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Handle fork(2) failures #22

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Merged
merged 1 commit into from
Jan 21, 2020
+10 −0
Merged

Handle fork(2) failures #22

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5 changes: 5 additions & 0 deletions src/mp/gen.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -7,10 +7,12 @@

#include <algorithm>
#include <capnp/schema-parser.h>
#include <errno.h>
#include <fstream>
#include <map>
#include <set>
#include <sstream>
#include <system_error>
#include <unistd.h>
#include <vector>

Expand Down Expand Up @@ -137,6 +139,9 @@ void Generate(kj::StringPtr src_prefix,
args.emplace_back("--output=" capnp_PREFIX "/bin/capnpc-c++");
args.emplace_back(src_file);
int pid = fork();
if (pid == -1) {
throw std::system_error(errno, std::system_category(), "fork");
}
if (!pid) {
mp::ExecProcess(args);
}
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5 changes: 5 additions & 0 deletions src/mp/util.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,7 @@

#include <mp/util.h>

#include <errno.h>
#include <kj/array.h>
#include <pthread.h>
#include <sstream>
Expand All @@ -13,6 +14,7 @@
#include <sys/types.h>
#include <sys/un.h>
#include <sys/wait.h>
#include <system_error>
#include <unistd.h>

#if __linux__
Expand Down Expand Up @@ -83,6 +85,9 @@ int SpawnProcess(int& pid, FdToArgsFn&& fd_to_args)
}

pid = fork();
if (pid == -1) {
throw std::system_error(errno, std::system_category(), "fork");
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Interesting, I didn't know system_error took a third string argument. It'd be good to start using this more places.

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Yes, it also occurred to me that if std::system_category() modifies errno then std::system_error() may get the wrong value of errno:

#include <errno.h>
#include <stdio.h>

int f1() {
    errno = 3;
    return 20;
}   

void f2(int e, int x) {
    printf("%d %d\n", e, x);
}   

int main(int, char**) {
    errno = 2; // resulted from fork() or whatever
    f2(errno, f1());
    return 0;
}

produces:

# clang 9.0.1
$ clang++90 t.cc -o t && ./t
2 20

# gcc 9.2.0
$ g++9 t.cc -o t && ./t
3 20

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It's a possible concern, but in practice if that constructor was setting errno, it would probably break a lot of things. Also in the worst case error message text would just be less clear

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Yes, safe to ignore in this case. Just the pattern func1(x, func2_may_possibly_modify_x()); is better to be avoided.

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system_error took a third string argument. It'd be good to start using this more places.

#23 Tell std::system_error() which function failed

Cheerz!

}
if (close(fds[pid ? 0 : 1]) != 0) {
throw std::system_error(errno, std::system_category());
}
Expand Down