One-byte process signaling through POSIX pipes

1// Copyright (c) 2021-present The Bitcoin Core developers
2// Distributed under the MIT software license, see the accompanying
3// file COPYING or http://www.opensource.org/licenses/mit-license.php.
4
5#include <bitcoin-build-config.h> // IWYU pragma: keep
6
7#include <util/tokenpipe.h>
8
9#ifndef WIN32
10
11#include <cerrno>
12#include <optional>
13
14#include <fcntl.h>
15#include <sys/types.h>
16#include <unistd.h>
17
18TokenPipeEnd TokenPipe::TakeReadEnd()
19{
20    TokenPipeEnd res(m_fds[0]);
21    m_fds[0] = -1;
22    return res;
23}
24
25TokenPipeEnd TokenPipe::TakeWriteEnd()
26{
27    TokenPipeEnd res(m_fds[1]);
28    m_fds[1] = -1;
29    return res;
30}
31
32TokenPipeEnd::TokenPipeEnd(int fd) : m_fd(fd)
33{
34}
35
36TokenPipeEnd::~TokenPipeEnd()
37{
38    Close();
39}
40
41int TokenPipeEnd::TokenWrite(uint8_t token)
42{
43    while (true) {
44        ssize_t result = write(m_fd, &token, 1);
45        if (result < 0) {
46            // Failure. It's possible that the write was interrupted by a signal,
47            // in that case retry.
48            if (errno != EINTR) {
49                return TS_ERR;
50            }
51        } else if (result == 0) {
52            return TS_EOS;
53        } else { // ==1
54            return 0;
55        }
56    }
57}
58
59int TokenPipeEnd::TokenRead()
60{
61    uint8_t token;
62    while (true) {
63        ssize_t result = read(m_fd, &token, 1);
64        if (result < 0) {
65            // Failure. Check if the read was interrupted by a signal,
66            // in that case retry.
67            if (errno != EINTR) {
68                return TS_ERR;
69            }
70        } else if (result == 0) {
71            return TS_EOS;
72        } else { // ==1
73            return token;
74        }
75    }
76    return token;
77}
78
79void TokenPipeEnd::Close()
80{
81    if (m_fd != -1) close(m_fd);
82    m_fd = -1;
83}
84
85std::optional<TokenPipe> TokenPipe::Make()
86{
87    int fds[2] = {-1, -1};
88#if HAVE_O_CLOEXEC && HAVE_DECL_PIPE2
89    if (pipe2(fds, O_CLOEXEC) != 0) {
90        return std::nullopt;
91    }
92#else
93    if (pipe(fds) != 0) {
94        return std::nullopt;
95    }
96#endif
97    return TokenPipe(fds);
98}
99
100TokenPipe::~TokenPipe()
101{
102    Close();
103}
104
105void TokenPipe::Close()
106{
107    if (m_fds[0] != -1) close(m_fds[0]);
108    if (m_fds[1] != -1) close(m_fds[1]);
109    m_fds[0] = m_fds[1] = -1;
110}
111
112#endif // WIN32
113