[PATCH 0/2] mingw: terminate child processes in a gentler way

[PATCH 0/2] mingw: terminate child processes in a gentler way

[Johannes Schindelin via GitGitGadget]

This patch series consists of two patches that have been carried in Git for
Windows since 2017 an 2018, respectively.

The problem they work around is a fundamental mismatch between Git's
understanding how processes can be terminated and Windows' multi-threading
centric world view (where multi-process architectures are quite, quite
rare), where processes do not tell other processes to terminate gently
(meaning: giving them a chance to run their atexit() handlers).

As such, Git thinks that it can send processes signals to terminate or
force-stop ("kill") them. There are no signals in the Unix sense on Windows,
though. So we try to emulate them. At present, in vanilla Git that means
that we use the TerminateProcess() Win32 API functions, which is most
similar to Unix' SIGKILL and is typically frowned upon because it does not
allow an orderly shutdown of multi-threaded applications. That's definitely
not what Git wants to do: If it wants to terminate a child process, it wants
that child process to clean up any .lock files, for example. And therefore
it wants to send a SIGTERM.

But the SIGTERM signal does not really have any equivalent on Windows. The
closest is to somehow get the target process to call the ExitProcess() Win32
API function. There is a trick that we employ here to do precisely that: we
create a remote thread in the target process, and specify the ExitProcess()
function as the callee. This works because that function matches the
function signature of thread functions enough that we can get away with it,
and because the address of that function is identical between processes
matching the same CPU architecture. Read: This approach does not work when
trying to terminate i686 processes from an x86_64 git.exe. But since it is
rare to mix and match processes of different CPU architectures on Windows
(certainly in Git scenarios), we kind of resort to this best effort that
works often enough to make it worthwhile.

It's a different story for SIGINT: That signal matches most closely what
Windows calls a ConsoleCtrlEvent. It is different, though, in that a
ConsoleCtrlEvent is not sent to a process, but to a Console, and is handled
by all processes that are attached to said Console. In the MSYS2 runtime
that provides the POSIX emulation layer required by the Bash distributed
with Git for Windows, we work around that by using a similar trick as the
SIGTERM/ExitProcess() injection: a thread is injected into the remote
process, passing the address of the (undocumented) kernel32!CtrlRoutine.
This is quite hacky and requires spawning a separate process to just to
figure out the address of said function, which only works in the MSYS2
runtime because it acquires that address once, and then remembers it for the
rest of its lifetime. Git also simply has no business emulating a Ctrl+C and
instead sends child processes SIGTERM. Therefore, there is no support for
sending SIGINT in this patch series. But patch number 2 implements reacting
to the emulated SIGINT "sent" by the MSYS2 runtime.

Johannes Schindelin (2):
  mingw: kill child processes in a gentler way
  mingw: really handle SIGINT

 compat/mingw.c              |  38 +++++++--
 compat/win32/exit-process.h | 165 ++++++++++++++++++++++++++++++++++++
 2 files changed, 195 insertions(+), 8 deletions(-)
 create mode 100644 compat/win32/exit-process.h


base-commit: 94f057755b7941b321fd11fec1b2e3ca5313a4e0
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2130%2Fdscho%2Fmingw-kill-gentle-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2130/dscho/mingw-kill-gentle-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/2130
-- 
gitgitgadget

[PATCH 1/2] mingw: kill child processes in a gentler way

[Johannes Schindelin via GitGitGadget]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

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              |  29 +++++--
 compat/win32/exit-process.h | 165 ++++++++++++++++++++++++++++++++++++
 2 files changed, 186 insertions(+), 8 deletions(-)
 create mode 100644 compat/win32/exit-process.h

diff --git a/compat/mingw.c b/compat/mingw.c
index 2023c16db6..973049ffe3 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -13,6 +13,7 @@
 #include "symlinks.h"
 #include "trace2.h"
 #include "win32.h"
+#include "win32/exit-process.h"
 #include "win32/lazyload.h"
 #include "wrapper.h"
 #include <aclapi.h>
@@ -2208,16 +2209,28 @@ 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);
-
-		if (TerminateProcess(h, -1)) {
+		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 (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);
+		}
+		if (ret) {
+			errno = err_win_to_posix(GetLastError());
 			CloseHandle(h);
-			return 0;
 		}
-
-		errno = err_win_to_posix(GetLastError());
-		CloseHandle(h);
-		return -1;
+		return ret;
 	} else if (pid > 0 && sig == 0) {
 		HANDLE h = OpenProcess(PROCESS_QUERY_INFORMATION, FALSE, pid);
 		if (h) {
diff --git a/compat/win32/exit-process.h b/compat/win32/exit-process.h
new file mode 100644
index 0000000000..d53989884c
--- /dev/null
+++ b/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
-- 
gitgitgadget

[PATCH 2/2] mingw: really handle SIGINT

[Johannes Schindelin via GitGitGadget]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

Previously, we did not install any handler for Ctrl+C, but now we really
want to because the MSYS2 runtime learned the trick to call the
ConsoleCtrlHandler when Ctrl+C was pressed.

With this, hitting Ctrl+C while `git log` is running will only terminate
the Git process, but not the pager. This finally matches the behavior on
Linux and on macOS.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 compat/mingw.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/compat/mingw.c b/compat/mingw.c
index 973049ffe3..f2b6c51f98 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -3580,7 +3580,14 @@ static void adjust_symlink_flags(void)
 		symlink_file_flags |= 2;
 		symlink_directory_flags |= 2;
 	}
+}
 
+static BOOL WINAPI handle_ctrl_c(DWORD ctrl_type)
+{
+	if (ctrl_type != CTRL_C_EVENT)
+		return FALSE; /* we did not handle this */
+	mingw_raise(SIGINT);
+	return TRUE; /* we did handle this */
 }
 
 #ifdef _MSC_VER
@@ -3617,6 +3624,8 @@ int wmain(int argc, const wchar_t **wargv)
 #endif
 #endif
 
+	SetConsoleCtrlHandler(handle_ctrl_c, TRUE);
+
 	maybe_redirect_std_handles();
 	adjust_symlink_flags();
 
-- 
gitgitgadget