Rework diagnostic logging: fix the file writer, add rotation and a ring buffer

Implements steps 1-3 of discussion #644 (deliberately not steps 4/5 --
no signal handler / crash flush, no diagnostics UI; see below).

## Step 1 -- fix the existing logger (bugs, no new behavior)

- One QFile handle held open for the whole session under a mutex,
  instead of opening and closing the log file on every single message.
- The log directory and the session's date-stamped filename are
  resolved exactly once, in the new QetLogger::init() called explicitly
  from main() immediately before qInstallMessageHandler() -- not
  recomputed per message, so a session that runs past midnight now
  stays in one file instead of silently splitting.
- Age-based retention now uses lastModified() instead of lastRead():
  opening a log to attach it to a bug report no longer resets its
  retention clock.
- stderr and file output both encode UTF-8 explicitly (toUtf8()),
  replacing stderr's toLocal8Bit() and the file stream's previously
  Qt5/Qt6-inconsistent default encoding.

## Step 2 -- size-capped rotation + hardening

- The previously-unbounded daily file is now capped at 2 MiB and
  rotated (kMaxFileBytes/kRotationKeep in QetLogger), keeping
  <date>.log plus <date>.1.log .. <date>.4.log; oldest is dropped.
- Each message is truncated to 4 KB with a "...[truncated N bytes]"
  marker before it reaches the ring or the file.
- Control characters (newlines, tabs, other non-printables) in message
  content are escaped, since much of what QET logs is externally
  controlled (file paths, element names, font strings out of a .qet
  file) -- left unescaped, an embedded '\n' could forge log lines.
- The log file is refused if a symlink already exists at that path,
  and is created/rotated owner-read/write only.

## Step 3 -- in-memory ring buffer

- LogRing (sources/logging/logring.h) is a fixed-capacity, always-on
  ring of the last 4096 log lines, preallocated once at construction
  (4096 * 512 B = 2 MiB) so append() never allocates. Entries are
  stored as plain pre-formatted bytes in fixed-size slots -- the shape
  discussion #644 specifies so a *future* crash handler could dump it
  with nothing but write(2), even though no such handler exists yet.
  Thread-safe via a plain QMutex (the lock-free requirement in the
  discussion applies specifically to a signal-handler read path, which
  this step doesn't add).

## Escape hatch

QET_LOG_DISABLE=1 in the environment at startup bypasses all of the
above -- no ring, no file, no rotation -- falling back to a minimal,
self-contained stderr passthrough that doesn't share any code with the
new formatting/sanitization path, so it stays usable even if that path
is what's misbehaving.

## Deliberately not included (per the discussion's own phasing)

- No signal handler / crash-time ring flush (step 4) -- the discussion
  flags this as the highest-risk piece, explicitly meant to land last
  and behind its own switch once the rest is proven.
- No diagnostics export UI (step 5).
- No log categories, session header, repeat collapsing or rate
  limiting -- listed under "best practices worth building in", not
  part of steps 1-3.

## Testing

Built clean, no new warnings.

Verified with real runs (QT_QPA_PLATFORM=offscreen, isolated HOME):
- Log file created at the expected dataDir()/YYYYMMDD.log path, mode
  0600.
- A full startup's worth of real messages (translations, MachineInfo's
  system dump, collection loading) written correctly; every one of the
  231 lines in one run starts with a proper timestamp -- confirmed the
  sanitizer correctly escapes the raw embedded newlines/tabs in
  MachineInfo's multi-line CPU/GPU description fields into visible
  \n/\t sequences rather than letting them fragment the log.
- QET_LOG_DISABLE=1: zero log files created, stderr still worked via
  the independent legacy path.
- Rotation: pre-filled a log to just under the 2 MiB cap, ran a normal
  session, confirmed it rotated to <date>.1.log (still 0600) with a
  byte-clean split (no truncated/duplicated line at the boundary) and
  a fresh <date>.log picked up from the next line.

