Userland: Add a basic xargs

This adds a basic implementation of xargs.
The implemenation is missing quite a few options, and is not entirely
POSIX-compliant, but it gets the job done :^)

Userland/xargs.cpp

@@ -0,0 +1,298 @@
+/*
+ * Copyright (c) 2020, The SerenityOS developers.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/Function.h>
+#include <AK/StdLibExtras.h>
+#include <AK/StringBuilder.h>
+#include <LibCore/ArgsParser.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/wait.h>
+
+bool run_command(Vector<char*>&& child_argv, bool verbose, bool is_stdin, int devnull_fd);
+
+enum Decision {
+    Unget,
+    Continue,
+    Stop,
+};
+bool read_items(FILE* fp, char entry_separator, Function<Decision(StringView)>);
+
+class ParsedInitialArguments {
+public:
+    ParsedInitialArguments(Vector<const char*>&, const StringView& placeholder);
+
+    void for_each_joined_argument(const StringView&, Function<void(const String&)>) const;
+
+    size_t size() const { return m_all_parts.size(); }
+
+private:
+    Vector<Vector<StringView>> m_all_parts;
+};
+
+int main(int argc, char** argv)
+{
+    if (pledge("stdio rpath proc exec", nullptr) < 0) {
+        perror("pledge");
+        return 1;
+    }
+
+    const char* placeholder = nullptr;
+    bool split_with_nulls = false;
+    const char* specified_delimiter = "\n";
+    Vector<const char*> arguments;
+    bool verbose = false;
+    const char* file_to_read = "-";
+    int max_lines_for_one_command = 0;
+    int max_bytes_for_one_command = ARG_MAX;
+
+    Core::ArgsParser args_parser;
+    args_parser.add_option(placeholder, "Placeholder string to be replaced in arguments", "replace", 'I', "placeholder");
+    args_parser.add_option(split_with_nulls, "Split input items with the null character instead of newline", "null", '0');
+    args_parser.add_option(specified_delimiter, "Split the input items with the specified character", "delimiter", 'd', "delim");
+    args_parser.add_option(verbose, "Display each command before executing it", "verbose", 'v');
+    args_parser.add_option(file_to_read, "Read arguments from the specified file instead of stdin", "arg-file", 'a', "file");
+    args_parser.add_option(max_lines_for_one_command, "Use at most max-lines lines to create a command", "line-limit", 'L', "max-lines");
+    args_parser.add_option(max_bytes_for_one_command, "Use at most max-chars characters to create a command", "char-limit", 's', "max-chars");
+    args_parser.add_positional_argument(arguments, "Command and any initial arguments for it", "command", Core::ArgsParser::Required::No);
+    args_parser.parse(argc, argv);
+
+    size_t max_bytes = min(ARG_MAX, max_bytes_for_one_command);
+    size_t max_lines = max(max_lines_for_one_command, 0);
+
+    if (!split_with_nulls && strlen(specified_delimiter) > 1) {
+        fprintf(stderr, "xargs: the delimiter must be a single byte\n");
+        return 1;
+    }
+
+    char entry_separator = split_with_nulls ? '\0' : *specified_delimiter;
+
+    StringView placeholder_view { placeholder };
+
+    if (!placeholder_view.is_empty())
+        max_lines = 1;
+
+    if (arguments.is_empty())
+        arguments.append("echo");
+
+    ParsedInitialArguments initial_arguments(arguments, placeholder_view);
+
+    FILE* fp = stdin;
+    bool is_stdin = true;
+
+    if (StringView { "-" } != file_to_read) {
+        // A file was specified, try to open it.
+        fp = fopen(file_to_read, "re");
+        if (!fp) {
+            perror("fopen");
+            return 1;
+        }
+        is_stdin = false;
+    }
+
+    StringBuilder builder;
+    Vector<char*> child_argv;
+
+    int devnull_fd = 0;
+
+    if (is_stdin) {
+        devnull_fd = open("/dev/null", O_RDONLY | O_CLOEXEC);
+        if (devnull_fd < 0) {
+            perror("open");
+            return 1;
+        }
+    }
+
+    size_t total_command_length = 0;
+    size_t items_used_for_this_command = 0;
+
+    auto fail = read_items(fp, entry_separator, [&](StringView item) {
+        if (item.ends_with('\n'))
+            item = item.substring_view(0, item.length() - 1);
+
+        if (item.is_empty())
+            return Continue;
+
+        // The first item is processed differently, as all the initial-arguments are processed _with_ that item
+        // as their substitution target (assuming that substitution is enabled).
+        // Note that if substitution is not enabled, we manually insert a substitution target at the end of initial-arguments,
+        // so this item has a place to go.
