Introduce dup2_list_t

This represents a "resolved" io_chain_t, where all of the different io_data_t
types have been reduced to a sequence of dup2() and close(). This will
eliminate a lot of the logic duplication around posix_spawn vs fork, and pave
the way for in-process redirections.

CMakeLists.txt

@@ -77,7 +77,7 @@ SET(FISH_SRCS
     src/postfork.cpp src/proc.cpp src/reader.cpp src/sanity.cpp src/screen.cpp
     src/signal.cpp src/tinyexpr.cpp src/tnode.cpp src/tokenizer.cpp src/utf8.cpp src/util.cpp
     src/wcstringutil.cpp src/wgetopt.cpp src/wildcard.cpp src/wutil.cpp
-    src/future_feature_flags.cpp
+    src/future_feature_flags.cpp src/redirection.cpp
 )
 
 # Header files are just globbed.

Makefile.in

@@ -119,7 +119,7 @@ FISH_OBJS := obj/autoload.o obj/builtin.o obj/builtin_bg.o obj/builtin_bind.o ob
 	obj/parser_keywords.o obj/path.o obj/postfork.o obj/proc.o obj/reader.o \
 	obj/sanity.o obj/screen.o obj/signal.o obj/tinyexpr.o obj/tokenizer.o obj/tnode.o obj/utf8.o \
 	obj/util.o obj/wcstringutil.o obj/wgetopt.o obj/wildcard.o obj/wutil.o \
-	obj/future_feature_flags.o
+	obj/future_feature_flags.o obj/redirection.o
 
 FISH_INDENT_OBJS := obj/fish_indent.o obj/print_help.o $(FISH_OBJS)
 

src/fish_tests.cpp

@@ -64,6 +64,7 @@
 #include "path.h"
 #include "proc.h"
 #include "reader.h"
+#include "redirection.h"
 #include "screen.h"
 #include "signal.h"
 #include "tnode.h"
@@ -2339,6 +2340,31 @@ static void test_wcstod() {
     tod_test(L"nope", "nope");
 }
 
+static void test_dup2s() {
+    using std::make_shared;
+    io_chain_t chain;
+    chain.push_back(make_shared<io_close_t>(17));
+    chain.push_back(make_shared<io_fd_t>(3, 19, true));
+    auto list = dup2_list_t::resolve_chain(chain);
+    do_test(list.has_value());
+    do_test(list->get_actions().size() == 2);
+
+    auto act1 = list->get_actions().at(0);
+    do_test(act1.src == 17);
+    do_test(act1.target == -1);
+
+    auto act2 = list->get_actions().at(1);
+    do_test(act2.src == 19);
+    do_test(act2.target == 3);
+
+    // Invalid files should fail to open.
+    // Suppress the debug() message.
+    scoped_push<int> saved_debug_level(&debug_level, -1);
+    chain.push_back(make_shared<io_file_t>(2, L"/definitely/not/a/valid/path/for/this/test", 0666));
+    list = dup2_list_t::resolve_chain(chain);
+    do_test(!list.has_value());
+}
+
 /// Testing colors.
 static void test_colors() {
     say(L"Testing colors");
@@ -5071,6 +5097,7 @@ int main(int argc, char **argv) {
     if (should_test_function("abbreviations")) test_abbreviations();
     if (should_test_function("test")) test_test();
     if (should_test_function("wcstod")) test_wcstod();
+    if (should_test_function("dup2s")) test_dup2s();
     if (should_test_function("path")) test_path();
     if (should_test_function("pager_navigation")) test_pager_navigation();
     if (should_test_function("pager_layout")) test_pager_layout();

src/io.cpp

@@ -163,11 +163,8 @@ void io_print(const io_chain_t &chain)
 }
 #endif
 
-/// If the given fd is used by the io chain, duplicates it repeatedly until an fd not used in the io
-/// chain is found, or we run out. If we return a new fd or an error, closes the old one. Any fd
-/// created is marked close-on-exec. Returns -1 on failure (in which case the given fd is still
-/// closed).
-static int move_fd_to_unused(int fd, const io_chain_t &io_chain) {
+
+int move_fd_to_unused(int fd, const io_chain_t &io_chain, bool cloexec) {
     if (fd < 0 || io_chain.get_io_for_fd(fd).get() == NULL) {
         return fd;
     }
@@ -188,7 +185,7 @@ static int move_fd_to_unused(int fd, const io_chain_t &io_chain) {
         // Ok, we have a new candidate fd. Recurse. If we get a valid fd, either it's the same as
         // what we gave it, or it's a new fd and what we gave it has been closed. If we get a
         // negative value, the fd also has been closed.
-        set_cloexec(tmp_fd);
+        if (cloexec) set_cloexec(tmp_fd);
         new_fd = move_fd_to_unused(tmp_fd, io_chain);
     }
 

src/io.h

@@ -295,6 +295,12 @@ shared_ptr<io_data_t> io_chain_get(io_chain_t &src, int fd);
 /// set to -1).
 bool pipe_avoid_conflicts_with_io_chain(int fds[2], const io_chain_t &ios);
 