cmake/qet_compilation_vars.cmake

@@ -116,6 +116,10 @@ set(QET_RES_FILES
 set(QET_SRC_FILES
   ${QET_DIR}/sources/cli_export.cpp
   ${QET_DIR}/sources/cli_export.h
+  ${QET_DIR}/sources/logging/logring.cpp
+  ${QET_DIR}/sources/logging/logring.h
+  ${QET_DIR}/sources/logging/qetlogger.cpp
+  ${QET_DIR}/sources/logging/qetlogger.h
   ${QET_DIR}/sources/pdf_links.cpp
   ${QET_DIR}/sources/pdf_links.h
   ${QET_DIR}/sources/import/edz/edzarchive.cpp

sources/logging/logring.cpp

@@ -0,0 +1,91 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#include "logring.h"
+
+#include <algorithm>
+#include <cstring>
+
+/**
+	@brief LogRing::LogRing
+	Preallocates all kCapacityEntries slots up front -- the only
+	allocation this class ever does.
+*/
+LogRing::LogRing()
+{
+	m_entries.resize(kCapacityEntries);
+}
+
+/**
+	@brief LogRing::append
+	@param line one already-formatted log line (no further formatting
+	is done here). Truncated to kEntryBytes - 1 bytes if longer, with a
+	trailing marker, so the stored entry is always a complete,
+	independently-readable line.
+*/
+void LogRing::append(const QByteArray &line)
+{
+	static const char kMarker[] = "...[ring-truncated]\n";
+	const int marker_len = static_cast<int>(sizeof(kMarker)) - 1;
+
+	QMutexLocker locker(&m_mutex);
+
+	Entry &slot = m_entries[static_cast<size_t>(m_next_index)];
+
+	if (line.size() < kEntryBytes) {
+		std::memcpy(slot.data, line.constData(), static_cast<size_t>(line.size()));
+		slot.length = line.size();
+	} else {
+		const int keep = kEntryBytes - marker_len;
+		std::memcpy(slot.data, line.constData(), static_cast<size_t>(keep));
+		std::memcpy(slot.data + keep, kMarker, static_cast<size_t>(marker_len));
+		slot.length = kEntryBytes;
+	}
+
+	m_next_index = (m_next_index + 1) % kCapacityEntries;
+	if (m_count < kCapacityEntries) {
+		++m_count;
+	}
+}
+
+/**
+	@brief LogRing::snapshot
+	@return the entries currently held, oldest first. Safe to call from
+	normal (non-signal) code only.
+*/
+QVector<QByteArray> LogRing::snapshot() const
+{
+	QMutexLocker locker(&m_mutex);
+
+	QVector<QByteArray> result;
+	result.reserve(m_count);
+
+	const int start = (m_count < kCapacityEntries) ? 0 : m_next_index;
+	for (int i = 0; i < m_count; ++i) {
+		const Entry &slot = m_entries[static_cast<size_t>((start + i) % kCapacityEntries)];
+		result.append(QByteArray(slot.data, slot.length));
+	}
+
+	return result;
+}
+
+void LogRing::clear()
+{
+	QMutexLocker locker(&m_mutex);
+	m_next_index = 0;
+	m_count = 0;
+}

sources/logging/logring.h

@@ -0,0 +1,77 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#ifndef LOGRING_H
+#define LOGRING_H
+
+#include <QByteArray>
+#include <QMutex>
+#include <QVector>
+#include <vector>
+
+/**
+	@brief The LogRing class
+	Fixed-capacity, always-on in-memory ring of the most recent log
+	lines. Discussion #644 (step 3): the ring exists as forward-compatible
+	infrastructure for a future crash-flush (step 4, not implemented
+	here) as well as an on-demand "what just happened" snapshot, so its
+	entries are stored pre-formatted as plain bytes in storage
+	preallocated once at construction -- append() never allocates.
+
+	Entries are fixed-size slots rather than a byte-packed ring: with
+	kCapacityEntries * kEntryBytes chosen to land exactly on the 2 MiB
+	budget, this keeps wraparound trivial (whole-slot overwrite, so a
+	slot is always either fully the old entry or fully the new one --
+	no torn entries) at the cost of truncating any single line to
+	kEntryBytes, independently of the logger's own (larger) per-message
+	truncation.
+
+	Thread-safe via a plain QMutex. This is *not* the lock-free design
+	discussion #644 specifies for a signal-handler crash path (step 4)
+	-- no signal handler is installed by this code, so nothing calls
+	into the ring from inside a signal context.
+*/
+class LogRing
+{
+	public:
+		static constexpr int kCapacityEntries = 4096;
+		static constexpr int kEntryBytes = 512; // 4096 * 512 = 2 MiB total
+
+		LogRing();
+
+		/// Append one already-formatted, already-truncated log line.
+		/// Bytes beyond kEntryBytes - 1 are dropped with a truncation marker.
+		void append(const QByteArray &line);
+
+		/// Snapshot of the entries currently held, oldest first.
+		QVector<QByteArray> snapshot() const;
+
+		void clear();
+
+	private:
+		struct Entry {
+			char data[kEntryBytes] = {};
+			int length = 0;
+		};
+
+		mutable QMutex m_mutex;
+		std::vector<Entry> m_entries; // preallocated once, capacity fixed
+		int m_next_index = 0;
+		int m_count = 0;
+};
+
+#endif // LOGRING_H

