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Use pipe instead of 'sh -c' to spawn subprocesses #2580

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Use pipe instead of 'sh -c' to spawn subprocesses #2580

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23 changes: 23 additions & 0 deletions src/subprocess-posix.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -51,6 +51,7 @@ Subprocess::~Subprocess() {

bool Subprocess::Start(SubprocessSet* set, const string& command) {
int output_pipe[2];
int input_pipe[2] = { -1, -1 };
if (pipe(output_pipe) < 0)
Fatal("pipe: %s", strerror(errno));
fd_ = output_pipe[0];
Expand Down Expand Up @@ -119,10 +120,32 @@ bool Subprocess::Start(SubprocessSet* set, const string& command) {
Fatal("posix_spawnattr_setflags: %s", strerror(err));

const char* spawned_args[] = { "/bin/sh", "-c", command.c_str(), NULL };
if (!use_console_) {
if (pipe(input_pipe) < 0)
Fatal("pipe: %s", strerror(errno));
SetCloseOnExec(input_pipe[0]);
err = posix_spawn_file_actions_adddup2(&action, input_pipe[0], 0);
if (err != 0)
Fatal("posix_spawn_file_actions_adddup2: %s", strerror(err));
err = posix_spawn_file_actions_addclose(&action, input_pipe[0]);
if (err != 0)
Fatal("posix_spawn_file_actions_addclose: %s", strerror(err));
err = posix_spawn_file_actions_addclose(&action, input_pipe[1]);
if (err != 0)
Fatal("posix_spawn_file_actions_addclose: %s", strerror(err));
spawned_args[1] = NULL;
}
err = posix_spawn(&pid_, "/bin/sh", &action, &attr,
const_cast<char**>(spawned_args), environ);
if (err != 0)
Fatal("posix_spawn: %s", strerror(err));

if (!use_console_) {
ssize_t written = write(input_pipe[1], command.c_str(), command.size());
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Does that work for large commands?

Most systems use a default pipe buffer size of 64 kiB, some even smaller (see https://github.com/afborchert/pipebuf) so this write() call will block infinitely if command.size() is larger than that (and the bug referenced by this PR talks about command sizes of 128 kiB).

Linux has fcntl(F_SETPIPE_SZ) to change that the pipe buffer size, but I am not aware of similar features on other systems. Even the Linux pipe manpage tsates that applications should not rely on a specific capacity.

It might be more portable and simple to write long commands to a file that is passed as the first argument to the shell instead. And that would also work for console processes too.

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friendly ping?

if (written == -1)
Fatal("write to input pipe: %s", strerror(errno));
close(input_pipe[1]);
}

err = posix_spawnattr_destroy(&attr);
if (err != 0)
Expand Down
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