Implement poorman's log rotating

This implements a very simple circular buffer that will take the last N
lines in a log file, and keep those instead.  Right now the maximum N is
hardcoded, but the code is written in a fashion that we can revisit some
of this, and add toggles where we see fit (for example in a settings
dialog or so).

I tested out the functionality this way:

        $ yes some-log-line | nl | head -5000 > log.txt

And then running PTE2 with a max log size of 1000, I noticed the logfile
like so:

        $ head -10 log.txt
        4001  some-log-line
        4002  some-log-line
        4003  some-log-line
        4004  some-log-line
        4005  some-log-line
        4006  some-log-line
        4007  some-log-line
        4008  some-log-line
        4009  some-log-line
        4010  some-log-line

and the bottom:

        $ tail -10 log.txt
        2023-12-30T14:38:32Z: [debug]: finding translations for locale:
        2023-12-30T14:38:32Z: [debug]:   locale: en-US
        2023-12-30T14:38:32Z: [debug]:   locale: en
        2023-12-30T14:38:32Z: [debug]:   locale: en-Latn-US
        2023-12-30T14:38:32Z: [debug]:   - checking: /home/psyomn/.local/share/powertab/powertabeditor/translations
        2023-12-30T14:38:32Z: [debug]:   - checking: /usr/local/share/powertab/powertabeditor/translations
        2023-12-30T14:38:32Z: [debug]:   - checking: /usr/share/powertab/powertabeditor/translations
        2023-12-30T14:38:32Z: [debug]:   - checking: /home/psyomn/programming/cc/fork/powertabeditor/build/bin/data/translations
        2023-12-30T14:38:32Z: [debug]:   - checking: /usr/share/qt/translations
        2023-12-30T14:38:32Z: [debug]: loaded qt base translations from /usr/share/qt/translations/qtbase_en.qm

and last checks:

        $ nl log.txt | head -1
             1    4001  some-log-line
        $ nl log.txt | tail -1
          1011  2023-12-30T14:38:32Z: [debug]: loaded qt base translations from /usr/share/qt/translations/qtbase_en.qm

source/util/log.cpp

@@ -25,10 +25,47 @@ std::filesystem::path logPath;
 
 std::mutex lock;
 
+void trim(const unsigned int count)
+{
+    /* a poorman's circular buffer that keeps the last 'count' log lines. */
+    std::vector<std::string> v(count);
+
+    std::ifstream input(logPath);
+    std::size_t index = 0;
+    std::string s;
+    while (std::getline(input, s)) {
+        v[index % count] = s;
+	index++;
+    }
+
+    input.close();
+
+    // required because of len/cap discrepancy in vector used as a circular
+    // buffer.
+    const auto upto = count < index ? index : count;
+
+    std::ofstream output(logPath, std::ios_base::out);
+    if (output.good()) {
+        for (size_t i = 0; i < upto; ++i)
+            output << v[(index + i) % count] << std::endl;
+    } else {
+        std::cerr << "could not open log file for trimming" << std::endl;
+    }
+    output.close();
+}
+
 void init(enum Level lvl, std::filesystem::path lp)
 {
     FilterLevel = lvl;
     logPath = lp;
+
+    // keep only the last X lines
+    trim(1000);
+
+    if (!logFile) {
+        const auto mode = std::ios_base::out | std::ios_base::app;
+        logFile = std::ofstream(logPath, mode);
+    }
 }
 
 std::string all()

source/util/log.h

@@ -24,8 +24,8 @@
 #include <sstream>
 #include <filesystem>
 #include <optional>
-#include <vector>
 #include <mutex>
+#include <vector>
 
 /* TODO: fmtlib: replace with std, when all compilers support `format'. */
 #include <fmt/core.h>
@@ -59,6 +59,7 @@ void init(enum Level lvl, std::filesystem::path logPath);
  */
 std::string all();
 
+
 template <typename... Args>
 void backend(enum Level level, std::string_view fmt, Args&&... args)
 {
@@ -94,11 +95,6 @@ void backend(enum Level level, std::string_view fmt, Args&&... args)
         << fmt::vformat(fmt, fmt::make_format_args(args...))
         << std::endl;
 
-    if (!logFile) {
-        const auto mode = std::ios_base::out | std::ios_base::app;
-        logFile = std::ofstream(logPath, mode);
-    }
-
     if (logFile.value().good()) {
         logFile.value()
             << timestamp_now_str_fn() << ": "