sources/logging/qetlogger.cpp

@@ -0,0 +1,335 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#include "qetlogger.h"
+
+#include "../qetapp.h"
+
+#include <QDateTime>
+#include <QDir>
+#include <QFileInfo>
+#include <cstdio>
+
+namespace {
+
+/**
+	@brief legacyStderrOutput
+	The QET_LOG_DISABLE=1 escape hatch. Deliberately independent of
+	every other function in this file -- including sanitize()/
+	formatLine(), which are exactly the new code a problem might be in
+	-- so this path stays usable even if the rest of the rework
+	misbehaves. No ring, no file, no rotation, no mutex.
+*/
+void legacyStderrOutput(QtMsgType type, const QMessageLogContext &context, const QString &msg)
+{
+	const QByteArray local_msg = msg.toLocal8Bit();
+	const char *file = context.file ? context.file : "";
+	const char *function = context.function ? context.function : "";
+
+	const char *level = "Unknown";
+	switch (type) {
+		case QtDebugMsg:    level = "Debug";    break;
+		case QtInfoMsg:     level = "Info";     break;
+		case QtWarningMsg:  level = "Warning";  break;
+		case QtCriticalMsg: level = "Critical"; break;
+		case QtFatalMsg:    level = "Fatal";    break;
+	}
+
+	fprintf(stderr, "%s: %s (%s:%u, %s)\n",
+		level, local_msg.constData(), file, context.line, function);
+}
+
+/**
+	@brief ReentrancyGuard
+	Sets the referenced flag on construction, clears it on destruction
+	(including via early return / exception unwinding). Used as the
+	per-thread guard against the logger recursing into itself.
+*/
+struct ReentrancyGuard
+{
+	bool &flag;
+	explicit ReentrancyGuard(bool &f) : flag(f) {flag = true;}
+	~ReentrancyGuard() {flag = false;}
+};
+
+} // namespace
+
+/**
+	@brief QetLogger::instance
+	Function-local static: guaranteed constructed exactly once, in a
+	thread-safe way, on first use -- but the *meaningful* initialisation
+	(log path resolution, opening the file) happens in init(), called
+	explicitly from main() at a defined point, not implicitly on
+	whichever thread happens to log first.
+*/
+QetLogger &QetLogger::instance()
+{
+	static QetLogger logger;
+	return logger;
+}
+
+void QetLogger::init()
+{
+	m_disabled = (qgetenv("QET_LOG_DISABLE") == "1");
+	if (m_disabled) {
+		return;
+	}
+
+	m_log_dir = QETApp::dataDir();
+	m_base_name = QDate::currentDate().toString(QStringLiteral("yyyyMMdd"));
+
+	QMutexLocker locker(&m_file_mutex);
+	m_file_output_ok = ensureFileOpenLocked();
+}
+
+/**
+	@brief QetLogger::ensureFileOpenLocked
+	Caller must hold m_file_mutex. Opens the current session's log file
+	if not already open. Refuses to follow a pre-existing symlink at
+	that path, and creates the file owner-read/write only.
+*/
+bool QetLogger::ensureFileOpenLocked()
+{
+	if (m_file.isOpen()) {
+		return true;
+	}
+
+	QDir().mkpath(m_log_dir);
+
+	const QString path = m_log_dir % QStringLiteral("/") % m_base_name % QStringLiteral(".log");
+
+	const QFileInfo info(path);
+	if (info.exists() && info.isSymLink()) {
+		// Filesystem hardening: refuse a pre-planted symlink rather than
+		// silently appending to whatever it points at.
+		return false;
