
--- a/llvm/lib/Support/Unix/Program.inc
+++ b/llvm/lib/Support/Unix/Program.inc
@@ -173,10 +173,11 @@ toNullTerminatedCStringArray(ArrayRef<StringRef> Strings, StringSaver &Saver) {
 }
 
 static bool Execute(ProcessInfo &PI, StringRef Program,
-                    ArrayRef<StringRef> Args, std::optional<ArrayRef<StringRef>> Env,
+                    ArrayRef<StringRef> Args,
+                    std::optional<ArrayRef<StringRef>> Env,
                     ArrayRef<std::optional<StringRef>> Redirects,
                     unsigned MemoryLimit, std::string *ErrMsg,
-                    BitVector *AffinityMask) {
+                    BitVector *AffinityMask, bool DetachProcess) {
   if (!llvm::sys::fs::exists(Program)) {
     if (ErrMsg)
       *ErrMsg = std::string("Executable \"") + Program.str() +
@@ -202,7 +203,8 @@ static bool Execute(ProcessInfo &PI, StringRef Program,
   // If this OS has posix_spawn and there is no memory limit being implied, use
   // posix_spawn.  It is more efficient than fork/exec.
 #ifdef HAVE_POSIX_SPAWN
-  if (MemoryLimit == 0) {
+  // Cannot use posix_spawn if you would like to detach the process
+  if (MemoryLimit == 0 && !DetachProcess) {
     posix_spawn_file_actions_t FileActionsStore;
     posix_spawn_file_actions_t *FileActions = nullptr;
 
@@ -270,7 +272,7 @@ static bool Execute(ProcessInfo &PI, StringRef Program,
 
     return true;
   }
-#endif
+#endif // HAVE_POSIX_SPAWN
 
   // Create a child process.
   int child = fork();
@@ -307,6 +309,14 @@ static bool Execute(ProcessInfo &PI, StringRef Program,
       }
     }
 
+    if (DetachProcess) {
+      // Detach from controlling terminal
+      if (::setsid() == -1) {
+        MakeErrMsg(ErrMsg, "Could not detach process, ::setsid failed");
+        return false;
+      }
+    }
+
     // Set memory limits
     if (MemoryLimit != 0) {
       SetMemoryLimits(MemoryLimit);