+/// If the given fd is used by the io chain, duplicates it repeatedly until an fd not used in the io
+/// chain is found, or we run out. If we return a new fd or an error, closes the old one.
+/// If \p cloexec is set, any fd created is marked close-on-exec.
+/// \returns -1 on failure (in which case the given fd is still closed).
+int move_fd_to_unused(int fd, const io_chain_t &io_chain, bool cloexec = true);
+
 /// Class representing the output that a builtin can generate.
 class output_stream_t {
    private:

src/redirection.cpp

@@ -0,0 +1,90 @@
+#include "config.h"  // IWYU pragma: keep
+
+#include "redirection.h"
+#include "wutil.h"
+
+#include <fcntl.h>
+
+/// File descriptor redirection error message.
+#define FD_ERROR "An error occurred while redirecting file descriptor %s"
+
+/// Pipe error message.
+#define LOCAL_PIPE_ERROR "An error occurred while setting up pipe"
+
+#define NOCLOB_ERROR _(L"The file '%s' already exists")
+
+#define FILE_ERROR _(L"An error occurred while redirecting file '%s'")
+
+/// Base open mode to pass to calls to open.
+#define OPEN_MASK 0666
+
+dup2_list_t::~dup2_list_t() = default;
+
+maybe_t<dup2_list_t> dup2_list_t::resolve_chain(const io_chain_t &io_chain) {
+    ASSERT_IS_NOT_FORKED_CHILD();
+    dup2_list_t result;
+    for (const auto &io_ref : io_chain) {
+        switch (io_ref->io_mode) {
+            case io_mode_t::file: {
+                // Here we definitely do not want to set CLO_EXEC because our child needs access.
+                // Open the file.
+                const io_file_t *io_file = static_cast<const io_file_t *>(io_ref.get());
+                int file_fd = open(io_file->filename_cstr, io_file->flags, OPEN_MASK);
+                if (file_fd < 0) {
+                    if ((io_file->flags & O_EXCL) && (errno == EEXIST)) {
+                        debug(1, NOCLOB_ERROR, io_file->filename_cstr);
+                    } else {
+                        debug(1, FILE_ERROR, io_file->filename_cstr);
+                        if (should_debug(1)) wperror(L"open");
+                    }
+                    return none();
+                }
+
+                // If by chance we got the file we want, we're done. Otherwise move the fd to an unused place and dup2 it.
+                // Note move_fd_to_unused() will close the incoming file_fd.
+                if (file_fd != io_file->fd) {
+                    file_fd = move_fd_to_unused(file_fd, io_chain, false /* cloexec */);
+                    if (file_fd < 0) {
+                        debug(1, FILE_ERROR, io_file->filename_cstr);
+                        if (should_debug(1)) wperror(L"dup");
+                        return none();
+                    }
+                }
+
+                // Record that we opened this file, so we will auto-close it.
+                assert(file_fd >= 0 && "Should have a valid file_fd");
+                result.opened_fds_.emplace_back(file_fd);
+
+                // Mark our dup2 and our close actions.
+                result.add_dup2(file_fd, io_file->fd);
+                result.add_close(file_fd);
+                break;
+            }
+
+            case io_mode_t::close: {
+                const io_close_t *io = static_cast<const io_close_t *>(io_ref.get());
+                result.add_close(io->fd);
+                break;
+            }
+
+            case io_mode_t::fd: {
+                const io_fd_t *io = static_cast<const io_fd_t *>(io_ref.get());
+                result.add_dup2(io->old_fd, io->fd);
+                break;
+            }
+
+            case io_mode_t::buffer:
+            case io_mode_t::pipe: {
+                const io_pipe_t *io = static_cast<const io_pipe_t *>(io_ref.get());
+                // If write_pipe_idx is 0, it means we're connecting to the read end (first pipe
+                // fd). If it's 1, we're connecting to the write end (second pipe fd).
+                unsigned int write_pipe_idx = (io->is_input ? 0 : 1);
+                result.add_dup2(io->pipe_fd[write_pipe_idx], io->fd);
+                if (io->pipe_fd[0] >= 0) result.add_close(io->pipe_fd[0]);
+                if (io->pipe_fd[1] >= 0) result.add_close(io->pipe_fd[1]);
+                break;
+            }
+        }
+    }
+    return {std::move(result)};
+}

src/redirection.h

@@ -0,0 +1,62 @@
+#ifndef FISH_REDIRECTION_H
+#define FISH_REDIRECTION_H
+
+#include "common.h"
+#include "maybe.h"
+#include "io.h"
+
+#include <vector>
+
+/// This file supports "applying" redirections.
+
+/// A class representing a sequence of basic redirections.
+class dup2_list_t {
+    /// A type that represents the action dup2(src, target).
+    /// If target is negative, this represents close(src).
+    /// Note none of the fds here are considered 'owned'.
+    struct action_t {
+        int src;
+        int target;
+    };
+
+    /// The list of actions.
+    std::vector<action_t> actions_;
+
+    /// The list of fds that we opened, and are responsible for closing.
+    std::vector<autoclose_fd_t> opened_fds_;
+
+    /// Append a dup2 action.
+    void add_dup2(int src, int target) {
+        assert(src >= 0 && target >= 0 && "Invalid fd in add_dup2");
+        if (src != target) {
+            actions_.push_back(action_t{src, target});
+        }
+    }
+
+    /// Append a close action.
+    void add_close(int fd) {
+        assert(fd >= 0 && "Invalid fd in add_close");
+        actions_.push_back(action_t{fd, -1});
+    }
+
+    dup2_list_t() = default;
+
+public:
+    ~dup2_list_t();
+
+    /// Disable copying because we own our fds.
+    dup2_list_t(const dup2_list_t &) = delete;
+    void operator=(const dup2_list_t &) = delete;
+
+    dup2_list_t(dup2_list_t &&) = default;
+    dup2_list_t &operator=(dup2_list_t &&) = default;
+
+    /// \return the list of dup2 actions.
+    const std::vector<action_t> &get_actions() const { return actions_; }
+
+    /// Produce a dup_fd_list_t from an io_chain. This may not be called before fork().
+    /// The result contains the list of fd actions (dup2 and close), as well as the list of fds opened.
+    static maybe_t<dup2_list_t> resolve_chain(const io_chain_t &);
+};
+
+#endif