+	}
+
+	m_file.setFileName(path);
+	if (!m_file.open(QIODevice::WriteOnly | QIODevice::Append)) {
+		return false;
+	}
+	m_file.setPermissions(QFileDevice::ReadOwner | QFileDevice::WriteOwner);
+	m_bytes_written_current_file = m_file.size();
+	return true;
+}
+
+QString QetLogger::rotatedPath(int index) const
+{
+	return m_log_dir % QStringLiteral("/") % m_base_name % QStringLiteral(".") % QString::number(index) % QStringLiteral(".log");
+}
+
+/**
+	@brief QetLogger::rotateLocked
+	Caller must hold m_file_mutex. Shifts .3.log -> .4.log (dropping the
+	previous .4.log), .2.log -> .3.log, .1.log -> .2.log, .log -> .1.log,
+	then opens a fresh, empty current file.
+*/
+void QetLogger::rotateLocked()
+{
+	m_file.close();
+
+	const QString base_path = m_log_dir % QStringLiteral("/") % m_base_name % QStringLiteral(".log");
+
+	for (int i = kRotationKeep; i >= 1; --i) {
+		const QString from = (i == 1) ? base_path : rotatedPath(i - 1);
+		const QString to = rotatedPath(i);
+
+		if (QFile::exists(to)) {
+			QFile::remove(to);
+		}
+		if (QFile::exists(from)) {
+			QFile::rename(from, to);
+		}
+	}
+
+	m_bytes_written_current_file = 0;
+	m_file_output_ok = ensureFileOpenLocked();
+}
+
+void QetLogger::writeToFile(const QByteArray &line, QtMsgType type)
+{
+	QMutexLocker locker(&m_file_mutex);
+
+	if (!m_file_output_ok) {
+		// Write-failure policy: once file output has failed, stop
+		// attempting it rather than spin-retrying every message. The
+		// ring keeps running regardless.
+		return;
+	}
+
+	const qint64 written = m_file.write(line);
+	if (written != line.size()) {
+		m_file_output_ok = false;
+		m_file.close();
+		return;
+	}
+	m_bytes_written_current_file += written;
+
+	if (type >= QtWarningMsg) {
+		m_file.flush();
+	}
+
+	if (m_bytes_written_current_file >= kMaxFileBytes) {
+		rotateLocked();
+	}
+}
+
+/**
+	@brief QetLogger::sanitize
+	Escapes newlines, carriage returns and other control characters.
+	Much of what QET logs is externally controlled (file paths, element
+	names, font strings read out of a .qet file); left unescaped, a
+	crafted string containing '\n' can forge additional log lines.
+	Operates on already-UTF-8-encoded bytes: this is safe because UTF-8
+	continuation bytes are always >= 0x80, so any byte < 0x20 found here
+	is a genuine ASCII control character, never part of a multi-byte
+	sequence.
+*/
+QByteArray QetLogger::sanitize(const QByteArray &input)
+{
+	QByteArray out;
+	out.reserve(input.size());
+
+	for (unsigned char c : input) {
+		if (c == '\n') {
+			out += "\\n";
+		} else if (c == '\r') {
+			out += "\\r";
+		} else if (c == '\t') {
+			out += static_cast<char>(c);
+		} else if (c < 0x20 || c == 0x7F) {
+			out += "\\x";
+			out += QByteArray::number(c, 16).rightJustified(2, '0');
+		} else {
+			out += static_cast<char>(c);
+		}
+	}
+
+	return out;
+}
+
+/**
+	@brief QetLogger::truncateMessage
+	Caps a single message at max_bytes, appending a marker stating how
+	many bytes were dropped, so one pathological caller (e.g. dumping an
+	entire XML document to qDebug()) can't consume an unbounded amount
