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std: Avoid deadlocking in ChildProcess.exec

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Reading stdin&stderr at different times may lead to nasty deadlocks (eg.
when stdout is read before stderr and the child process doesn't write
anything onto stdout).

Implement a polling mechanism to make sure this won't happen: we read
data from stderr/stdout as it becomes ready and then it's copied into an
ArrayList provided by the user, avoiding any kind of blocking read.
This commit is contained in:
LemonBoy 2020-10-20 08:51:21 +02:00 committed by Andrew Kelley
parent 1590ed9d6a
commit dc810eb73b
3 changed files with 208 additions and 11 deletions
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173
lib/std/child_process.zig
View File

@ -186,6 +186,106 @@ pub const ChildProcess = struct {
pub const exec2 = @compileError("deprecated: exec2 is renamed to exec");
fn collectOutputPosix(child: *const ChildProcess, stdout: *std.ArrayList(u8), stderr: *std.ArrayList(u8), max_output_bytes: usize) !void {
var poll_fds = [_]os.pollfd{
.{ .fd = child.stdout.?.handle, .events = os.POLLIN, .revents = undefined },
.{ .fd = child.stderr.?.handle, .events = os.POLLIN, .revents = undefined },
};
var dead_fds: usize = 0;
var loop_buf: [4096]u8 = undefined;
while (dead_fds < poll_fds.len) {
const events = try os.poll(&poll_fds, std.math.maxInt(i32));
if (events == 0) continue;
// Try reading whatever is available before checking the error
// conditions.
if (poll_fds[0].revents & os.POLLIN != 0) {
// stdout is ready.
const n = try os.read(poll_fds[0].fd, &loop_buf);
try stdout.appendSlice(loop_buf[0..n]);
}
if (poll_fds[1].revents & os.POLLIN != 0) {
// stderr is ready.
const n = try os.read(poll_fds[1].fd, &loop_buf);
try stderr.appendSlice(loop_buf[0..n]);
}
// Exclude the fds that signaled an error.
if (poll_fds[0].revents & (os.POLLERR | os.POLLNVAL | os.POLLHUP) != 0) {
poll_fds[0].fd = -1;
dead_fds += 1;
}
if (poll_fds[1].revents & (os.POLLERR | os.POLLNVAL | os.POLLHUP) != 0) {
poll_fds[1].fd = -1;
dead_fds += 1;
}
}
}
fn collectOutputWindows(child: *const ChildProcess, stdout: *std.ArrayList(u8), stderr: *std.ArrayList(u8), max_output_bytes: usize) !void {
// The order of the objects here is important, WaitForMultipleObjects
// uses the same order when scanning the events.
var wait_objects = [_]windows.kernel32.HANDLE{
child.handle, child.stdout.?.handle, child.stderr.?.handle,
};
// XXX: Calling zeroes([2]windows.OVERLAPPED) causes the stage1 compiler
// to crash and burn.
var overlapped = [_]windows.OVERLAPPED{
mem.zeroes(windows.OVERLAPPED),
mem.zeroes(windows.OVERLAPPED),
};
var temp_buf: [2][4096]u8 = undefined;
// Kickstart the loop by issuing two async reads.
// ReadFile returns false and GetLastError returns ERROR_IO_PENDING if
// everything is ok.
_ = windows.kernel32.ReadFile(wait_objects[1], &temp_buf[0], temp_buf[0].len, null, &overlapped[0]);
_ = windows.kernel32.ReadFile(wait_objects[2], &temp_buf[1], temp_buf[1].len, null, &overlapped[1]);
while (true) {
const status = windows.kernel32.WaitForMultipleObjects(wait_objects.len, &wait_objects, 0, windows.INFINITE);
std.debug.print("status {x}\n", .{status});
switch (status) {
windows.WAIT_OBJECT_0 + 0 => {
// The child process was terminated.
break;
},
windows.WAIT_OBJECT_0 + 1 => {
// stdout is ready.
var read_bytes: u32 = undefined;
if (windows.kernel32.GetOverlappedResult(wait_objects[1], &overlapped[0], &read_bytes, 0) == 0) {
switch (windows.kernel32.GetLastError()) {
else => |err| return windows.unexpectedError(err),
}
}
try stdout.appendSlice(temp_buf[0][0..read_bytes]);
_ = windows.kernel32.ReadFile(wait_objects[1], &temp_buf[0], temp_buf[0].len, null, &overlapped[0]);
},
windows.WAIT_OBJECT_0 + 2 => {
// stderr is ready.
var read_bytes: u32 = undefined;
if (windows.kernel32.GetOverlappedResult(wait_objects[2], &overlapped[1], &read_bytes, 0) == 0) {
