mingw: kill child processes in a gentler way

The TerminateProcess() function does not actually leave the child
processes any chance to perform any cleanup operations. This is bad
insofar as Git itself expects its signal handlers to run.

A symptom is e.g. a left-behind .lock file that would not be left behind
if the same operation was run, say, on Linux.

To remedy this situation, we use an obscure trick: we inject a thread
into the process that needs to be killed and to let that thread run the
ExitProcess() function with the desired exit status. Thanks J Wyman for
describing this trick.

The advantage is that the ExitProcess() function lets the atexit
handlers run. While this is still different from what Git expects (i.e.
running a signal handler), in practice Git sets up signal handlers and
atexit handlers that call the same code to clean up after itself.

In case that the gentle method to terminate the process failed, we still
fall back to calling TerminateProcess(), but in that case we now also
make sure that processes spawned by the spawned process are terminated;
TerminateProcess() does not give the spawned process a chance to do so
itself.

Please note that this change only affects how Git for Windows tries to
terminate processes spawned by Git's own executables. Third-party
software that *calls* Git and wants to terminate it *still* need to make
sure to imitate this gentle method, otherwise this patch will not have
any effect.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -8,6 +8,7 @@
 #include "../strbuf.h"
 #include "../run-command.h"
 #include "../cache.h"
+#include "win32/exit-process.h"
 #include "win32/lazyload.h"
 #include "../config.h"
 #include "dir.h"
@@ -2133,16 +2134,28 @@ int mingw_execvp(const char *cmd, char *const *argv)
 int mingw_kill(pid_t pid, int sig)
 {
 	if (pid > 0 && sig == SIGTERM) {
-		HANDLE h = OpenProcess(PROCESS_TERMINATE, FALSE, pid);
+		HANDLE h = OpenProcess(PROCESS_CREATE_THREAD |
+				       PROCESS_QUERY_INFORMATION |
+				       PROCESS_VM_OPERATION | PROCESS_VM_WRITE |
+				       PROCESS_VM_READ | PROCESS_TERMINATE,
+				       FALSE, pid);
+		int ret;
 
-		if (TerminateProcess(h, -1)) {
-			CloseHandle(h);
-			return 0;
+		if (h)
+			ret = exit_process(h, 128 + sig);
+		else {
+			h = OpenProcess(PROCESS_TERMINATE, FALSE, pid);
+			if (!h) {
+				errno = err_win_to_posix(GetLastError());
+				return -1;
+			}
+			ret = terminate_process_tree(h, 128 + sig);
 		}
-
-		errno = err_win_to_posix(GetLastError());
-		CloseHandle(h);
-		return -1;
+		if (ret) {
+			errno = err_win_to_posix(GetLastError());
+			CloseHandle(h);
+		}
+		return ret;
 	} else if (pid > 0 && sig == 0) {
 		HANDLE h = OpenProcess(PROCESS_QUERY_INFORMATION, FALSE, pid);
 		if (h) {

