LibC: Implement scandir(...) to enumerate directories.

I ran into a need for this when running  stress-ng against the system.
This change implements the full functionality of scandir, where it
accepts a selection callback, as well as a comparison callback.
These can be used to trim and sort the entries from the directory
that we are being asked to enumerate. A test was also included to
validate the new functionality.

Userland/Libraries/LibC/dirent.cpp

@@ -5,7 +5,9 @@
  */
 
 #include <AK/Assertions.h>
+#include <AK/ScopeGuard.h>
 #include <AK/StdLibExtras.h>
+#include <AK/Vector.h>
 #include <dirent.h>
 #include <errno.h>
 #include <fcntl.h>
@@ -184,4 +186,64 @@ int dirfd(DIR* dirp)
     VERIFY(dirp);
     return dirp->fd;
 }
+
+int scandir(const char* dir_name,
+    struct dirent*** namelist,
+    int (*select)(const struct dirent*),
+    int (*compare)(const struct dirent**, const struct dirent**))
+{
+    auto dir = opendir(dir_name);
+    if (dir == nullptr)
+        return -1;
+    ScopeGuard guard = [&] {
+        closedir(dir);
+    };
+
+    Vector<struct dirent*> tmp_names;
+    ScopeGuard names_guard = [&] {
+        tmp_names.remove_all_matching([&](auto& entry) {
+            free(entry);
+            return true;
+        });
+    };
+
+    while (true) {
+        errno = 0;
+        auto entry = readdir(dir);
+        if (!entry)
+            break;
+
+        // Omit entries the caller chooses to ignore.
+        if (select && !select(entry))
+            continue;
+
+        auto entry_copy = (struct dirent*)malloc(entry->d_reclen);
+        if (!entry_copy)
+            break;
+        memcpy(entry_copy, entry, entry->d_reclen);
+        tmp_names.append(entry_copy);
+    }
+
+    // Propagate any errors encountered while accumulating back to the user.
+    if (errno) {
+        return -1;
+    }
+
+    // Sort the entries if the user provided a comparator.
+    if (compare) {
+        qsort(tmp_names.data(), tmp_names.size(), sizeof(struct dirent*), (int (*)(const void*, const void*))compare);
+    }
+
+    const int size = tmp_names.size();
+    auto names = (struct dirent**)malloc(size * sizeof(struct dirent*));
+    for (auto i = 0; i < size; i++) {
+        names[i] = tmp_names[i];
+    }
+
+    // Disable the scope guard which free's names on error.
+    tmp_names.clear();
+
+    *namelist = names;
+    return size;
+}
 }

Userland/Libraries/LibC/dirent.h

@@ -55,4 +55,8 @@ struct dirent* readdir(DIR*);
 int readdir_r(DIR*, struct dirent*, struct dirent**);
 int dirfd(DIR*);
 
+int scandir(const char* dirp, struct dirent*** namelist,
+    int (*filter)(const struct dirent*),
+    int (*compar)(const struct dirent**, const struct dirent**));
+
 __END_DECLS

Userland/Tests/LibC/CMakeLists.txt

@@ -4,6 +4,7 @@ set(TEST_SOURCES
     ${CMAKE_CURRENT_SOURCE_DIR}/TestLibCTime.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/TestLibCMkTemp.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/TestLibCExec.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/TestLibCDirEnt.cpp
 )
 
 file(GLOB CMD_SOURCES  CONFIGURE_DEPENDS "*.cpp")

Userland/Tests/LibC/TestLibCDirEnt.cpp

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2021, Brian Gianforcaro <bgianf@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestCase.h>
+#include <dirent.h>
+#include <string.h>
+
+TEST_CASE(scandir_basic_scenario)
+{
+    struct dirent** namelist = nullptr;
+    auto entries = scandir("/etc", &namelist, nullptr, nullptr);
+    EXPECT(entries > 0);
+    EXPECT_NE(namelist, nullptr);
+
+    bool found_passwd = false;
+    for (auto i = 0; i < entries; i++) {
+        if (strcmp(namelist[i]->d_name, "passwd") == 0) {
+            found_passwd = true;
+            break;
+        }
+        free(namelist[i]);
+    }
+    EXPECT(found_passwd);
+    free(namelist);
+}