Add getpwent() family of functions to LibC.

Also add a little /etc/passwd database. There's just me in there.

LibC/pwd.cpp

@@ -0,0 +1,117 @@
+#include <pwd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include <AK/String.h>
+
+struct passwd_with_strings : public passwd {
+    char name_buffer[256];
+    char passwd_buffer[256];
+    char gecos_buffer[256];
+    char dir_buffer[256];
+    char shell_buffer[256];
+};
+
+static FILE* __pwdb_stream = nullptr;
+static unsigned __pwdb_line_number = 0;
+static struct passwd_with_strings* __pwdb_entry = nullptr;
+
+void setpwent()
+{
+    __pwdb_line_number = 0;
+    if (__pwdb_stream) {
+        rewind(__pwdb_stream);
+    } else {
+        __pwdb_stream = fopen("/etc/passwd", "r");
+        __pwdb_entry = (struct passwd_with_strings*)mmap(nullptr, getpagesize());
+    }
+}
+
+void endpwent()
+{
+    __pwdb_line_number = 0;
+    if (__pwdb_stream) {
+        fclose(__pwdb_stream);
+        __pwdb_stream = nullptr;
+    }
+    if (__pwdb_entry) {
+        munmap(__pwdb_entry, getpagesize());
+        __pwdb_entry = nullptr;
+    }
+}
+
+struct passwd* getpwuid(uid_t uid)
+{
+    setpwent();
+    while (auto* pw = getpwent()) {
+        if (pw->pw_uid == uid)
+            return pw;
+    }
+    return nullptr;
+}
+
+struct passwd* getpwnam(const char* name)
+{
+    setpwent();
+    while (auto* pw = getpwent()) {
+        if (!strcmp(pw->pw_name, name))
+            return pw;
+    }
+    return nullptr;
+}
+
+struct passwd* getpwent()
+{
+    if (!__pwdb_stream)
+        setpwent();
+
+    if (feof(__pwdb_stream))
+        return nullptr;
+
+next_entry:
+    char buffer[1024];
+    ++__pwdb_line_number;
+    char* s = fgets(buffer, sizeof(buffer), __pwdb_stream);
+    if (!s)
+        return nullptr;
+    if (feof(__pwdb_stream))
+        return nullptr;
+    String line(s);
+    auto parts = line.split(':');
+    if (parts.size() != 7) {
+        fprintf(stderr, "getpwent(): Malformed entry on line %u\n", __pwdb_line_number);
+        goto next_entry;
+    }
+    auto& e_name = parts[0];
+    auto& e_passwd = parts[1];
+    auto& e_uid_string = parts[2];
+    auto& e_gid_string = parts[3];
+    auto& e_gecos = parts[4];
+    auto& e_dir = parts[5];
+    auto& e_shell = parts[6];
+    bool ok;
+    uid_t e_uid = e_uid_string.toUInt(ok);
+    if (!ok) {
+        fprintf(stderr, "getpwent(): Malformed UID on line %u\n", __pwdb_line_number);
+        goto next_entry;
+    }
+    gid_t e_gid = e_gid_string.toUInt(ok);
+    if (!ok) {
+        fprintf(stderr, "getpwent(): Malformed GID on line %u\n", __pwdb_line_number);
+        goto next_entry;
+    }
+    __pwdb_entry->pw_uid = e_uid;
+    __pwdb_entry->pw_gid = e_gid;
+    __pwdb_entry->pw_name = __pwdb_entry->name_buffer;
+    __pwdb_entry->pw_passwd = __pwdb_entry->passwd_buffer;
+    __pwdb_entry->pw_gecos = __pwdb_entry->gecos_buffer;
+    __pwdb_entry->pw_dir = __pwdb_entry->dir_buffer;
+    __pwdb_entry->pw_shell = __pwdb_entry->shell_buffer;
+    strncpy(__pwdb_entry->name_buffer, e_name.characters(), e_name.length());
+    strncpy(__pwdb_entry->passwd_buffer, e_passwd.characters(), e_passwd.length());
+    strncpy(__pwdb_entry->gecos_buffer, e_gecos.characters(), e_gecos.length());
+    strncpy(__pwdb_entry->dir_buffer, e_dir.characters(), e_dir.length());
+    strncpy(__pwdb_entry->shell_buffer, e_shell.characters(), e_shell.length());
+    return __pwdb_entry;
+}