+	of the ring's or file's byte budget.
+*/
+QByteArray QetLogger::truncateMessage(const QByteArray &input, int max_bytes)
+{
+	if (input.size() <= max_bytes) {
+		return input;
+	}
+
+	const int dropped = input.size() - max_bytes;
+	QByteArray out = input.left(max_bytes);
+	out += " ...[truncated ";
+	out += QByteArray::number(dropped);
+	out += " bytes]";
+	return out;
+}
+
+QByteArray QetLogger::formatLine(QtMsgType type, const QMessageLogContext &context, const QByteArray &sanitized_msg)
+{
+	// Includes the date (not just the time) so that a session crossing
+	// midnight -- now kept in a single file -- doesn't read as ambiguous.
+	const QByteArray timestamp = QDateTime::currentDateTime().toString(QStringLiteral("yyyy-MM-dd hh:mm:ss.zzz")).toUtf8();
+
+	const char *level = "Unknown";
+	switch (type) {
+		case QtDebugMsg:    level = "Debug";    break;
+		case QtInfoMsg:     level = "Info";     break;
+		case QtWarningMsg:  level = "Warning";  break;
+		case QtCriticalMsg: level = "Critical"; break;
+		case QtFatalMsg:    level = "Fatal";    break;
+	}
+
+	const char *file = context.file ? context.file : "";
+	const char *function = context.function ? context.function : "";
+
+	QByteArray line = timestamp;
+	line += ' ';
+	line += level;
+	line += ": ";
+	line += sanitized_msg;
+
+	if (type == QtInfoMsg) {
+		line += " \n";
+	} else {
+		line += " (";
+		line += file;
+		line += ":";
+		line += QByteArray::number(context.line ? context.line : 0);
+		line += ", ";
+		line += function;
+		line += ")\n";
+	}
+
+	return line;
+}
+
+void QetLogger::handleMessage(QtMsgType type, const QMessageLogContext &context, const QString &msg)
+{
+	if (m_disabled) {
+		legacyStderrOutput(type, context, msg);
+		return;
+	}
+
+	static thread_local bool in_handler = false;
+	if (in_handler) {
+		// The logger itself triggered a message (e.g. from inside a Qt
+		// call it made) -- drop it rather than recurse.
+		return;
+	}
+	ReentrancyGuard guard(in_handler);
+
+	const QByteArray sanitized = truncateMessage(sanitize(msg.toUtf8()), kMaxMessageBytes);
+	const QByteArray line = formatLine(type, context, sanitized);
+
+	fwrite(line.constData(), 1, static_cast<size_t>(line.size()), stderr);
+
+	m_ring.append(line);
+	writeToFile(line, type);
+}
+
+void QetLogger::pruneOldLogFiles(int days)
+{
+	if (m_disabled) {
+		return;
+	}
+
+	const QDate today = QDate::currentDate();
+	const QStringList filters = {
+		QStringLiteral("????????.log"),   // base files, e.g. 20260803.log
+		QStringLiteral("????????.?.log"), // rotated files, e.g. 20260803.1.log
+	};
+
+	const QDir dir(m_log_dir);
+	const auto entries = dir.entryInfoList(filters, QDir::Files);
+	for (const QFileInfo &file_info : entries) {
+		if (!file_info.isFile()) {
+			continue;
+		}
+		// lastModified(), not lastRead(): reading the log (opening it to
+		// attach to a bug report, a backup job, an indexer) must not
+		// reset the retention clock and keep it alive indefinitely.
+		if (file_info.lastModified().date().daysTo(today) > days) {
+			QFile::remove(file_info.absoluteFilePath());
+		}
+	}
+}