compat/win32/exit-process.h

@@ -0,0 +1,165 @@
+#ifndef EXIT_PROCESS_H
+#define EXIT_PROCESS_H
+
+/*
+ * This file contains functions to terminate a Win32 process, as gently as
+ * possible.
+ *
+ * At first, we will attempt to inject a thread that calls ExitProcess(). If
+ * that fails, we will fall back to terminating the entire process tree.
+ *
+ * For simplicity, these functions are marked as file-local.
+ */
+
+#include <tlhelp32.h>
+
+/*
+ * Terminates the process corresponding to the process ID and all of its
+ * directly and indirectly spawned subprocesses.
+ *
+ * This way of terminating the processes is not gentle: the processes get
+ * no chance of cleaning up after themselves (closing file handles, removing
+ * .lock files, terminating spawned processes (if any), etc).
+ */
+static int terminate_process_tree(HANDLE main_process, int exit_status)
+{
+	HANDLE snapshot = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);
+	PROCESSENTRY32 entry;
+	DWORD pids[16384];
+	int max_len = sizeof(pids) / sizeof(*pids), i, len, ret = 0;
+	pid_t pid = GetProcessId(main_process);
+
+	pids[0] = (DWORD)pid;
+	len = 1;
+
+	/*
+	 * Even if Process32First()/Process32Next() seem to traverse the
+	 * processes in topological order (i.e. parent processes before
+	 * child processes), there is nothing in the Win32 API documentation
+	 * suggesting that this is guaranteed.
+	 *
+	 * Therefore, run through them at least twice and stop when no more
+	 * process IDs were added to the list.
+	 */
+	for (;;) {
+		int orig_len = len;
+
+		memset(&entry, 0, sizeof(entry));
+		entry.dwSize = sizeof(entry);
+
+		if (!Process32First(snapshot, &entry))
+			break;
+
+		do {
+			for (i = len - 1; i >= 0; i--) {
+				if (pids[i] == entry.th32ProcessID)
+					break;
+				if (pids[i] == entry.th32ParentProcessID)
+					pids[len++] = entry.th32ProcessID;
+			}
+		} while (len < max_len && Process32Next(snapshot, &entry));
+
+		if (orig_len == len || len >= max_len)
+			break;
+	}
+
+	for (i = len - 1; i > 0; i--) {
+		HANDLE process = OpenProcess(PROCESS_TERMINATE, FALSE, pids[i]);
+
+		if (process) {
+			if (!TerminateProcess(process, exit_status))
+				ret = -1;
+			CloseHandle(process);
+		}
+	}
+	if (!TerminateProcess(main_process, exit_status))
+		ret = -1;
+	CloseHandle(main_process);
+
+	return ret;
+}
+
+/**
+ * Determine whether a process runs in the same architecture as the current
+ * one. That test is required before we assume that GetProcAddress() returns
+ * a valid address *for the target process*.
+ */
+static inline int process_architecture_matches_current(HANDLE process)
+{
+	static BOOL current_is_wow = -1;
+	BOOL is_wow;
+
+	if (current_is_wow == -1 &&
+	    !IsWow64Process (GetCurrentProcess(), &current_is_wow))
+		current_is_wow = -2;
+	if (current_is_wow == -2)
+		return 0; /* could not determine current process' WoW-ness */
+	if (!IsWow64Process (process, &is_wow))
+		return 0; /* cannot determine */
+	return is_wow == current_is_wow;
+}
+
+/**
+ * Inject a thread into the given process that runs ExitProcess().
+ *
+ * Note: as kernel32.dll is loaded before any process, the other process and
+ * this process will have ExitProcess() at the same address.
+ *
+ * This function expects the process handle to have the access rights for
+ * CreateRemoteThread(): PROCESS_CREATE_THREAD, PROCESS_QUERY_INFORMATION,
+ * PROCESS_VM_OPERATION, PROCESS_VM_WRITE, and PROCESS_VM_READ.
+ *
+ * The idea comes from the Dr Dobb's article "A Safer Alternative to
+ * TerminateProcess()" by Andrew Tucker (July 1, 1999),
+ * http://www.drdobbs.com/a-safer-alternative-to-terminateprocess/184416547
+ *
+ * If this method fails, we fall back to running terminate_process_tree().
+ */
+static int exit_process(HANDLE process, int exit_code)
+{
+	DWORD code;
+
+	if (GetExitCodeProcess(process, &code) && code == STILL_ACTIVE) {
+		static int initialized;
+		static LPTHREAD_START_ROUTINE exit_process_address;
+		PVOID arg = (PVOID)(intptr_t)exit_code;
+		DWORD thread_id;
+		HANDLE thread = NULL;
+
+		if (!initialized) {
+			HINSTANCE kernel32 = GetModuleHandleA("kernel32");
+			if (!kernel32)
+				die("BUG: cannot find kernel32");
+			exit_process_address =
+				(LPTHREAD_START_ROUTINE)(void (*)(void))
+				GetProcAddress(kernel32, "ExitProcess");
+			initialized = 1;
+		}
+		if (!exit_process_address ||
+		    !process_architecture_matches_current(process))
+			return terminate_process_tree(process, exit_code);
+
+		thread = CreateRemoteThread(process, NULL, 0,
+					    exit_process_address,
+					    arg, 0, &thread_id);
+		if (thread) {
+			CloseHandle(thread);
+			/*
+			 * If the process survives for 10 seconds (a completely
+			 * arbitrary value picked from thin air), fall back to
+			 * killing the process tree via TerminateProcess().
+			 */
+			if (WaitForSingleObject(process, 10000) ==
+			    WAIT_OBJECT_0) {
+				CloseHandle(process);
+				return 0;
+			}
+		}
+
+		return terminate_process_tree(process, exit_code);
+	}
+
+	return 0;
+}
+
+#endif