Implement Process.ppid on Windows

spec/std/process_spec.cr

@@ -358,21 +358,31 @@ describe Process do
 
   typeof(Process.new(*standing_command).terminate(graceful: false))
 
-  pending_win32 ".exists?" do
-    # We can't reliably check whether it ever returns false, since we can't predict
-    # how PIDs are used by the system, a new process might be spawned in between
-    # reaping the one we would spawn and checking for it, using the now available
-    # pid.
-    Process.exists?(Process.ppid).should be_true
+  it ".exists?" do
+    # On Windows killing a parent process does not reparent its children to
+    # another existing process, so the following isn't guaranteed to work
+    {% unless flag?(:win32) %}
+      # We can't reliably check whether it ever returns false, since we can't predict
+      # how PIDs are used by the system, a new process might be spawned in between
+      # reaping the one we would spawn and checking for it, using the now available
+      # pid.
+      Process.exists?(Process.ppid).should be_true
+    {% end %}
 
     process = Process.new(*standing_command)
     process.exists?.should be_true
     process.terminated?.should be_false
 
     # Kill, zombie now
     process.terminate
-    process.exists?.should be_true
-    process.terminated?.should be_false
+    {% if flag?(:win32) %}
+      # Windows has no concept of zombie processes
+      process.exists?.should be_false
+      process.terminated?.should be_true
+    {% else %}
+      process.exists?.should be_true
+      process.terminated?.should be_false
+    {% end %}
 
     # Reap, gone now
     process.wait

src/crystal/system/win32/process.cr

@@ -1,6 +1,7 @@
 require "c/processthreadsapi"
 require "c/handleapi"
 require "c/synchapi"
+require "c/tlhelp32"
 require "process/shell"
 require "crystal/atomic_semaphore"
 
@@ -71,7 +72,28 @@ struct Crystal::System::Process
   end
 
   def self.ppid
-    raise NotImplementedError.new("Process.ppid")
+    pid = self.pid
+    each_entry do |pe|
+      return pe.th32ParentProcessID if pe.th32ProcessID == pid
+    end
+    raise RuntimeError.new("Cannot locate current process")
+  end
+
+  private def self.each_entry(&)
+    h = LibC.CreateToolhelp32Snapshot(LibC::TH32CS_SNAPPROCESS, 0)
+    raise RuntimeError.from_winerror("CreateToolhelp32Snapshot") if h == LibC::INVALID_HANDLE_VALUE
+
+    begin
+      pe = LibC::PROCESSENTRY32W.new(dwSize: sizeof(LibC::PROCESSENTRY32W))
+      if LibC.Process32FirstW(h, pointerof(pe)) != 0
+        while true
+          yield pe
+          break if LibC.Process32NextW(h, pointerof(pe)) == 0
+        end
+      end
+    ensure
+      LibC.CloseHandle(h)
+    end
   end
 
   def self.signal(pid, signal)
@@ -129,7 +151,7 @@ struct Crystal::System::Process
 
   def self.exists?(pid)
     handle = LibC.OpenProcess(LibC::PROCESS_QUERY_INFORMATION, 0, pid)
-    return false if handle.nil?
+    return false unless handle
     begin
       if LibC.GetExitCodeProcess(handle, out exit_code) == 0
         raise RuntimeError.from_winerror("GetExitCodeProcess")

src/lib_c/x86_64-windows-msvc/c/tlhelp32.cr

@@ -0,0 +1,20 @@
+lib LibC
+  TH32CS_SNAPPROCESS = 0x00000002
+
+  struct PROCESSENTRY32W
+    dwSize : DWORD
+    cntUsage : DWORD
+    th32ProcessID : DWORD
+    th32DefaultHeapID : ULONG_PTR
+    th32ModuleID : DWORD
+    cntThreads : DWORD
+    th32ParentProcessID : DWORD
+    pcPriClassBase : LONG
+    dwFlags : DWORD
+    szExeFile : WCHAR[MAX_PATH]
+  end
+
+  fun CreateToolhelp32Snapshot(dwFlags : DWORD, th32ProcessID : DWORD) : HANDLE
+  fun Process32FirstW(hSnapshot : HANDLE, lppe : PROCESSENTRY32W*) : BOOL
+  fun Process32NextW(hSnapshot : HANDLE, lppe : PROCESSENTRY32W*) : BOOL
+end

src/process.cr

@@ -37,6 +37,10 @@ class Process
   end
 
   # Returns the process identifier of the parent process of the current process.
+  #
+  # On Windows, the parent is associated only at process creation time, and the
+  # system does not re-parent the current process if the parent terminates; thus
+  # `Process.exists?(Process.ppid)` is not guaranteed to be true.
   def self.ppid : Int64
     Crystal::System::Process.ppid.to_i64
   end