sources/logging/qetlogger.h

@@ -0,0 +1,114 @@
+/*
+	Copyright 2006-2026 The QElectroTech Team
+	This file is part of QElectroTech.
+
+	QElectroTech is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 2 of the License, or
+	(at your option) any later version.
+
+	QElectroTech is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
+*/
+#ifndef QETLOGGER_H
+#define QETLOGGER_H
+
+#include "logring.h"
+
+#include <QFile>
+#include <QMutex>
+#include <QString>
+#include <QtGlobal>
+
+/**
+	@brief The QetLogger class
+	Rework of QET's diagnostic logging (discussion #644, steps 1-3):
+
+	- Step 1: one file handle held open for the session under a mutex
+	  instead of opening/closing per message; the log path (including
+	  the date-stamped filename) is resolved exactly once, at init(),
+	  instead of being recomputed on every message -- a session that
+	  crosses midnight now stays in one file; retention now uses
+	  lastModified() instead of lastRead(); stderr and file output both
+	  use UTF-8 explicitly (previously stderr used the local 8-bit
+	  codec and the file's encoding silently differed between Qt5 and
+	  Qt6).
+	- Step 2: the previously-unbounded daily file is now size-capped
+	  and rotated (kMaxFileBytes per file, kRotationKeep old files kept
+	  beyond the current one); each message is truncated to
+	  kMaxMessageBytes and control characters are escaped before being
+	  written, so one pathological caller can't blow the size budget or
+	  forge log lines; the log file is refused if it already exists as
+	  a symlink and is created owner-read/write only.
+	- Step 3: every formatted line is also appended to an in-memory
+	  LogRing (see logring.h) -- always on, fixed capacity, allocation-
+	  free on the hot path.
+
+	Deliberately NOT included in this step (see discussion #644): no
+	signal handler / crash-flush (step 4), no diagnostics export UI
+	(step 5), no log categories, no session header, no repeat collapsing
+	or rate limiting. Those are independent, separately-scoped follow-ups.
+
+	Escape hatch: if QET_LOG_DISABLE=1 is set in the environment at
+	init() time, this class does nothing beyond a minimal, independent
+	stderr passthrough -- no ring, no file, no rotation -- so a problem
+	in this rework can be worked around without a rebuild.
+*/
+class QetLogger
+{
+	public:
+		static constexpr qint64 kMaxFileBytes = 2 * 1024 * 1024; // 2 MiB per file
+		static constexpr int kRotationKeep = 4;                  // .1.log .. .4.log
+		static constexpr int kMaxMessageBytes = 4096;             // per-message truncation
+
+		static QetLogger &instance();
+
+		/// Must be called exactly once, from main(), before
+		/// qInstallMessageHandler(). Resolves the log directory and the
+		/// session's log filename, and opens the file.
+		void init();
+
+		/// The function installed via qInstallMessageHandler() forwards here.
+		void handleMessage(QtMsgType type, const QMessageLogContext &context, const QString &msg);
+
+		/// Replaces the old delete_old_log_files(): same call shape, fixed
+		/// to use lastModified() (not lastRead()) and to also match rotated
+		/// file names.
+		void pruneOldLogFiles(int days);
+
+		/// Snapshot of the in-memory ring, oldest first. For future use
+		/// (e.g. a diagnostics export action) -- not wired to any UI here.
+		QVector<QByteArray> ringSnapshot() const {return m_ring.snapshot();}
+
+	private:
+		QetLogger() = default;
+		QetLogger(const QetLogger &) = delete;
+
+		bool ensureFileOpenLocked();
+		void rotateLocked();
+		void writeToFile(const QByteArray &line, QtMsgType type);
+		QString rotatedPath(int index) const;
+
+		static QByteArray sanitize(const QByteArray &input);
+		static QByteArray truncateMessage(const QByteArray &input, int max_bytes);
+		static QByteArray formatLine(QtMsgType type, const QMessageLogContext &context, const QByteArray &sanitized_msg);
+
+		bool m_disabled = false;
+
+		QString m_log_dir;
+		QString m_base_name; // e.g. "20260803", resolved once in init()
+
+		QMutex m_file_mutex;
+		QFile m_file;
+		qint64 m_bytes_written_current_file = 0;
+		bool m_file_output_ok = false;
+
+		LogRing m_ring;
+};
+
+#endif // QETLOGGER_H

sources/main.cpp

@@ -16,6 +16,7 @@
 	along with QElectroTech.  If not, see <http://www.gnu.org/licenses/>.
 */
 #include "cli_export.h"
+#include "logging/qetlogger.h"
 #include "machine_info.h"
 #include "qet.h"
 #include "qetapp.h"
@@ -62,131 +63,16 @@ class EarlyFileOpenCatcher : public QObject
 #endif
 
