LibC: syslog and lots of compat stuff for it

This is an implementation of syslog with some OpenBSD extensions.
There is no syslogd support (so it only logs to dbgprintf/stderr),
but otherwise is functional.

Many weird defines are always present, because some syslog users in
the wild check for their existence.

Libraries/LibC/syslog.cpp

@@ -0,0 +1,132 @@
+// Has to be defined before including due to legacy Unices
+#define SYSLOG_NAMES 1
+
+#include <AK/StringBuilder.h>
+#include <stdio.h>
+#include <string.h>
+#include <syslog.h>
+#include <unistd.h>
+
+// This implementation doesn't talk to a syslog server. Any options related to
+// that are no-ops.
+
+extern "C" {
+
+// For implementation simplicity, we actually only use the re-entrant version
+// of each function, and the version that isn't just redirects with a static
+// struct to share.
+static struct syslog_data global_log_data = {
+    .ident = nullptr,
+    .logopt = 0,
+    .facility = LOG_USER,
+    .maskpri = LOG_UPTO(LOG_DEBUG)
+};
+
+// Used when ident is null, since syslog traditionally prints the program's
+// own name; the process name will always be the same unless we exec.
+static char program_name_buffer[256];
+static bool program_name_set = false;
+
+// Convenience function for initialization and checking what string to use
+// for the program name.
+static const char* get_syslog_ident(struct syslog_data* data)
+{
+    if (!program_name_set && data->ident == nullptr)
+        program_name_set = get_process_name(program_name_buffer, sizeof(program_name_buffer)) >= 0;
+
+    if (data->ident != nullptr)
+        return data->ident;
+    else if (program_name_set)
+        return program_name_buffer;
+
+    ASSERT_NOT_REACHED();
+}
+
+void openlog_r(const char* ident, int logopt, int facility, struct syslog_data* data)
+{
+    data->ident = ident;
+    data->logopt = logopt;
+    data->facility = facility;
+    // default value
+    data->maskpri = LOG_UPTO(LOG_DEBUG);
+    // would be where we connect to a daemon
+}
+
+void openlog(const char* ident, int logopt, int facility)
+{
+    openlog_r(ident, logopt, facility, &global_log_data);
+}
+
+void closelog_r(struct syslog_data* data)
+{
+    // would be where we disconnect from a daemon
+    // restore defaults
+    data->ident = nullptr;
+    data->logopt = 0;
+    data->facility = LOG_USER;
+    data->maskpri = LOG_UPTO(LOG_DEBUG);
+}
+
+void closelog(void)
+{
+    closelog_r(&global_log_data);
+}
+
+int setlogmask_r(int maskpri, struct syslog_data* data)
+{
+    // Remember, this takes the input of LOG_MASK/LOG_UPTO
+    int old_maskpri = data->maskpri;
+    data->maskpri = maskpri;
+    return old_maskpri;
+}
+
+int setlogmask(int maskpri)
+{
+    return setlogmask_r(maskpri, &global_log_data);
+}
+
+void syslog_r(int priority, struct syslog_data* data, const char* message, ...)
+{
+    va_list ap;
+    va_start(ap, message);
+    vsyslog_r(priority, data, message, ap);
+    va_end(ap);
+}
+
+void syslog(int priority, const char* message, ...)
+{
+    va_list ap;
+    va_start(ap, message);
+    vsyslog_r(priority, &global_log_data, message, ap);
+    va_end(ap);
+}
+
+void vsyslog_r(int priority, struct syslog_data* data, const char* message, va_list args)
+{
+    StringBuilder combined;
+
+    int real_priority = LOG_PRI(priority);
+    // Lots of parens, but it just extracts the priority from combo and masks.
+    if (!(data->maskpri & LOG_MASK(real_priority)))
+        return;
+
+    // Some metadata would be consumed by a syslog daemon, if we had one.
+    if (data->logopt & LOG_PID)
+        combined.appendf("%s[%d]: ", get_syslog_ident(data), getpid());
+    else
+        combined.appendf("%s: ", get_syslog_ident(data));
+
+    combined.appendvf(message, args);
+    String combined_string = combined.build();
+
+    if (data->logopt & LOG_CONS)
+        dbgputstr(combined_string.characters(), combined_string.length());
+    if (data->logopt & LOG_PERROR)
+        fputs(combined_string.characters(), stderr);
+}
+
+void vsyslog(int priority, const char* message, va_list args)
+{
+    vsyslog_r(priority, &global_log_data, message, args);
+}
+}