switch (windows.kernel32.GetLastError()) {
else => |err| return windows.unexpectedError(err),
}
}
try stdout.appendSlice(temp_buf[1][0..read_bytes]);
_ = windows.kernel32.ReadFile(wait_objects[2], &temp_buf[1], temp_buf[1].len, null, &overlapped[1]);
},
windows.WAIT_FAILED => {
switch (windows.kernel32.GetLastError()) {
else => |err| return windows.unexpectedError(err),
}
},
// We're waiting with an infinite timeout
windows.WAIT_TIMEOUT => unreachable,
else => unreachable,
}
}
}
/// Spawns a child process, waits for it, collecting stdout and stderr, and then returns.
/// If it succeeds, the caller owns result.stdout and result.stderr memory.
pub fn exec(args: struct {
@ -210,19 +310,21 @@ pub const ChildProcess = struct {
try child.spawn();
const stdout_in = child.stdout.?.reader();
const stderr_in = child.stderr.?.reader();
var stdout = std.ArrayList(u8).init(args.allocator);
var stderr = std.ArrayList(u8).init(args.allocator);
// TODO https://github.com/ziglang/zig/issues/6343
const stdout = try stdout_in.readAllAlloc(args.allocator, args.max_output_bytes);
errdefer args.allocator.free(stdout);
const stderr = try stderr_in.readAllAlloc(args.allocator, args.max_output_bytes);
errdefer args.allocator.free(stderr);
// XXX: Respect max_output_bytes
// XXX: Smarter reading logic, read directly into the ArrayList
if (builtin.os.tag == .windows) {
try collectOutputWindows(child, &stdout, &stderr, args.max_output_bytes);
} else {
try collectOutputPosix(child, &stdout, &stderr, args.max_output_bytes);
}
return ExecResult{
.term = try child.wait(),
.stdout = stdout,
.stderr = stderr,
.stdout = stdout.toOwnedSlice(),
.stderr = stderr.toOwnedSlice(),
};
}
@ -555,7 +657,7 @@ pub const ChildProcess = struct {
var g_hChildStd_OUT_Wr: ?windows.HANDLE = null;
switch (self.stdout_behavior) {
StdIo.Pipe => {
try windowsMakePipeOut(&g_hChildStd_OUT_Rd, &g_hChildStd_OUT_Wr, &saAttr);
try windowsMakePipe(&g_hChildStd_OUT_Rd, &g_hChildStd_OUT_Wr, &saAttr);
},
StdIo.Ignore => {
g_hChildStd_OUT_Wr = nul_handle;
@ -575,7 +677,7 @@ pub const ChildProcess = struct {
var g_hChildStd_ERR_Wr: ?windows.HANDLE = null;
switch (self.stderr_behavior) {
StdIo.Pipe => {
try windowsMakePipeOut(&g_hChildStd_ERR_Rd, &g_hChildStd_ERR_Wr, &saAttr);
try windowsMakePipe(&g_hChildStd_ERR_Rd, &g_hChildStd_ERR_Wr, &saAttr);
},
StdIo.Ignore => {
g_hChildStd_ERR_Wr = nul_handle;
@ -808,6 +910,55 @@ fn windowsDestroyPipe(rd: ?windows.HANDLE, wr: ?windows.HANDLE) void {
if (wr) |h| os.close(h);
}
var pipe_name_counter = std.atomic.Int(u32).init(1);
fn windowsMakePipe(rd: *?windows.HANDLE, wr: *?windows.HANDLE, sattr: *const windows.SECURITY_ATTRIBUTES) !void {
var tmp_buf: [128]u8 = undefined;
// Forge a random path for the pipe.
const pipe_path = std.fmt.bufPrintZ(
&tmp_buf,
"\\\\.\\pipe\\zig-childprocess-{d}-{d}",
.{ windows.kernel32.GetCurrentProcessId(), pipe_name_counter.fetchAdd(1) },
) catch unreachable;
// Create the read handle that can be used with overlapped IO ops.
const read_handle = windows.kernel32.CreateNamedPipeA(
pipe_path,
windows.PIPE_ACCESS_INBOUND | windows.FILE_FLAG_OVERLAPPED,
windows.PIPE_TYPE_BYTE,
1,
0x1000,
0x1000,
0,
sattr,
);
if (read_handle == windows.INVALID_HANDLE_VALUE) {
switch (windows.kernel32.GetLastError()) {
else => |err| return windows.unexpectedError(err),
}
}
const write_handle = windows.kernel32.CreateFileA(
pipe_path,
windows.GENERIC_WRITE,
0,
sattr,
windows.OPEN_EXISTING,
windows.FILE_ATTRIBUTE_NORMAL,
null,
);
if (write_handle == windows.INVALID_HANDLE_VALUE) {
switch (windows.kernel32.GetLastError()) {
else => |err| return windows.unexpectedError(err),
}
}
try windows.SetHandleInformation(read_handle, windows.HANDLE_FLAG_INHERIT, 0);
rd.* = read_handle;
wr.* = write_handle;
}
fn windowsMakePipeIn(rd: *?windows.HANDLE, wr: *?windows.HANDLE, sattr: *const windows.SECURITY_ATTRIBUTES) !void {
var rd_h: windows.HANDLE = undefined;
var wr_h: windows.HANDLE = undefined;