+        if (items_used_for_this_command == 0) {
+            child_argv.ensure_capacity(initial_arguments.size());
+
+            initial_arguments.for_each_joined_argument(item, [&](const String& string) {
+                total_command_length += string.length();
+                child_argv.append(strdup(string.characters()));
+            });
+
+            ++items_used_for_this_command;
+        } else {
+            if ((max_lines > 0 && items_used_for_this_command + 1 > max_lines) || total_command_length + item.length() + 1 >= max_bytes) {
+                // Note: This `move' does not actually move-construct a new Vector at the callsite, it only allows perfect-forwarding
+                //       and does not invalidate `child_argv' in this scope.
+                //       The same applies for the one below.
+                if (!run_command(move(child_argv), verbose, is_stdin, devnull_fd))
+                    return Stop;
+                items_used_for_this_command = 0;
+                total_command_length = 0;
+                return Unget;
+            } else {
+                child_argv.append(strndup(item.characters_without_null_termination(), item.length()));
+                total_command_length += item.length();
+                ++items_used_for_this_command;
+            }
+        }
+
+        return Continue;
+    });
+
+    if (!fail && !child_argv.is_empty())
+        fail = !run_command(move(child_argv), verbose, is_stdin, devnull_fd);
+
+    if (!is_stdin)
+        fclose(fp);
+
+    return fail ? 1 : 0;
+}
+
+bool read_items(FILE* fp, char entry_separator, Function<Decision(StringView)> callback)
+{
+    bool fail = false;
+
+    for (;;) {
+        char* item = nullptr;
+        size_t buffer_size = 0;
+
+        auto item_size = getdelim(&item, &buffer_size, entry_separator, fp);
+
+        if (item_size < 0) {
+            // getdelim() will return -1 and set errno to 0 on EOF.
+            if (errno != 0) {
+                perror("getdelim");
+                fail = true;
+            }
+            break;
+        }
+
+        Decision decision;
+        do {
+            decision = callback(item);
+            if (decision == Stop) {
+                free(item);
+                return true;
+            }
+        } while (decision == Unget);
+
+        free(item);
+    }
+
+    return fail;
+}
+
+bool run_command(Vector<char*>&& child_argv, bool verbose, bool is_stdin, int devnull_fd)
+{
+    child_argv.append(nullptr);
+
+    if (verbose) {
+        StringBuilder builder;
+        builder.join(" ", child_argv);
+        fprintf(stderr, "xargs: %s\n", builder.to_string().characters());
+        fflush(stderr);
+    }
+
+    auto pid = fork();
+    if (pid < 0) {
+        perror("fork");
+        return false;
+    }
+
+    if (pid == 0) {
+        if (is_stdin)
+            dup2(devnull_fd, STDIN_FILENO);
+
+        execvp(child_argv[0], child_argv.data());
+        exit(1);
+    }
+
+    for (auto* ptr : child_argv)
+        free(ptr);
+
+    child_argv.clear_with_capacity();
+
+    int wstatus = 0;
+    if (waitpid(pid, &wstatus, 0) < 0) {
+        perror("waitpid");
+        return false;
+    }
+
+    if (WIFEXITED(wstatus)) {
+        if (WEXITSTATUS(wstatus) != 0)
+            return false;
+    } else {
+        return false;
+    }
+
+    return true;
+}
+
+ParsedInitialArguments::ParsedInitialArguments(Vector<const char*>& arguments, const StringView& placeholder)
+{
+    m_all_parts.ensure_capacity(arguments.size());
+    bool some_argument_has_placeholder = false;
+
+    for (auto argument : arguments) {
+        StringView arg { argument };
+
+        if (placeholder.is_empty()) {
+            m_all_parts.append({ arg });
+        } else {
+            auto parts = arg.split_view(placeholder, true);
+            some_argument_has_placeholder = some_argument_has_placeholder || parts.size() > 1;
+            m_all_parts.append(move(parts));
+        }
+    }
+
+    // Append an implicit placeholder at the end if no argument has any placeholders.
+    if (!some_argument_has_placeholder) {
+        Vector<StringView> parts;
+        parts.append("");
+        parts.append("");
+        m_all_parts.append(move(parts));
+    }
+}
+
+void ParsedInitialArguments::for_each_joined_argument(const StringView& separator, Function<void(const String&)> callback) const
+{
+    StringBuilder builder;
+    for (auto& parts : m_all_parts) {
+        builder.clear();
+        builder.join(separator, parts);
+        callback(builder.to_string());
+    }
+}