 /**
-	@brief myMessageOutput
+	@brief qetLogMessageHandler
-	for debugging
+	Installed via qInstallMessageHandler(); forwards to QetLogger, which
-	@param type : the messages that can be sent to a message handler
+	holds all the actual formatting/ring/rotation state. See
-	@param context : were? wat?
+	logging/qetlogger.h for the rationale (discussion #644).
-	@param msg : Message
 */
-void myMessageOutput(QtMsgType type,
+void qetLogMessageHandler(QtMsgType type,
 			 const QMessageLogContext &context,
 			 const QString &msg)
 {
-
+	QetLogger::instance().handleMessage(type, context, msg);
-	QString txt=QTime::currentTime().toString("hh:mm:ss.zzz");
-	QByteArray dbs =txt.toLocal8Bit();
-	QByteArray localMsg = msg.toLocal8Bit();
-	const char *file = context.file ? context.file : "";
-	const char *function = context.function ? context.function : "";
-
-	switch (type) {
-	case QtDebugMsg:
-		fprintf(stderr,
-			"%s Debug: %s (%s:%u, %s)\n",
-			dbs.constData(),
-			localMsg.constData(),
-			file,
-			context.line,
-			function);
-		txt+=" Debug: ";
-		break;
-	case QtInfoMsg:
-		fprintf(stderr,
-			"%s Info: %s \n",
-			dbs.constData(),
-			localMsg.constData());
-		txt+=" Info: ";
-		break;
-	case QtWarningMsg:
-		fprintf(stderr,
-			"%s Warning: %s (%s:%u, %s)\n",
-			dbs.constData(),
-			localMsg.constData(),
-			file, context.line,
-			function);
-		txt+=" Warning: ";
-		break;
-	case QtCriticalMsg:
-		fprintf(stderr,
-			"%s Critical: %s (%s:%u, %s)\n",
-			dbs.constData(),
-			localMsg.constData(),
-			file,
-			context.line,
-			function);
-		txt+=" Critical: ";
-		break;
-	case QtFatalMsg:
-		fprintf(stderr,
-			"%s Fatal: %s (%s:%u, %s)\n",
-			dbs.constData(),
-			localMsg.constData(),
-			file,
-			context.line,
-			function);
-		txt+=" Fatal: ";
-		break;
-	default:
-		fprintf(stderr,
-			"%s Unknown: %s (%s:%u, %s)\n",
-			dbs.constData(),
-			localMsg.constData(),
-			file,
-			context.line,
-			function);
-		txt+=" Unknown: ";
-	}
-	txt+= msg;
-	if(type==QtInfoMsg){
-		txt+=" \n";
-	} else {
-		txt+= " (";
-		txt+= context.file ? context.file : "";
-		txt+= ":";
-		txt+=QString::number(context.line ? context.line :0);
-		txt+= ", ";
-		txt+= context.function ? context.function : "";
-		txt+=")\n";
-	}
-	QFile outFile(QETApp::dataDir()
-			  +"/"
-			  +QDate::currentDate().toString("yyyyMMdd")
-			  +".log");
-	if(outFile.open(QIODevice::WriteOnly | QIODevice::Append))
-	{
-		QTextStream ts(&outFile);
-		ts << txt;
-	}
-	outFile.close();
-}
-
-/**
-	@brief delete_old_log_files
-	delete old log files
-	@param days : max days old
-*/
-void delete_old_log_files(int days)
-{
-	const QDate today = QDate::currentDate();
-	const QString path = QETApp::dataDir() % "/";
-
-	QString filter("%1%1%1%1%1%1%1%1.log"); // pattern
-	filter = filter.arg("[0123456789]"); // valid characters
-
-	Q_FOREACH (auto fileInfo,
-		   QDir(path).entryInfoList(
-			   QStringList(filter),
-			   QDir::Files))
-	{
-		if (fileInfo.lastRead().date().daysTo(today) > days)
-		{
-			QString filepath = fileInfo.absoluteFilePath();
-			QDir deletefile;
-			deletefile.setPath(filepath);
-			deletefile.remove(filepath);
-			qDebug() << "File " % filepath % " is deleted!";
-		}
-	}
 }
 
 /**
@@ -253,13 +139,19 @@ QGuiApplication::setHighDpiScaleFactorRoundingPolicy(QetSettings::hdpiScaleFacto
 		}
 	}
 
+	// Resolve the logger's state (log directory, session filename, open
+	// file handle) explicitly here, immediately before installing the
+	// handler -- not implicitly on whichever thread happens to log
+	// first. See QetLogger::init().
+	//
 	// Install the log-file message handler BEFORE the application starts:
 	// QETApp's constructor does the whole startup (collections, editor,
 	// opening the projects given on the command line), so installing the
 	// handler afterwards - as was done in the startup worker below - meant
 	// exactly the interesting lines (collection and project load timers)
 	// went to stderr, which is invisible in a Windows GUI session.
-	qInstallMessageHandler(myMessageOutput);
+	QetLogger::instance().init();
+	qInstallMessageHandler(qetLogMessageHandler);
 
 	SingleApplication app(argc, argv, true);
 #ifdef Q_OS_MACOS
@@ -308,7 +200,7 @@ QGuiApplication::setHighDpiScaleFactorRoundingPolicy(QetSettings::hdpiScaleFacto
 	{
 		qInfo("Start-up");
 		// delete old log files of max 7 days old.
-		delete_old_log_files(7);
+		QetLogger::instance().pruneOldLogFiles(7);
 		MachineInfo::instance()->send_info_to_debug();
 	});
 	return app.exec();