13
lib/std/os/windows/bits.zig
View File

@ -438,6 +438,19 @@ pub const SECURITY_ATTRIBUTES = extern struct {
pub const PSECURITY_ATTRIBUTES = *SECURITY_ATTRIBUTES;
pub const LPSECURITY_ATTRIBUTES = *SECURITY_ATTRIBUTES;
pub const PIPE_ACCESS_INBOUND = 0x00000001;
pub const PIPE_ACCESS_OUTBOUND = 0x00000002;
pub const PIPE_ACCESS_DUPLEX = 0x00000003;
pub const PIPE_TYPE_BYTE = 0x00000000;
pub const PIPE_TYPE_MESSAGE = 0x00000004;
pub const PIPE_READMODE_BYTE = 0x00000000;
pub const PIPE_READMODE_MESSAGE = 0x00000002;
pub const PIPE_WAIT = 0x00000000;
pub const PIPE_NOWAIT = 0x00000001;
pub const GENERIC_READ = 0x80000000;
pub const GENERIC_WRITE = 0x40000000;
pub const GENERIC_EXECUTE = 0x20000000;

33
lib/std/os/windows/kernel32.zig
View File

@ -15,6 +15,29 @@ pub extern "kernel32" fn CloseHandle(hObject: HANDLE) callconv(WINAPI) BOOL;
pub extern "kernel32" fn CreateDirectoryW(lpPathName: [*:0]const u16, lpSecurityAttributes: ?*SECURITY_ATTRIBUTES) callconv(WINAPI) BOOL;
pub extern "kernel32" fn SetEndOfFile(hFile: HANDLE) callconv(WINAPI) BOOL;
pub extern "kernel32" fn GetCurrentProcessId() callconv(.Stdcall) DWORD;
pub extern "kernel32" fn CreateNamedPipeA(
lpName: [*:0]const u8,
dwOpenMode: DWORD,
dwPipeMode: DWORD,
nMaxInstances: DWORD,
nOutBufferSize: DWORD,
nInBufferSize: DWORD,
nDefaultTimeOut: DWORD,
lpSecurityAttributes: ?*const SECURITY_ATTRIBUTES,
) callconv(.Stdcall) HANDLE;
pub extern "kernel32" fn CreateNamedPipeW(
lpName: LPCWSTR,
dwOpenMode: DWORD,
dwPipeMode: DWORD,
nMaxInstances: DWORD,
nOutBufferSize: DWORD,
nInBufferSize: DWORD,
nDefaultTimeOut: DWORD,
lpSecurityAttributes: ?*const SECURITY_ATTRIBUTES,
) callconv(.Stdcall) HANDLE;
pub extern "kernel32" fn CreateEventExW(
lpEventAttributes: ?*SECURITY_ATTRIBUTES,
lpName: [*:0]const u16,
@ -32,6 +55,16 @@ pub extern "kernel32" fn CreateFileW(
hTemplateFile: ?HANDLE,
) callconv(WINAPI) HANDLE;
pub extern "kernel32" fn CreateFileA(
lpFileName: [*:0]const u8,
dwDesiredAccess: DWORD,
dwShareMode: DWORD,
lpSecurityAttributes: ?*const SECURITY_ATTRIBUTES,
dwCreationDisposition: DWORD,
dwFlagsAndAttributes: DWORD,
hTemplateFile: ?HANDLE,
) callconv(.Stdcall) HANDLE;
pub extern "kernel32" fn CreatePipe(
hReadPipe: *HANDLE,
hWritePipe: *HANDLE,